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Electrodeposition and corrosion study of nanocrystalline cobalt and cobalt-iron alloy coatings Nik Mohd Masdek, Nik Rozlin 2014

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Electrodeposition and Corrosion Study of Nanocrystalline Cobalt and Cobalt-Iron Alloy Coatings   by Nik Rozlin Nik Mohd Masdek B. Eng.  Universiti of Malaya, 2002 M. Eng. Universiti of Malaya, 2007   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES  (Materials Engineering)   THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  September 2014 © Nik Rozlin Nik Mohd Masdek, 2014 	  	    ii Abstract Nanocrystalline materials with grain sizes less than 100 nm, have attracted considerable attention due to their enhanced properties as compared to their polycrystalline counterparts. However, investigation on the corrosion resistance of these materials is still lacking. In order to further expand their future applications, their corrosion behaviour is of great importance. Hence, in this study, nanocrystalline Co and CoFe alloy coatings were prepared through the electrodeposition process and their electrochemical corrosion behaviour were investigated. Depending on the environment, the effect of the nanocrystaline grain sizes as well as Fe alloying resulted in different corrosion responses. A decrease in grain size was observed with an increase in iron concentration that also leads to a change in crystal structure from HCP to BCC phase structure. Meanwhile, microhardness was first seen to increase gradually with the increase in iron content before it decreases with higher iron concentration of more than 18 wt%Fe. The corrosion properties of these electrodeposited nanocrystalline Co and CoFe alloy coatings were also studied in solutions ranging from acidic to alkaline. Nanocrystalline Co and CoFe exhibit higher corrosion rates in 0.1 M H2SO4 than in an alkaline environment. In deaerated acidic solutions, both nanocrystalline Co and CoFe alloy coatings showed only active anodic dissolution without any transition to passivation. Meanwhile, two stages of passivation were present in the alkaline solution which is reported to be due to the formation of a duplex passive film.  	  	    iii The presence and absence of saccharin as an additive in the electrolyte both showed detrimental and beneficial effect on the corrosion behaviour of these nanostructured deposited coatings. By employing the electrochemical quartz crystal microbalance (EQCM), it was observed that the mass decrease was significant with an increase in saccharin concentration indicating an active dissolution. However, saccharin was observed to hinder the formation of a protective passive film on CoFe alloy coatings in alkaline solution. The presence of the sulphide ion also significantly decreased the corrosion resistance of deposited CoFe alloy coatings. The presence of sulphide accelerates markedly the anodic reaction. The dissolution rate was two times faster when S2− was added.      	  	    iv Preface All research work presented in this thesis was performed in the corrosion laboratory in the Department of Materials Engineering, University of British Columbia under the supervision of Professor Akram Alfantazi. The following papers were published and presented in this dissertation. I am the primary author of all the published work in which Professor Akram Alfantazi co-authored. He had provided me with suggestions for revision and had extensively helped with all the aspects of the research work.  Journal papers: 1. Nik Masdek, N.R., and Alfantazi, A.M. Electrochemical behavior of electrodeposited nanocrystalline CoFe alloy coating in 0.1M Na2SO4 with different pH and sulphide ion concentration. Submitted 2. Nik Masdek, N.R., and Alfantazi, A.M. (2014) An EQCM study on the influence of saccharin on the corrosion properties of nanostructured cobalt and cobalt-iron alloy coatings, Journal of Solid State Electrochemistry. doi:10.1007/s10008-014-2417-z 3. Nik Masdek, N.R., and Alfantazi, A.M. (2013) Electrochemical Properties of Electrodeposited Nanocrystalline Cobalt and Cobalt-Iron Alloys in Acidic and Alkaline Solutions, Journal of Applied Electrochemistry, Volume 43, Issue 7, pp. 721-734. doi: 10.1007/s10800-013-0562-1 	  	    v 4. Nik Masdek, N.R., and Alfantazi, A.M.  (2012) Nanocrystalline Cobalt-Iron Alloy: Synthesis and Characterization, Material Science and Engineering A, Volume 550, pp. 388–394. doi:http://dx.doi.org/10.1016/j.msea.2012.04.092  Conference transaction and presentation 1. Nik Masdek, N.R., and Alfantazi, A.M., (2010) Review of Studies on Corrosion of Electrodeposited Nanocrystalline Metals and Alloys, ECS Transactions, 28, 249. 2. Nik Masdek, N.R., and Alfantazi, A.M., Synthesis of Nanocrystalline Cobalt-Iron Alloy and Its Characterization, 221st ECS Meeting, Seattle, Washington, May 2012.      	  	    vi Table of Contents Abstract	  ....................................................................................................................................	  ii	  Preface	  ......................................................................................................................................	  iv	  Table	  of	  Contents	  ..................................................................................................................	  vi	  List	  of	  Tables	  ..........................................................................................................................	  xi	  List	  of	  Figures	  ......................................................................................................................	  xiv	  List	  of	  Symbols	  ..................................................................................................................	  xxiii	  List	  of	  Acronyms	  ................................................................................................................	  xxv	  Acknowledgements	  ........................................................................................................	  xxvii	  Dedication	  .......................................................................................................................	  xxviii	  1	   Introduction	  .....................................................................................................................	  1	  2	   Literature	  Review	  ...........................................................................................................	  4	  2.1	   Nanomaterials	  ....................................................................................................................	  4	  2.2	   Synthesis	  of	  Nanomaterials	  ..........................................................................................	  8	  2.3	   Electrodeposited	  Nanocrystalline	  Co	  and	  CoFe	  Alloy	  ....................................	  13	  2.3.1	   Phase	  Diagram	  of	  CoFe	  Alloy	  ...........................................................................	  13	  2.3.2	   Electrodeposition	  Mechanism	  ........................................................................	  17	  2.3.3	   Electrodeposition	  Parameters	  ........................................................................	  20	  2.3.4	   Applications	  ............................................................................................................	  24	  2.4	   Corrosion	  Behaviour	  of	  Nanocrystalline	  Materials	  .........................................	  25	  	  	    vii 2.4.1	   Corrosion	  Properties	  of	  Pure	  Metals	  ...........................................................	  27	  2.4.2	  	   Corrosion	  Properties	  of	  Alloys	  .......................................................................	  35	  2.5	   Other	  Properties	  of	  Nanomaterials	  ........................................................................	  40	  2.6	   Previous	  Studies	  on	  Electrodeposited	  CoFe	  Alloy	  ...........................................	  44	  3	   Objectives	  .......................................................................................................................	  53	  4	   Experimental	  Approach	  and	  Methodology	  .........................................................	  55	  4.1	   Electrodeposition	  of	  Nanocrystalline	  Co	  and	  CoFe	  Alloy	  Coatings	  .............................................................................................................................	  55	  4.1.1	   Surface	  and	  Chemical	  Characterization	  ......................................................	  58	  4.1.2	   XRD	  .............................................................................................................................	  59	  4.1.3	   TEM	  ............................................................................................................................	  60	  4.1.4	   Microhardness	  .......................................................................................................	  60	  4.2	   Electrochemical	  Corrosion	  Measurements	  of	  Nanocrystalline	  Co	  and	  CoFe	  Alloy	  Coatings	  .......................................................................................	  61	  4.2.1	   Electrolytes	  .............................................................................................................	  61	  4.2.2	   Potentiodynamic	  Polarization	  Test	  ..............................................................	  62	  4.2.3	   EIS	  ...............................................................................................................................	  62	  4.2.4	   LPR	  ..............................................................................................................................	  63	  4.2.5	   Potentiostatic	  Polarization	  ...............................................................................	  63	  4.2.6	   XPS	  ..............................................................................................................................	  63	  4.3	   Electrochemical	  Quartz	  Crystal	  Microbalance	  ..................................................	  64	  	  	    viii 5	   Nanocrystalline	  Cobalt-­‐Iron	  Alloy:	  Synthesis	  and	  Characterization	  .......................................................................................................	  68	  5.1	   Effect	  of	  FeSO4	  Concentration	  ..................................................................................	  69	  5.2	   Current	  Efficiency	  ..........................................................................................................	  71	  5.3	   SEM	  and	  TEM	  ...................................................................................................................	  72	  5.4	   XRD	  Analysis	  ....................................................................................................................	  75	  5.5	   Grain	  Size	  Decreases	  With	  Iron	  Content	  ..............................................................	  77	  5.6	   Microhardness	  ................................................................................................................	  78	  5.7	   Summary	  ...........................................................................................................................	  82	  6	   Electrochemical	  Properties	  of	  Electrodeposited	  Nanocrystalline	  Cobalt	  and	  Cobalt-­‐Iron	  Alloys	  in	  Acidic	  and	  Alkaline	  Solutions	  ...............	  84	  6.1	   Chemical	  Composition,	  Grain	  Size	  and	  Surface	  Morphology	  of	  CoFe	  Alloy	  Coatings	  ......................................................................................................	  85	  6.2	   Acidic	  Conditions	  ...........................................................................................................	  88	  6.2.1	  	   Open	  Circuit	  Potential	  Measurements	  in	  0.1	  M	  H2SO4	  .........................	  88	  6.2.2	   Potentiodynamic	  Polarization	  in	  0.1	  M	  H2SO4	  .........................................	  89	  6.2.3	   Electrochemical	  Impedance	  Spectroscopy	  (EIS)	  in	  0.1	  M	  H2SO4	  .......................................................................................................................	  93	  6.2.4	   X-­‐ray	  Photoelectron	  Spectroscopy	  Analysis	  .............................................	  97	  6.2.5	   Corrosion	  Morphologies	  .................................................................................	  103	  6.3	   Alkaline	  Condition	  ......................................................................................................	  105	  	  	    ix 6.3.1	   Open	  Circuit	  Potential	  Measurements	  in	  0.1	  M	  NaOH	  ......................	  105	  6.3.2	   Potentiodynamic	  Polarization	  in	  0.1	  M	  NaOH	  ......................................	  106	  6.3.2	   Electrochemical	  Impedance	  Spectroscopy	  (EIS)	  in	  0.1	  M	  NaOH	  .....................................................................................................................	  109	  6.3.3	   X-­‐ray	  Photoelectron	  Spectroscopy	  Analysis	  ..........................................	  111	  6.3.4	   Corrosion	  Morphologies	  .................................................................................	  117	  6.4	   Summary	  ........................................................................................................................	  119	  7	   An	  EQCM	  Study	  on	  the	  Influence	  of	  Saccharin	  on	  the	  Corrosion	  Properties	  of	  Nanostructured	  Cobalt	  and	  Cobalt-­‐Iron	  Alloy	  Coatings	  ....................................................................................................................	  120	  7.1	   Characterization	  of	  Nanostructured	  Co	  and	  CoFe	  Alloy	  Coatings	  ..........................................................................................................................	  121	  7.2	   Electrochemical	  Quartz	  Crystal	  Microbalance	  Study	  ..................................	  129	  7.2.1	   CV	  and	  Mass	  Variation:	  Influence	  of	  Saccharin	  Concentration	  ...................................................................................................	  129	  7.2.2	   Mass	  and	  Potential	  Changes:	  Influence	  of	  Saccharin	  .........................	  135	  7.2.3	   Electrochemical	  Polarization	  Measurements	  .......................................	  143	  7.2.4	   Electrochemical	  Impedance	  Spectroscopy	  .............................................	  150	  7.2.5	   X-­‐ray	  Photoelectron	  Spectroscopy	  (XPS)	  ...............................................	  158	  7.3	   Summary	  ........................................................................................................................	  164	  	  	    x 8	   Electrochemical	  Behavior	  of	  Electrodeposited	  Nanocrystalline	  CoFe	  Alloy	  Coating	  in	  0.1	  M	  Na2SO4	  With	  Different	  pH	  and	  Sulphide	  Ion	  Concentration	  ...............................................................................	  166	  8.1	   Effect	  of	  pH	  Solution	  ..................................................................................................	  167	  8.2	   EQCM	  Measurements	  ................................................................................................	  170	  8.3	   Electrochemical	  Polarization	  Measurements	  .................................................	  172	  8.4	   EIS	  ......................................................................................................................................	  174	  8.5	   Effect	  of	  Sulphide	  Ion	  ................................................................................................	  179	  8.6	   EQCM	  Measurements	  ................................................................................................	  180	  8.7	   Potentiodynamic	  Polarization	  ...............................................................................	  181	  8.8	   EIS	  ......................................................................................................................................	  185	  8.9	   Summary	  ........................................................................................................................	  189	  9	   Conclusions	  .................................................................................................................	  190	  10	   Recommendations	  and	  Future	  Work	  ..............................................................	  194	  11	   References	  ................................................................................................................	  196	  Appendix	  A	  .........................................................................................................................	  228	  Appendix	  B	  .........................................................................................................................	  232	     	  	    xi List of Tables Table	  2.	  1	   List	  of	  various	  techniques	  used	  to	  produce	  nanocrystalline	  materials	  ............................................................................................................................	  9	  	  Table	  4.	  1	   Bath	  composition	  and	  electrodepositing	  parameters	  for	  Co	  and	  CoFe	  alloy	  deposits	  ............................................................................................	  57	  	  Table	  6.	  1	   Iron	  composition	  and	  grain	  size	  of	  nanocrystalline	  Co	  and	  CoFe	  alloy	  .......................................................................................................................	  85	  Table	  6.	  2	   Potentiodynamic	  polarization	  data	  of	  Co	  and	  CoFe	  alloys	  in	  0.1M	  H2SO4	  .....................................................................................................................	  91	  Table	  6.	  3	   EIS	  data	  obtained	  by	  equivalent	  circuit	  simulation	  in	  0.1	  M	  H2SO4	  ................................................................................................................................	  96	  Table	  6.	  4	   Atom	  concentration	  ratio	  (at%)	  of	  elements	  in	  XPS	  analysis	  for	  as-­‐deposited	  samples	  and	  polarized	  in	  0.1	  M	  H2SO4	  .........................	  102	  Table	  6.	  5	   Potentiodynamic	  polarization	  data	  of	  CoFe	  alloys	  in	  0.1	  M	  NaOH	  .............................................................................................................................	  108	  Table	  6.	  6	   EIS	  data	  obtained	  by	  equivalent	  circuit	  simulation	  in	  0.1	  M	  NaOH	  .............................................................................................................................	  111	  	  	    xii Table	  6.	  7	   Atom	  concentration	  ratio	  (at.%)	  of	  elements	  in	  XPS	  analysis	  for	  as-­‐deposited	  samples	  and	  polarized	  in	  0.1	  M	  NaOH	  .............................................................................................................................	  113	  	  Table	  7.	  1	   Rate	  of	  mass	  change	  of	  immersion	  test	  in	  the	  absence	  and	  presence	  of	  saccharin	  at	  different	  concentrations	  in	  0.1	  M	  H2SO4	  and	  0.1	  M	  NaOH	  ..........................................................................................	  140	  Table	  7.	  2	   Corrosion	  potential	  and	  corrosion	  current	  density	  of	  Co	  and	  CoFe	  with	  different	  saccharin	  concentrations	  obtained	  from	  potentiodynamic	  polarization	  curves	  in	  0.1	  M	  H2SO4	  and	  0.1	  M	  NaOH	  ........................................................................................................	  146	  Table	  7.	  3	   Equivalent	  circuit	  fitting	  for	  the	  EIS	  data	  of	  the	  nanostructured	  Co	  and	  CoFe	  deposits	  ............................................................	  156	  Table	  7.	  4	   Surface	  film	  composition	  (at%)	  of	  as-­‐deposited	  and	  polarized	  nanostructured	  CoFe	  alloy	  coating	  ..............................................	  159	  	  Table	  8.	  1	   Corrosion	  parameters	  of	  CoFe	  alloys	  after	  60	  minutes	  of	  electrode	  immersion	  in	  stagnant	  0.1	  M	  Na2SO4	  aqueous	  solutions	  of	  pH	  5,	  7,	  9,	  and	  11,	  at	  25°C	  ...........................................................	  173	  Table	  8.	  2	   Equivalent	  circuit	  parameters	  of	  CoFe	  alloy	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11,	  after	  1	  hour	  of	  OCP.	  .............................	  178	  	  	    xiii Table	  8.	  3	   Corrosion	  parameters	  of	  CoFe	  alloy	  coating	  in	  0.1	  M	  Na2SO4	  solutions	  with	  different	  sulphide	  ion	  concentrations.	  .............................	  184	  Table	  8.	  4	   Equivalent	  circuit	  parameters	  of	  CoFe	  alloy	  in	  0.1	  M	  Na2SO4	  solutions	  with	  different	  sulphide	  ion	  concentrations	  after	  1	  hour	  OCP.	  .....................................................................................................................	  186	  	    	  	    xiv List of Figures Figure	  2.	  1	   SEM	  microstructure	  of	  a	  polycrystalline	  and	  nanocrystalline	  (left)	  and	  high	  resolution	  TEM	  image	  of	  a	  chemically-­‐similar	  material	  [courtesy	  of	  Integran	  Technologies	  Inc.]	  (Brooks,	  2012)	  .........................................................................	  6	  Figure	  2.	  2	   Schematic	  illustration	  of	  typical	  microstructures	  produced	  by	  the	  pre-­‐eminent	  nanomaterials	  synthesis	  methods;	  (a)	  nanopowders,	  (b)	  consolidated	  nanopowders,	  (c)	  deformation-­‐induced	  nanostructures,	  (d)	  nanograins	  crystallized	  from	  an	  amorphous	  phase,	  (e)	  nanotwinned	  material,	  (f)	  electrodeposited	  nanocrystalline	  material	  [courtesy	  of	  U.	  Erb]	  (Brooks,	  	  2012)	  ......................................................................	  7	  Figure	  2.	  3	   Schematic	  diagram	  of	  the	  various	  stages	  of	  electrocrystallization	  (Choo	  et	  al.,	  1995).	  ...........................................................	  8	  Figure	  2.	  4	   Various	  types	  of	  electrodeposited	  coatings	  (Lu	  and	  Lu,	  1999).	  ...............................................................................................................................	  12	  Figure	  2.	  5	   Schematic	  diagram	  of	  the	  components	  of	  the	  electrodeposition	  cell	  ................................................................................................	  18	  Figure	  2.	  6	   Potentiodynamic	  polarization	  curves	  of	  nickel	  of	  different	  grain	  sizes	  in	  1	  mol/L	  H2SO4	  at	  a	  scan	  rate	  of	  0.5mV/s.	  The	  	  	    xv experiments	  were	  conducted	  after	  stabilization	  of	  free	  corrosion	  potentials	  (Mishra	  and	  Balasubramaniam,	  2004).	  ..................	  30	  Figure	  2.	  7	   Images	  of	  nc	  zinc	  (a)	  and	  EG	  steel	  (b)	  in	  deaerated	  0.5	  N	  NaOH	  after	  potentiodynamic	  polarization	  (Youssef	  et	  al.,	  2004).	  ...............................................................................................................................	  32	  Figure	  2.	  8	   Polarization	  curves	  obtained	  from	  polycrystalline	  and	  nanocrystalline	  Co	  in	  (a)	  Na2SO4	  (pH	  6.5)	  and	  (b)	  NaOH	  (pH13)	  at	  scan	  rate:0.2	  mV/s	  (Kim	  et	  al.,	  2003)	  ...........................................	  34	  Figure	  2.	  9	   SEM	  micrographs	  of	  the	  corroded	  surfaces	  of:	  (a)	  micro	  Co;	  (b)	  nano	  Co	  after	  potentiodynamic	  polarization	  scan	  in	  deaerated	  0.1	  M	  H2SO4	  (Gonzalez	  et	  al.,	  1996).	  .............................................	  35	  Figure	  2.	  10	   Cathodic	  polarization	  curves	  of	  Co-­‐W	  electrodeposition	  from	  electrolytes	  with	  different	  additives	  (Wei	  et	  al.,	  2007)	  ..................	  39	  Figure	  2.	  11	   Schematic	  diagram	  of	  the	  variation	  of	  yield	  strength	  as	  a	  function	  of	  grain	  size	  (Kumar	  et	  al.,	  2003).	  .....................................................	  43	  	  Figure	  4.	  1	   Schematic	  diagram	  of	  the	  electrodeposition	  process	  .................................	  56	  Figure	  4.	  2	   Schematic	  diagram	  of	  AT	  cut	  quartz	  crystal	  ...................................................	  65	  Figure	  4.	  3	   Interconnections	  between	  EQCM,	  potentiostat,	  and	  computer	  ........................................................................................................................	  67	  	  	  	    xvi Figure	  5.	  1	   Iron	  content	  (wt%)	  of	  the	  CoFe	  alloy	  coatings	  as	  a	  function	  of	  the	  FeSO4	  in	  the	  solution.	  ...................................................................................	  69	  Figure	  5.	  2	   EDS	  spectrum	  of	  CoFe	  alloy	  (a)	  8	  wt%	  Fe.	  (b)	  25	  wt%	  Fe.	  .......................	  70	  Figure	  5.	  3	   Current	  efficiency	  (%)	  as	  a	  function	  of	  current	  density	  (A/cm2)	  ...........................................................................................................................	  72	  Figure	  5.	  4	   SEM	  images	  of	  CoFe	  alloy	  electrodeposited	  with	  different	  iron	  contents.	  (a)	  0	  wt	  %	  Fe,	  (b)	  5	  wt%	  Fe	  (c)	  11	  wt%	  Fe	  (d)17	  wt%	  Fe	  (e)	  25	  wt%	  Fe	  (f)	  Deposit	  consisting	  of	  multiple	  cracks.	  ............................................................................................................	  73	  Figure	  5.	  5	   TEM	  images	  of	  CoFe	  alloy	  electrodeposited	  with	  different	  iron	  contents.	  (a)	  17	  wt%	  Fe,	  (b)	  25	  wt%	  Fe	  ..................................................	  74	  Figure	  5.	  6	   X-­‐ray	  diffraction	  patterns	  for	  Co	  and	  CoFe	  alloy	  coatings;	  (a)	  100	  wt%Co	  (b)	  	  5	  wt%	  Fe	  (c)	  11	  wt%	  Fe	  (d)	  17	  wt%	  Fe	  (e)	  25	  wt%	  Fe	  ...............................................................................................................	  76	  Figure	  5.	  7	   Variation	  of	  grain	  sizes	  of	  CoFe	  alloy	  coatings	  as	  a	  function	  of	  iron	  content	  (wt%).	  ..............................................................................................	  78	  Figure	  5.	  8	   Variation	  of	  microhardness	  of	  CoFe	  alloy	  coatings	  as	  a	  function	  of	  Fe	  content	  (wt%).	  ................................................................................	  80	  Figure	  5.	  9	   Variation	  of	  microhardness	  of	  CoFe	  alloy	  coatings	  as	  a	  function	  of	  grain	  size.	  ................................................................................................	  81	  	  	  	    xvii Figure	  6.	  1	   TEM	  images	  of	  CoFe	  alloy	  electrodeposited	  with	  different	  iron	  contents.	  (a)	  11	  wt%	  Fe	  and	  (b)	  25	  wt%	  Fe.	  .........................................	  86	  Figure	  6.	  2	   SEM	  images	  of	  CoFe	  alloy	  electrodeposited	  with	  different	  iron	  contents.	  (a)	  100	  wt	  %	  Co	  (b)	  5	  wt%	  Fe	  (c)	  11	  wt%	  Fe	  (d)	  25	  wt%	  Fe	  ...............................................................................................................	  87	  Figure	  6.	  3	   Eoc	  time	  behavior	  of	  CoFe	  alloys	  with	  various	  iron	  content	  in	  0.1	  M	  H2SO4	  ..............................................................................................................	  88	  Figure	  6.	  4	   Potentiodynamic	  polarization	  curves	  of	  CoFe	  with	  various	  iron	  content	  in	  deaerated	  0.1	  M	  H2SO4	  with	  a	  scan	  rate	  of	  0.5	  mVs-­‐1	  .........................................................................................................................	  90	  Figure	  6.	  5	   Nyquist	  plots	  for	  nano	  Co	  and	  CoFe	  alloys	  with	  various	  iron	  content	  in	  deaerated	  0.1	  M	  H2SO4	  .......................................................................	  94	  Figure	  6.	  6	   Equivalent	  circuit	  proposed	  for	  the	  electrochemical	  impedance	  response	  of	  Co	  and	  CoFe	  alloy	  coatings	  in	  0.1	  M	  H2SO4.	  ...............................................................................................................................	  95	  Figure	  6.	  7	   XPS	  spectra	  (full	  range)	  of	  (a)	  as-­‐deposited	  and	  (b)	  nano	  Co	  and	  CoFe	  with	  various	  iron	  content	  in	  0.1M	  H2SO4	  after	  potentiostatic	  treatment	  at	  −300	  mVSCE	  for	  10	  minutes.	  ...........................	  98	  Figure	  6.	  8	   Comparison	  of	  Co	  2p	  peaks	  of	  	  (a)	  as-­‐deposited	  Co	  and	  Co	  after	  polarization	  in	  0.1	  M	  H2SO4	  and	  (b)	  as-­‐deposited	  and	  Co	  25wt%	  Fe	  after	  polarization	  in	  0.1	  M	  H2SO4	  ............................................	  99	  	  	    xviii Figure	  6.	  9	   Measured	  high	  resolution	  spectra	  of	  O	  1s	  of	  the	  Co	  25wt%Fe	  alloy	  coating	  after	  polarization	  in	  0.1	  M	  H2SO4	  .....................	  100	  Figure	  6.	  10	   Measured	  high	  resolution	  spectra	  of	  Fe	  2p	  of	  the	  Co	  25wt%Fe	  alloy	  coating	  after	  polarization	  in	  0.1	  M	  H2SO4	  .....................	  101	  Figure	  6.	  11	   SEM	  morphologies	  of	  the	  CoFe	  alloy	  coatings	  after	  potentiodynamic	  polarization	  test	  in	  deaerated	  0.1	  M	  H2SO4	  (a)	  Co,	  (b)	  Co	  5wt%Fe,	  (c)	  Co	  11wt%Fe,	  (d)	  Co	  25wt%Fe	  .....................................................................................................................	  104	  Figure	  6.	  12	   Eoc	  time	  behaviour	  of	  Co	  and	  CoFe	  alloys	  with	  various	  	  iron	  content	  in	  0.1	  M	  NaOH	  ...........................................................................................	  105	  Figure	  6.	  13	   Potentiodynamic	  polarization	  curves	  of	  Co	  and	  CoFe	  with	  various	  iron	  content	  in	  deaerated	  0.1	  M	  NaOH	  with	  a	  scan	  rate	  of	  1	  mVs-­‐1	  ...........................................................................................................	  106	  Figure	  6.	  14	   Nyquist	  plots	  for	  nano	  Co	  and	  CoFe	  with	  various	  iron	  content	  in	  deaerated	  0.1	  M	  NaOH	  .....................................................................	  110	  Figure	  6.	  15	   Equivalent	  circuit	  proposed	  for	  the	  electrochemical	  impedance	  response	  of	  Co	  and	  CoFe	  alloy	  coatings	  in	  0.1	  M	  NaOH	  .............................................................................................................................	  110	  Figure	  6.	  16	   XPS	  spectra	  (full	  range)	  of	  nano	  Co	  and	  CoFe	  with	  various	  iron	  content	  in	  0.1	  M	  NaOH	  .................................................................................	  112	  	  	    xix Figure	  6.	  17	   Comparison	  of	  Co	  2p	  peaks	  of	  	  (a)	  as-­‐deposited	  Co	  and	  Co	  after	  polarization	  in	  0.1	  M	  NaOH	  and	  (b)	  as-­‐deposited	  	  and	  Co	  25wt%	  Fe	  after	  polarization	  	  in	  0.1	  M	  NaOH	  .........................................	  114	  Figure	  6.	  18	   Measured	  high	  resolution	  spectra	  of	  O	  1s	  after	  polarization	  in	  0.1	  M	  NaOH	  ............................................................................................................	  115	  Figure	  6.	  19	   Measured	  high	  resolution	  spectra	  of	  Fe	  2p	  of	  the	  Co	  25wt%Fe	  alloy	  coating	  after	  polarization	  in	  0.1	  M	  NaOH	  ......................	  116	  Figure	  6.	  20	   SEM	  morphologies	  of	  the	  CoFe	  alloy	  coatings	  after	  potentiodynamic	  polarization	  test	  in	  deaerated	  0.1	  M	  NaOH	  (a)	  Co,	  (b)	  Co	  5wt%	  Fe,	  (c)	  Co	  11wt%	  Fe,	  (d)	  Co	  25	  wt%	  Fe	  ................	  118	  Figure	  7.	  1	   SEM	  images	  of	  Co	  deposited	  in	  plating	  solutions	  with	  and	  without	  saccharin	  (a)	  no	  saccharin,	  (b)	  1	  g	  L-­‐1	  saccharin,	  (c)	  3	  g	  L-­‐1	  saccharin,	  and	  (d)	  7	  g	  L-­‐1	  saccharin	  ....................................................	  122	  Figure	  7.	  2	   SEM	  images	  of	  CoFe	  deposited	  in	  plating	  solutions	  with	  and	  without	  saccharin	  (a)	  no	  saccharin,	  (b)	  1	  g	  L-­‐1	  saccharin,	  (c)	  3	  g	  L-­‐1	  saccharin,	  and	  (d)	  7	  g	  L-­‐1	  g	  saccharin	  ................................................	  124	  Figure	  7.	  3	   XRD	  patterns	  of	  (a)	  Co,	  (b)	  magnified	  XRD	  patterns	  of	  Co,	  (c)	  CoFe	  and	  (d)	  magnified	  XRD	  patterns	  of	  CoFe	  with	  various	  saccharin	  concentrations	  .....................................................................	  126	  Figure	  7.	  4	   Cyclic	  voltammogram	  of	  Co	  (a)	  and	  CoFe	  (c)	  and	  mass	  evolution	  curves	  corresponding	  to	  the	  cyclic	  voltammetry	  	  	    xx of	  Co	  (b)	  and	  CoFe	  (d)	  with	  different	  saccharin	  concentrations.	  Scan	  rate:	  10	  mV	  s-­‐1.	  ..............................................................	  132	  Figure	  7.	  5	   (a)	  Mass	  and	  (b)	  potential	  changes	  of	  nanostructured	  Co	  measured	  by	  EQCM	  in	  0.1	  M	  H2SO4	  ..................................................................	  136	  Figure	  7.	  6	   (a)	  Mass	  and	  (b)	  potential	  changes	  of	  nanostructured	  CoFe	  measured	  by	  EQCM	  in	  0.1	  M	  H2SO4	  ..................................................................	  139	  Figure	  7.	  7	   (a)	  Mass	  and	  (b)	  potential	  changes	  of	  nanostructured	  Co	  measured	  by	  EQCM	  in	  0.1	  M	  NaOH	  ..................................................................	  141	  Figure	  7.	  8	   (a)	  Mass	  and	  (b)	  potential	  changes	  of	  nanostructured	  CoFe	  measured	  by	  EQCM	  in	  0.1	  M	  NaOH	  ..................................................................	  142	  Figure	  7.	  9	   Potentiodynamic	  polarization	  of	  nanostructured	  (a)	  Co	  and	  (b)	  CoFe	  in	  0.1	  M	  H2SO4.	  Scan	  rate:	  0.167	  mV	  s-­‐1	  ........................................	  145	  Figure	  7.	  10	   Potentiodynamic	  polarization	  of	  nanostructured	  (a)	  Co	  and	  (b)	  CoFe	  in	  0.1	  M	  NaOH	  Scan	  rate:	  0.167	  mV	  s-­‐1	  ........................................	  149	  Figure	  7.	  11	   Bode	  phase	  obtained	  for	  nanostructured	  Co	  (a)	  and	  CoFe	  (b)	  in	  0.1	  M	  H2SO4	  after	  1	  hour	  immersion	  at	  OCP	  ....................................	  152	  Figure	  7.	  12	   Bode	  phase	  obtained	  for	  nanostructured	  Co	  (a)	  and	  CoFe	  (b)	  in	  0.1	  M	  NaOH	  after	  1	  hour	  immersion	  at	  OCP	  ....................................	  154	  Figure	  7.	  13	   Equivalent	  electrical	  circuit	  for	  coatings	  in	  (a)	  0.1	  M	  H2SO4	  	  and	  (b)	  0.1	  M	  NaOH	  .................................................................................................	  155	  	  	    xxi Figure	  7.	  14	   Deconvoluted	  XPS	  spectra	  for	  as-­‐deposited	  and	  polarized	  CoFe	  alloy	  coating	  in	  acidic	  and	  alkaline	  solutions:	  (a)	  Co	  2p,	  (b)	  O	  1s,	  (c)	  S	  2p,	  and	  (d)	  Fe	  2p	  ..................................................................	  162	  	  Figure	  8.	  1	   Potential	  changes	  of	  CoFe	  alloy	  coating	  measured	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11	  ........................................................	  169	  Figure	  8.	  2	   Rp	  of	  CoFe	  alloy	  coating	  after	  immersion	  for	  1	  hour	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11	  ..................................................	  170	  Figure	  8.	  3	   	  Mass	  changes	  of	  CoFe	  alloy	  coating	  after	  1	  hour	  OCP	  measured	  by	  EQCM	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11.	  ......................................................................................................................	  171	  Figure	  8.	  4	   Potentiodynamic	  polarization	  of	  CoFe	  alloy	  coating	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11.	  Scan	  rate:	  0.167	  mV/s	  ..............................................................................................................................	  173	  Figure	  8.	  5	   (a)	  Nyquist	  (b)	  Bode	  phase	  EIS	  and	  (c)	  Physical	  model	  and	  electrical	  equivalent	  circuit	  after	  1	  hour	  OCP	  in	  0.1	  M	  Na2SO4	  solutions	  at	  pH	  of	  5,	  7,	  9,	  and	  11.	  .......................................................	  176	  Figure	  8.	  6	   Potential	  changes	  of	  CoFe	  alloy	  coating	  measured	  by	  EQCM	  in	  0.1	  M	  NaSO4	  ...........................................................................................................	  179	  Figure	  8.	  7	   Mass	  changes	  of	  CoFe	  alloy	  coating	  measured	  by	  EQCM	  after	  1	  hour	  OCP	  .......................................................................................................	  180	  	  	    xxii Figure	  8.	  8	   Rate	  of	  mass	  change	  of	  CoFe	  alloy	  coating	  in	  different	  sulphide	  ion	  concentration	  ..................................................................................	  181	  Figure	  8.	  9	   Potentiodynamic	  polarization	  of	  CoFe	  alloy	  coating	  in	  0.1	  M	  Na2SO4	  solutions	  with	  different	  sulphide	  ion	  concentrations.	  Scan	  rate:	  0.167	  mV/s.	  ..........................................................	  182	  Figure	  8.	  10	   (a)	  Nyquist	  (b)	  Bode	  phase	  EIS	  and	  (c)	  Physical	  model	  and	  electrical	  equivalent	  circuit	  after	  1	  hour	  OCP	  in	  0.1	  M	  Na2SO4	  solutions	  with	  different	  sulphide	  ion	  concentrations	  ..............	  188	  	    	  	    xxiii List of Symbols A  peak current density (A/cm2) Ba  anodic Tafel slope Bc  cathodic Tafel slope Bs  magnetic saturation flux density β   Full Width at Half Maximum βred  cathodic Tafel constant βox  anodic Tafel constant CPE  constant phase element d  grain size Ecorr  corrosion potential (V) Eoc  Open circuit potential (V) ε   electrochemical equivalents f  frequency (Hz) I   current 	  	    xxiv Icorr  corrosion current density (A/cm2) Δm   change in mass  mt   theoretical mass 𝑛    harmonic number ηj  current efficiency OCP   open circuit potential Oe  coercivity 𝑝?    density of quartz Qdl  double layer capacitance RLPR  polarization resistance obtained from LPR curves Rp  polarization resistance Rs  solution resistance T  time (s) 𝜇?  shear modulus of quartz Z`  real impedance Z``  imaginary impedance 	  	    xxv List of Acronyms AC  Alternating Current BCC  Body Centered Cubic CPE   Constant Phase Element CV  Cyclic Voltammetry DC  Direct Current EDX  Energy Dispersive X-ray Spectroscopy EEC  Equivalent Electrical Circuit EIS   Electrochemical Impendance Spectroscopy EPMA Electron Probe Micro-Analyzer EQCM Electrochemical Quartz Crystal Microbalance FCC   Face Centered Cubic FWHM Full Width Half Maximum HCP   Hexagonal Close-Packed dHCP Double Hexagonal Close-Packed 	  	    xxvi LPR  Linear Polarization Resistance MEMS Microelectromechanical System NEMS Nanoelectromechanical Systems OCP   Open Circuit Potential PAR  Princeton Applied Research PDP  Potentiodynamic Polarization PSP  Potentiostatic Polarization SAC  Saccharin SCE   Satrurated Calomel Electrode (reference electrode) SEM  Scanning Electron Microscopy TEM  Transmission Electron Microscopy ULSI  Ultra Large Scale Integration XPS   X-ray Photoelectron Spectroscopy XRD  X-ray Diffractio  	  	    xxvii Acknowledgements I would like to express my deepest appreciation and sincere gratitude to my supervisor, Professor Akram Alfantazi for his skillful guidance, technical assistance, encouragement, opportunities, patience, support, and great concern, which made this work successful. Thank you for being the best supervisor anyone could have asked for. I would also like to thank my committee members, Professor Guangrui Xia and Professor John Madden for their insightful comments, suggestions, and time for serving as my committee members. I am thankful to all my colleagues from the corrosion group both past and present for their help. A scholarship and study leave granted by Ministry of High Education, Malaysia, and Universiti Teknologi MARA is also gratefully acknowledged.  Last but not least, I wish to express my immeasurable appreciation and thanks to my family. Words cannot express how grateful I am to be blessed with such amazing parents, Nik Masdek and Noraini. Thank you for your prayers and support. Most of all, I would like to thank my husband, Fairuzzaman for his patience and for staying up with me through all those sleepless nights. To my two precious daughters, Issya and Alyya, thank you for your unconditional love. All of you have been the source of my strength and motivation for success especially in periods of uncertainties and difficulties.  	  	    xxviii Dedication       TO MY FAMILY    	  	    1 Chapter 1 1	   Introduction	  Since its first introduction by Gleiter (1981), this novel advanced material known as nanocrystalline materials were found to have a huge impact on many material properties. These materials are defined as having grain sizes in the nanometer range, typically less than 100 nm. Grain refinement of materials into the nanometer range has been shown to result in unique and improved properties as compared to their conventional polycrystalline counterparts. An enhancement in hardness, ductility, fatigue behaviour, coercivity, wear, and corrosion resistance was often observed from the reduction in the grain size toward the sub-100 nm scale. Since the first production of nanocrystaline materials through the inert gas condensation method, several processing techniques have been currently developed and available for producing these materials. In general, it can be classified into four groups, which are the gas phase condensation of particulates, electrodeposition, mechanical alloying, and severe plastic deformation. From the four major groups, electrodeposition has been reported to be the oldest and most economical and viable technique to produce these very fine grain materials. Uniform depositions on complex shaped substrates, low cost, high production rates, good reproducibility, and the reduction of waste attributed to synthesizing nanocrystalline material are among the many advantages that have attracted many researches to employ this method in producing nanostructured metal and alloys. 	  	    2 On the other hand, earlier interest in cobalt (Co) and cobalt-iron (CoFe) alloys have been mainly due to their excellent decorative properties and corrosion resistance. They have also been identified as alternatives to the hazardous hexavalent chromium. The first research work on the CoFe alloy was first reported by Toepffer (1899). Since then, CoFe alloy coatings are gaining much attention in the applications of protective coatings, microelectrochemical systems (MEMS) such as actuators, magnetic sensors, step motors, and hard disk drives. Furthermore, the need for recording heads in hard disk drives with better properties than iron-nickel (FeNi) alloys has also lead to the search and development of new soft magnetic materials with higher saturation magnetization. Electrodeposited CoFe alloys are known to exhibit higher saturation magnetization and coercivity as compared to electrodeposited FeNi alloys making them the more suitable choice for future magnetic applications. Most investigations of the CoFe alloys have been directed to the magnetic characteristic of these alloys in terms of the effect of alloy composition and plating parameters (Alagiri et al., 2011; Buravikhin et al., 1974; Chang, et al. 1990; Yu et al., 2000). While the magnetic properties of CoFe alloys has been a main subject of attention, more interest should be placed on their corrosion behaviour. In addition, very few studies have focused on decreasing the grain size of these alloy coatings into the nanocrystalline range (< 100 nm) as a means for improving their corrosion properties. For many applications such as protective coatings and electrical contact components, corrosion damage of these nanocrystalline coatings can result in the development of cracks or pores and ultimately 	  	    3 malfunction of the components. Because minimal corrosion can have an adverse effect, understanding the corrosion behaviour of nanocrystalline Co and CoFe alloy coatings is of immediate practical importance. The results of the present work are aimed at providing some insight on the corrosion behaviour of these electrodeposited nanocrystalline coatings. Their corrosion properties were examined in highly acidic and alkaline solutions. The effects of iron concentration on the electrochemical response in these solutions were also investigated. An attempt to gain better understanding of the use of saccharin as an additive in the plating bath on the corrosion behaviour of the nanocrystalline electrodeposits was also undertaken. The presence of sulphide ions on their electrochemical corrosion behaviour was also investigated.  	  	  	    4 Chapter 2 2	   Literature	  Review	  2.1 Nanomaterials Nanocrystalline materials are rather arbitrarily described as a single or multi-phase polycrystalline material that possess an average grain size less than 100 nm. These novel materials are sometimes also referred to as nanostructured, nanophase, nanocrystals, or even nanometer-sized crystalline solids. Gleiter et al. initially proposed it in 1981 and later an extensive review on these nanocrystalline material as well as their unique properties was also reported (Gleiter, 1989). This has been considered as one of the major breakthroughs in the materials field in the past 30 years.  Figure 2.1 shows an example of the SEM microstructure of a polycrystalline, nanocrystalline and high resolution TEM image of chemically-similar material. Due to their very fine grain sizes, these nanocrystalline materials is mainly composed of interfaces namely grain boundaries. The volume fraction of interfacial atoms in the nanostructured material such as grain boundaries and triple junctions increased significantly as a result of the nanograin sizes. Metals, alloys, ceramics, and composites have been produced in the nanocrystalline structure. The synthesis, processing, and characterization of this ultra-fine grained material in many forms including filamentary structures, atomic cluster and bulk solids has been extensively researched and investigated. The significant interest in this 	  	    5 particular area is mainly due to its superior mechanical, chemical, and physical properties as compared to their polycrystalline counterparts.  The enhanced properties of the nanostructured material are controlled by their microstructure which is generally seen in the increase of their grain boundaries and triple junctions. It was first shown in the early 1900’s that the finer grain size microstructure resulted in an improved and increased strength and toughness of structural materials due to finer dispersion of precipitates. Since then, several studies have reported on this enhanced behaviour due to the decrease of grain sizes which included increased hardness (Gleiter, 1989; El-Sherik et al., 1992), increased electrical resistivity (Gleiter, 1989; Aus et al., 1994), and enhanced hydrogen solubility and diffusivity (Gleiter, 1989; Palumbo 1990).  Nanocrystalline materials are classified according to the number of dimensions which are atom clusters (zero dimensional), lamellar structures (one dimensional), filamentary structure (two dimensional), and equiaxed nanostructured material (three dimensional) (Siegel, 1994; Suryanarayana and Froes, 1992a). Figure 2.2 illustrates the schematic structures of nanomaterials. 	  	    6  Figure 2. 1 SEM microstructure of a polycrystalline and nanocrystalline (left) and high resolution TEM image of a chemically-similar material [courtesy of Integran Technologies Inc.] (Brooks, 2012) Formation of the nanocrystals depends on the controlling of nucleation and growth of the lattice. The importance in understanding and identifying the electrodeposition parameters that controls the massive nucleation and reduced grain growth during the crystallization stage will determine in obtaining the ultrafine grain size in the deposits (Choo et al., 1995). The various stages of electrocrystallization are shown in figure 2.3 together with some factors that control the grain size of the electrodeposits. Until now, nanocrystalline materials are being studied extensively to better understand the mechanism that controls the formation of this ultrafine material that results in improved performance over those demonstrated by conventional polycrystalline materials.   	  	    7  Figure 2. 2 Schematic illustration of typical microstructures produced by the pre-eminent nanomaterials synthesis methods; (a) nanopowders, (b) consolidated nanopowders, (c) deformation-induced nanostructures, (d) nanograins crystallized from an amorphous phase, (e) nanotwinned material, (f) electrodeposited nanocrystalline material [courtesy of U. Erb] (Brooks,  2012) 	  	    8  Figure 2. 3 Schematic diagram of the various stages of electrocrystallization (Choo et al., 1995). 2.2 Synthesis of Nanomaterials It is known that grain size significantly affects the properties of materials. Hence, it has led to many efforts and extensive research in investigating the best technique possible in producing the nanostructured material. A relatively large number of synthesis routes have been used to produce nanocrystalline materials resulting in a huge diversity of structures from these processing methods. In brief, the methods could also be classified into five major groups, which is solid state, vapour phase, liquid phase, chemical and electrochemical process as shown in table 2.1.  	  	    9 The earliest technique available in producing nanocrystalline materials was the inert gas condensation which employs an ultra-high vacuum apparatus (Gleiter, 1989). Although this method is well developed in processing nanostructured materials, however, several limitations such as complex setup, high operating cost, and low productivity rate is often associated with this technique. Table 2. 1 List of various techniques used to produce nanocrystalline materials Solid state processing • Ball milling • Mechanical attrition Vapor phase processing • Inert gas condensation • Sputtering Liquid phase processing • Rapid solidification • Thermal spray coating Chemical processing • Sol-gel processing • Mixed alloy processing Electrochemical processing • Electroless plating • Electrodeposition In general, there are two approaches that are used to process materials with enhanced physical and chemical properties through the use of grain boundaries. The two-step process involves the production of a nanopowder followed by a consolidation process. Examples of 	  	    10 this type of synthesis technique include sol-gel chemical processing, inert gas condensation, and mechanical attrition by ball milling. The reliable synthesis and consolidation of ball-milled nano-grains are rapidly progressing. Ball milling has been a popular choice in producing various nanocrystalline materials due to the relatively easy and low cost setup but the major drawback of this particular method is in the high porosity level of the final product that are in need for secondary consolidation to compact the powder. Through this processing technique, these materials retain some degree of porosity that has a significant effect on the mechanical and elastic properties (Koch et al., 2010; Witkin and Lavernia, 2006; Krstic et al., 1993.)  Rapid solidification is also an interesting method to synthesize nanocrystalline materials by rapidly solidifying a material into its amorphous state and then annealing it subcritically. The main problem in employing this technique is due to their difficult and very high cost procedure. The thickness of the material produced is also limited due to the heat transfer considerations.  Meanwhile, the single-step processing techniques, as the name implies, only involves a one-step process with the elimination of the subsequent powder consolidation. Severe plastic deformation, equal channel angular pressing, and electrodeposition are among the examples of this type of processing technique. While severe plastic deformation is capable in producing bulk quantities of material, it often results in very large dislocation densities. It is also unable to synthesize materials with an average grain size below 100 nm. Crystallization of amorphous precursors is another technique that has been widely used to 	  	    11 produce fully-dense nanocrystalline microstructures. Unfortunately, this process causes adverse impact on the properties of nanocrystalline materials produced by this route due to the formation of the highly non-equilibrium amorphous precursor by processes such as rapid solidification (Herzer, 1992). Electrodeposition, on the other hand, has been well developed and known for the production of fully-dense nanocrystalline materials. Due to its simple and feasible industrial application, this processing technique has been widely used in producing many different nanocrystalline materials (metals, alloys, ceramics, and composites). Figure 2.4 illustrates the various types of electrodeposited coatings (Lu and Lu, 1999). 	  	    12  Figure 2. 4 Various types of electrodeposited coatings (Lu and Lu, 1999). The properties of the metallic coatings (figure 2.4a) could be easily tailored by changing the experimental variables such as current density, electrolyte solution, electrolyte pH, electrolyte temperature, and additives. Composite coatings (figure 2.4b) consist of a metal or alloy matrix with a dispersion of second-phase particles that could comprise of 	  	    13 nanotubes, solid lubricants, or hard particles to increase wear and corrosion properties (Davis, 2001). Figures 2.4c and 2.4e show the deposition of sub-layers and gradient coatings, which could significantly reduce the stress between coatings and substrates (Warcholinski et al., 2010). Finally, the patterned films shown in figure 2.4d are generally applied in applications such as gas sensors, optoelectronic devices, and micro fluidic systems (Lai et al., 2010). Low initial cost in implementing this technology as well as the high production rate and few shape and size limitations, a dense end product without secondary processing steps are among the many advantages of electrodeposition over the other techniques available.  2.3 Electrodeposited Nanocrystalline Co and CoFe Alloy 2.3.1 Phase Diagram of CoFe Alloy The binary cobalt-iron phase diagram is shown in Figure 2.5. Table 2.2 describes the phases present. From the binary phase diagram there are 4 stable phases in the iron-cobalt system. At low temperatures the α phase has a BCC structure and is the stable phase in iron-rich alloys. At high temperatures the δ phase is a second BCC phase that becomes stable for iron-rich alloys. Meanwhile, at high cobalt compositions, or high temperatures, the FCC phase αCo (which is isomorphous to γFe) is the equilibrium phase. Furthermore. an additional equilibrium phase is present in the cobalt-rich alloys at low temperatures known as the ε phase. The ε phase has an HCP structure, however, it does not appear in Figure 2.5, because it transforms into the γFe/αCo phase at temperatures above 422°C 	  	    14 (Betteridge et al., 1982). The structure at room temperature as a function of iron content in cobalt-iron ailoys under equilibrium conditions has also been reported and listed in Table 2.3.   Figure 2.5 Iron-cobalt binary phase diagram (Okamoto, 2008)  From the binary phase diagram, the phases that might be expected in iron-cobalt thin-films at a particular composition could be determined. Electrodeposition, however, often produces films that are composed of phases that would not be expected to be stable based 	  	    15 on the iron-cobalt phase diagram (Cavallotti et al., 2005). Several studies have reported cobalt-iron deposits containing either only one phase, i.e., the bcc or fcc phase, or both phases co-existing. For instance Liu et al., 2000 electrodeposited CoFe without any additives while applying a magnetic field. They found that deposits containing less than 20% iron did not exhibit the bcc phase while only the hcp phase was present for pure cobalt at room temperature. Meanwhile, Mattoso et al., 2001 did not observe any α’ phase in the electrodeposited CoFe hin-films. Also the range in composition over which the FCC and BCC phases can be deposited often conflicts with the region shown in Figure 2.5 (Kakuno et al., 1997, Mattoso et al., 2001and Myung et al., 2001). However, it has also been reported that the fcc, bcc and mixedfcc-bcc phases exist in the form of disordered solid solutions of cobalt and iron rather than the mixture of crystallites of each element (Nam et al., 2001). Furthermore, it has also been found that metastable phases are also often produced through electrodeposition (Cavallotti et al., 2005). The kinetic effects during the electroplating process as well as to the incorporation of impurities in the deposits are among the main reasons these phases often appear (Tochitskii et al., 1996 and Cavallotti et al., 2005).    	  	    16 Table 2.2 Equilibrium phases of iron-cobalt binary system (Betteridge et al., 1982 and Baker et al., 1992) Phase Structure αFe BCC α’Fe CsCl γFe/αCo FCC δFe BCC εCo HCP  Table 2.3 Equilibrium phases of iron-cobalt binary system as a function of iron content Iron content (wt.%) Phase Crystal Structure 0 - 3 ε H.C.P. 3 - 5 Ε + γ H.C.P. + F.C.C. 5 - 11 γ F.C.C. 11 - 24 γ +α F.C.C. + B.C.C. 24 - 100 α B.C.C.   	  	    17 2.3.2 Electrodeposition Mechanism Electrodeposition has been a promising deposition technique for many of the emerging applications of nanotechnology. It is a powerful and versatile technique to synthesize dense coatings with improved properties. A variety of nanocrystalline pure metals (e.g. Co, Ni, Cu), alloys (e.g. Ni-Co, Co-P, NiFe, Co-W), and composites (e.g. Ni-Al2O3, Ni-SiC) could be prepared using the electrodeposition technique. Electrodeposition of metals or alloys involves the reduction of metal ions from either an aqueous, organic or fused-salt electrolyte. However, in this study only the deposition from aqueous solutions are of interest. Electrodeposition, which is also sometimes referred to as electroplating, is the deposition of a pure metal or alloy from an electrolyte solution by the passing of an electric current (Ross, 1994). The deposition of nanocrystalline materials consists of two fundamental processes: (a) nucleation rate and (b) growth of existing grains.  A low surface diffusion rate and high overpotential promote the formation of new nuclei resulting in refined grain size.  In order for an electrodeposition to take place, the main major components of the electroplating cell consist of the electrolytic plating solution, two electrodes where one acts as the anode and the other cathode, and a power source, as shown in figure 2.5.  	  	    18  Figure 2. 5 Schematic diagram of the components of the electrodeposition cell The cathode acts as the site for metal deposition. Metallic films are formed at the cathode when electrons are consumed which reduces the aqueous metal ions according to reaction (2.1) as follows: M n+(aq) + ne- M(s)    (2.1) where M indicates cobalt and iron for the deposition of the nanostructured coatings in the present study. Four types of fundamental issues are involved in the process represented by equation (2.1), which is listed as follows: 	  	    19 (1) metal-solution interface as the locus of the deposition process (2) kinetics and mechanism of the deposition process (3) nucleation and growth process of the metal lattice (4) structure and properties of the deposits Meanwhile, at the anode side, the oxidation reaction occurs where free electrons are produced and flow to the positive terminal of the power source as shown in reaction (2.2): M(s)  M n+(aq) + ne –     (2.2) The anode reaction oxidizes species in the electrolyte when an inert electrode is used in the electrodeposition process. It normally results in the evolution of the oxygen gas. These reactions are often pH-dependant where, in acid and neutral solutions, the anode reaction typically proceeds as reaction (2.3): 2H2O(l)          O2(g) + 4H+(aq) + 4e-   (2.3) Meanwhile, in basic solution, the anode reaction involves the oxidation of hydroxide ions  as shown in reaction (2.4): 4OH-(aq)   O2(g) + 2H2O(l) + 4e-  (2.4) The critical nucleation radius, rc, in an electrochemical deposition can be obtained by the Kelvin equation (2.5) as follows (Budevski, et al., 1996): 	  	    20 rc = 2γVm / ze0 |η|    (2.5) where γ is the specific surface energy, Vm is the atomic volume in the crystal, z is the number of elementary charges e0, and η the over voltage. According to equation (2.5), the use of higher voltage will result in smaller grains. The combination of both high voltage and high current will also result in higher deposition rates. Until now, electrodeposition is considered to be the most attractive method in synthesizing thin, protective, nanostructured coatings as well as magnetic materials. Electrodeposition provides many advantages as previously discussed compared to other processing techniques. In addition, electrodeposition allows the thickness and microstructure of the films to be controlled by adjusting the deposition parameters such as the electrolyte composition, pH, temperature, current density, and agitation. 2.3.3 Electrodeposition Parameters One of the prime advantages of electrodeposition is that there are many parameters such as current density, solution concentration, pH, and addition of additives to the plating bath that could be adjusted to obtain suitable nanostructured deposits. Factors accelerating the nucleation rate and inhibit grain growth are favourable to the formation of ultra-fine grained deposits.  As previously described, a large cathodic overpotential is important in achieving a high nucleation rate, which results in the formation of crystalline nuclei that is small in size and also in large quantities. The 	  	    21 reduction of grain size can be attributed to the higher overpotential which is related to the crystalline nucleus formation probability W expressed by the following equation (2.6) (Karakus and Chin, 1994): W = A exp (-b/ηk2)   (2.6) where A and b are constants. The overpotential increases with the increasing current density as described in the following equation (2.7) (Wang et al., 2006): ηk = a + b log i    (2.7) where a and b are constants and i is the current density. Therefore, with increasing current density reduction in grain size could be achieved. The current density should be controlled and held within a suitable range with respect to bath composition and temperature.  Insufficient or low current density will result in poor coverage of recesses and a low plating rate. On the other hand, the presence of an excessive current will cause other difficulties such as stress, high traces of impurities, and also may produce a burned plate (Milan and Mordechay, 2006). The effect of current density on the properties of nanocrystalline Ni-Co-W alloy was reported and the results obtained showed that, with increased current density, both the Co and W contents decreased. This resulted in better corrosion resistance of these nanocrystalline alloys (Farzaneh et al., 2010). The current efficiency for Ni-Co co-deposition in chloride solutions were also reported to increase up to 98% with increasing current density although the coating plated with high 	  	    22 current density of 100 A dm-2 was observed to be porous, fragile, and partially detached from the electrode (Fan and Piron, 1996). The presence of different ion concentrations could also lead to different properties of electrodeposited coatings. With a fixed concentration of cobalt ions, the iron content in nanostructured CoFe alloy deposits increased gradually with increasing the Fe2+ concentration in the electrolyte (Nik Masdek and Alfantazi, 2012). The increase in iron content in the electrodeposits resulted in a refinement of grain size and thus increased the microhardness of these electrodeposited nanocrystalline alloy coatings. Maintaining pH of the plating solution is crucial in providing stability of the bath during deposition process. Park et al. (2006) mentioned that instability of the chloride bath was observed with a higher pH value of 2.15 due to the precipitate formation. The optimal pH value has been reported to be in the range of 2 to 6 (Osaka, 2000). Very high pH values could cause the formation of hydroxide precipitates of the metal ions and thus results in unstable bath solutions. Meanwhile, low pH values result in excessive hydrogen evolution. Control of the deposition bath temperature is vital for the consistent performance of any deposition process. Small deviation of the bath temperature is sufficient to harm plating quality and deposition rates. Hence, bath temperature is one of the primary parameters that have a huge impact on the properties of deposited nanomaterials. The deposition temperature was observed to significantly change the microstructure, morphology and magnetic properties of electrodeposited films (Natter and Hempelmann, 1996; Dulal et al., 	  	    23 2007; Hongliang et al., 2007). The bath temperature showed a significant effect on the deposit composition due to a strong dependence of the surface adsorption on temperature (Adamson and Gast, 1997). The composition and grain size changes considerably with increasing temperature. Mixed phases of BCC and FCC was observed at high temperatures (90oC) while a solid solution FCC phase is present from low temperatures. The BCC α-Co7Fe3 phase is favored at intermediated temperature (RT~50 oC) (Wei et al., 2013). In order to ensure a nanostructured grain size, the grain growth of existing grains could also be surpressed by certain inhibiting molecules (Natter et al., 1996). Therefore, adding organic inhibitors to the electrolyte has been a common practice during the electrodeposition process. These organic additive molecules are believed to adsorb in a reversible way on active sites of the electrode surface and thus, block the active sites and reduce crystallite growth. In addition, the absorbed organic molecules also hinder the surface diffusion of adatoms. This results in fewer metal adatoms reaching growth sites and promotes formation of new nuclei (Natter et al., 1998).  In general, these additive molecules decomposed at the cathode surface promotes nucleation and impede grain growth, which refines the grain size further by at least two orders of magnitude (Safranek, 1974). It has been reported that shorter nucleation time was seen with the addition of organic additives. This was due to the partial coverage of the electrode surface by the additive, which blocks the active sites and thus decreases the nucleation rate. Moreover, adsorption of the additives on the growing deposits can also occur, blocking the electrocrystallization process (Guoying et al., 2007). 	  	    24 2.3.4 Applications The upsurge of interest in electrochemical deposition is due mainly to these new technologies: (a) fabrication of integrated circuits through metal deposition, (b) fabrication of magnetic recording devices, and (c) deposition of multilayer structures. Another application that is gaining many research activities of the metal deposition is from computer technology. The electrodeposition of magnetic materials for thin film recording heads and magnetic storage media is of great interest. Recently, due to their superior properties CoFe-based alloys have been considered as an alternative to FeNi which are being used as write head core materials in hard disk drives. Electrodeposited CoFe alloys are also gaining much attention for applications in magnetic recording (read/write heads) and ultra-large scale integration (ULSI) devices (Andricacos and Robertson, 1998;  Kohn et al., 2001).  Moreover, it has also been incorporated into microelectromechanical (MEMS) devices such as sensors, microactuators, microgears and micromotors (Judy et al., 1995). This is mainly due to their low magnetic loss, high saturation magnetic flux density, low coercive force, and high permeability. Recently these deposited alloys have also been found in nanoelectromechanical systems (NEMS) (Tabakovic et al., 2001; Romankiw et al., 1970; Osaka et al., 1998). In the manufacturing industries, CoFe alloys have been used intensively as free standing soft magnets and transformer core materials. CoFe alloys have also been identified to be the most suitable alloy in future magnetic applications. This has led to extensive research in producing these alloys with optimum properties to confirm their 	  	    25 optimum performance and reliability as an integral part of the device (Tabakovic et al., 2011). To make sure that these alloys meet all the expected requirements in their respective applications, their corrosion resistance has to be at its best. Therefore, understanding their corrosion behaviour in different environments is necessary. The next section will focus on the research studies of corrosion behaviours of deposited nanocrystalline metals and alloys that have been reported.  2.4 Corrosion Behaviour of Nanocrystalline Materials It is expected that the electrochemical corrosion behaviour of nanocrystalline materials to be different from polycrystalline materials due to the presence of higher volume of grain boundaries and triple junctions (more active sites available), and higher diffusivity of the alloying element. Other parameters such as solute atoms, impurities, grain size, surface defects, and precipitate distribution could lead to the difference of corrosion resistance between nanocrystalline and conventional materials (Yamashita et al., 1991; Mauer et al., 1984; Aust et al., 1994). Furthermore, the environment as well as the nature of the material also affects the electrochemical response of these fine grain materials. The intercrystalline surface area fraction maybe closely related to the corrosion properties since the intercrystalline defects are considered as preferential dissolution sites (Kirchheim, R., et al., 1992). If the intercrystalline surface area fraction is considered to be related to the cathode to anode area ratio, nanocrystalline materials is believed to show improved performance against corrosion than materials with microcrystalline structures. If the 	  	    26 corroding metal is assumed to be equivalent to a short circuited cell where energy is dissipated during consumption of a cathodic reagent there will be no external source of electrons. When the area of the anodic and cathodic site is large, the current densities need not be equal. However, when the same surface area between anodic and cathodic sites as for nanocrystalline materials, the anodic current density can equal the cathodic current density in absolute value. Thus, reduction in grain size will reduce the penetration current density and result in uniform and better localized corrosion resistance due to the highly distributed corrosion current. It is crucial in making sure that corrosion resistance is kept at a minimum level in engineering applications of nancrystalline materials. Therefore, investigation on the corrosion behaviour of these materials is of great importance. Over the years, there has been tremendous interest on the effect of very fine microstructures on the corrosion properties of nanocrystalline materials. A survey of existing literature on corrosion behaviour of electrodeposited nanostructured pure metals and their alloys as well as composites have been reported (Nik Masdek and Alfantazi, 2010). Both detrimental and beneficial effects have been reported on the corrosion performance of these nanostructured materials. It is expected that nanocrystalline materials exhibit a higher corrosion rate because of the high volume of grain boundaries, dislocations, and triple points that serve as anodic sites (Rofagha et al., 1991a; Rofagha et al., 1993a; Barbuci et al., 1999; Lu et al., 2006; Vinogradov et al., 1999). Although nanostructure may accelerate the corrosion rate by producing more electrochemical reaction sites between the large amount of grain 	  	    27 boundaries and the matrix, the high density of grain boundaries and lattice defects could also provide more nucleation sites for formation of protective passive films which, in return, results in a high fraction of passive layers and high corrosion resistance. This enhancement in corrosion resistance of some nanocrystalline materials has also been reported (Mohan et al., 2004; Kedim et al., 2004). The following studies will be made to present the corrosion properties of nanocrystalline pure metals and alloys. 2.4.1 Corrosion Properties of Pure Metals Early research on nanocrystalline materials have been mainly focused on single nanostructured metals for example nickel (Ni) (Rofagha et al., 1991b; Ghosh et al., 2006a; Liu et al., 2007; Tang et al., 1995), copper (Cu) (Yu et al., 2005; Tao and Li, 2006; Heim and Schwitzgebel, 1999), and cobalt (Co) (Wang et al., 2007; Cheng et al., 2001; Kim et al., 2003).  Rofagha was the first to explore the corrosion behaviour of electrodeposited nanocrystalline Ni in an acidic medium (Rofagha et al., 1991; Rofagha et al., 1992). Potentiodynamic and potentiostatic polarization tests were used to analyze the corrosion performance of nanocrystalline nickel in deaerated 2N H2SO4. The typical active-passive-transpassive behaviour was obtained for nanocrystalline nickel electrodeposits with grain sizes ranging from 500 to 32 nm, which was similar to the polycrystalline nickel, although the passive current density was seen to be one magnitude higher. The authors suggested that the increased electrochemical response for the nanocrystalline Ni was due to the higher 	  	    28 dissolution rate corresponding to the increased intercrystalline region that provided more active sites for corrosion activity. However, this passivation current density between the samples was negligible at high potentials. As mentioned above, in ultra-fine grain size materials, the large number of grain boundaries provide a path for faster diffusion of impurities that has a huge effect on the corrosion behaviour of these materials. The influence of these fine grains on the electrochemical corrosion of several metals has been a major interest for the past decade. Recently, the effect of grain size on the corrosion behaviour of bulk nanocrystalline nickel produced by direct current electrodeposition in different electrochemical solutions was investigated by Li-yuan et al. (2010).  The corrosion of both microcrystalline and nanocrystalline Ni with grain sizes ranging from 2 µm to 16 nm was studied in three different solutions (10% NaOH, 3% NaCl, and 1% H2SO4 solutions). Different corrosion resistance was observed for different solutions. In both NaCl and NaOH solutions, corrosion resistance was further improved with the formation of a protective passive film while only an active behaviour was displayed in the H2SO4 solution indicating a decrease in corrosion resistance. The lowest passive current was observed for nanocrystalline Ni with grain size of 16 nm in NaOH while the corrosion current of nanocrystalline Ni in NaCl was reported to be 10 times lower as the grain size decreased. The results indicate that the corrosion response of nanocrystalline materials not only depend on their grain sizes, but also the environment where the material is being exposed. 	  	    29 Diffusion of elemental and ionic species in nanostructured materials may occur with the presence of lattice defects such as vacancies or dislocations. This diffusivity is reported to affect the type of corrosion response (Mishra and Balasubramaniam, 2004). Corrosion behaviour of nanostructured Ni deposits was compared with bulk nickel in an H2SO4 solution. Various characterization techniques were employed and results confirmed that a higher number of defects on the passive film resulted in easier and more diffusion of nickel cations through the defected film and thus created a higher passive current density. A significant shift of zero current potential towards the noble direction was observed for nanostructured deposits as compared to bulk nickel due to the difference in grain size and catalysis of hydrogen processes. Polarization behaviour of these Ni deposits is shown in figure 2.6.  The effect of grain size and solute segregation for both polycrystalline and nanocrystalline Ni (grain size of 20 to 30 nm and sulphur impurities of 1000 ppm by weight) has also been reported (Kim et al., 2002). It was observed from SEM that both specimens exhibit extensive corrosion. However, surface morphologies after the polarization test revealed uniform corrosion morphology with high density of evenly distributed corrosion pits of less than 2 µm deep on the nanocrystalline Ni coatings. X-ray photoelectron spectroscopy of the specimens after polarization in the passive region showed that the passive film formed on the nanostructured specimen is more defective than that formed on the polycrystalline specimen, while the thickness of the passive layer on both specimens were similar. This resulted in a more uniform breakdown of the passive film. Meanwhile, a contrast on the 	  	    30 corrosion morphology was observed for the coarse-grained Ni which exhibited severe localized corrosion mainly along grain boundaries and triple junctions. The breakdown of the passive film occurs first at the grain boundaries and triple junctions rather than the crystal surface, leading to preferential attack at these defects.  Figure 2. 6 Potentiodynamic polarization curves of nickel of different grain sizes in 1 mol/L H2SO4 at a scan rate of 0.5mV/s. The experiments were conducted after stabilization of free corrosion potentials (Mishra and Balasubramaniam, 2004). The corrosion properties of nanocrystalline iron (Fe) were also investigated to determine the effect of surface nanocrystallization produced from pulse electrodeposition (Afshari and Dehghanian, 2009). The results obtained further verified the beneficial effect of grain size reduction on the formation of a superior protective passive film on the coating surface and thus significantly enhanced the corrosion resistance of the nanostructured Fe deposits.  	  	    31 The transport of ions in passive films takes place by migration and/or diffusion of defects (grain boundaries, linear dislocations, vacancies, or interstitials), so more protective passive layer is formed on nanocrystal electrodeposited coating. This may be attributed to the widening of the energy band by decreasing the grain size. Furthermore, higher corrosion resistance of the nanostructure deposits was said to be due to the rapid formation of continuous Fe oxy-hydroxide films at surface crystalline defects.  Improved corrosion resistance of nanocrystalline Fe in both HCl and H2SO4 has also been observed through the immersion test, potentiodynamic polarization curves and EIS scans (Wang et al., 2005). Passivation of nanocrystalline Fe has also been reported in 0.5 M H2SO4 solutions where an increase in passivation domain width was observed (Elkedim et al., 2002).  Several studies have been focused on the corrosion behaviour of electrodeposited nanocrystalline Zn (Gomes and da Silva Pereira, 2006; Saber et al., 2003; Vasilakopoulos et al., 2006). The corrosion performance of nanocrystalline zinc synthesized from pulsed electrodeposition was compared with electrogalvanized steel in a deaerated 0.5 M NaOH solution (Youssef et al., 2004). Again the positive effect of fine grain size of electrodeposited nanocrystalline materials could be observed through the reduction of the corrosion rate by about 60% as compared to electrogalvanized steel. The results were further clarified by SEM images as shown in figure 2.7. It can be seen that nanocrystalline zinc exhibited discrete, etch-pit-like morphology while a more uniform corrosion was present on electrogalvanized steel indicating a less protective oxide film was formed on the steels surface. Lower grain- sized dimensions, higher uniformity of grain distribution, and a 	  	    32 higher number of lattice imperfections yielded higher corrosion resistance due to the improved passivation behavior.  Figure 2. 7 Images of nc zinc (a) and EG steel (b) in deaerated 0.5 N NaOH after potentiodynamic polarization (Youssef et al., 2004). Meanwhile, Wang et al. (2007) reported on both the positive and negative effects of the fine grain size on the corrosion performance of nanocrystalline Co in different test solutions. The positive effect of high volume fraction of grain boundaries due to the small grain size of nanocrystalline Co produced by double pulse current electrodeposition could be observed in alkaline solutions (NaOH and NaCl). Formation of a protective passive film was enhanced by the high density of grain boundaries leading to a much higher corrosion resistance as compared to coarse grain Co coatings. On the other hand, negative effects were present for the corrosion performance of nanocrystalline Co in HCl solutions. The grain boundaries were preferential attack sites when exposed to high acidity environments 	  	    33 causing detrimental corrosion behavior. The results here were similar as observed for nanocrystalline Ni in both acidic and alkaline solutions (Li-yuan et al., 2010). Interestingly, the corrosion characteristics of electrodeposited nanocrystalline Co with grain sizes ranging from 13-15 nm in Na2SO4 (pH 6.5) and NaOH (pH 13) from anodic polarization tests revealed that the overall shape of the anodic polarization curve of nanocrystalline Co and polycrystalline Co were very similar to each other (Kim et al., 2003). The similarity in both Ecorr and lack of passivation suggested that corrosion behavior of Co was not affected by nanoprocessing. As a matter of fact, a slightly-enhanced anodic dissolution current (figure 2.8a) was obtained from nanocrystalline Co, although a more uniform corrosion was observed for these nanocrystalline deposits. Furthermore, potentiodynamic polarization tests in NaOH, as shown in figure 2.8b, indicates that the grain size does not affect the passivation behavior of Co. A similar observation was reported by Jung and Alfantazi (2006) for electroplated nanocrystalline Co (grain size 20 nm) in deaerated 0.1 M H2SO4. All samples exhibited active dissolution without any transition towards passivation as expected thermodynamically according to the Pourbaix diagram for the Co–water system similar with those reported by Helfand et al. (1992). SEM images as illustrated in figure 2.9 clearly indicate that preferential grain boundary attack was eliminated  	  	    34  Figure 2. 8 Polarization curves obtained from polycrystalline and nanocrystalline Co in (a) Na2SO4 (pH 6.5) and (b) NaOH (pH13) at scan rate:0.2 mV/s (Kim et al., 2003)    	  	    35 through the nanoprocessing that resulted in a uniformly corroded surface for nanocrystalline Co, unlike the severe corrosion seen along the intercrystalline defects of the micro Co. Furthermore, for both the micro-Co and nanocrystalline Co, an inductive loop was observed at low frequency and this was attributed to the adsorption behavior. However, at higher anodic potentials, corrosion resistance diminished due to the formation of a non-protective surface film. 	  Figure 2. 9 SEM micrographs of the corroded surfaces of: (a) micro Co; (b) nano Co after potentiodynamic polarization scan in deaerated 0.1 M H2SO4 (Gonzalez et al., 1996). 2.4.2  Corrosion Properties of Alloys Synthesis of nanostructured alloys through the electrodeposition technique has been possible since the early 1980’s. Adding alloying elements is believed to improve the corrosion behavior of these nanostructured alloy coatings. This is likely because the 	  	    36 diffusivity of alloying and impurity elements in nanocrystalline materials is much higher as compared to polycrystalline materials which leads to better corrosion resistance (Wang et al, 2003). As the current trend in engineering and technology supports the substitutes of individual metals by their alloys, the co-deposition of metals is of major interest as it features a wider spectrum of properties. Since the addition of alloying elements affects the performance and corrosion properties of nanocrystalline materials, this condition is extensively investigated in the studies mentioned below.  This positive effect of alloying can be seen from the study by Jung and Alfantazi (2006), where the addition of P leads to a remarkable increase in corrosion resistance in an acidic aqueous medium. A positive shift in corrosion potential (59 mV) was seen for nanocrystalline Co-1.1 wt%P which significantly reduced the anodic dissolution rates as compared to pure nanostructured Co deposits. In addition, at open circuit potential, total interfacial impedance of nanocrystalline Co–1.1 wt% P was significantly larger than that of nanocrystalline Co as observed from EIS measurements.  The corrosion rate of the nanocystalline Ni-P alloy was also considered to be significantly enhanced due to the enrichment of P on the surface coating which inhibited the dissolution rate further as reported (Rofagha et al., 1993b; Helfand et al., 1992). From the study, it was suggested that dissolution was limited by the formation and absorption of the hypophosphite anion on the alloy surface, that forms a barrier layer between the alloy and the electrolyte and prevents H2O from reaching the alloy surface.   	  	    37 Meanwhile, alloying nanostructured Ni with W has been found to be beneficial in several applications, for example, as a barrier coating in electronics and for wear protection in engineering components, owing to their superior strength and thermal stability. However, it was reported that the competing effects of W content and grain size evidently govern the corrosion behaviour of the nanocrystalline Ni-W alloy. While an increase in W content has been noted to promote the formation of a mixed oxide film that increases corrosion resistance (Sriraman et al., 2007), it also leads to finer grains (Detor and Schuh, 2007) providing more active sites for corrosion reaction. The electrodeposited nanocrystalline Ni microalloy containing P less than 0.3 wt% with an average grain size of 100 nm was developed in the early 1990’s as a sleeve material for in situ repair of a nuclear reactor steam generator tubing which was compromised by intergranular corrosion and stress corrosion cracking (Palumbo et al., 1997; Gonzalez et al., 1996). Various experimental results have shown that the nanostructured sleeve is intrinsically resistant to intergranular attack and intergranular stress corrosion cracking due to the P alloying. The material was also found to be resistant to pitting attack and only slightly susceptible to crevice corrosion. The addition of Cr nanoparticles affected the electrochemical behaviour, particularly pitting corrosion of the electrodeposited nanocrystalline Ni in a solution containing Cl− ions (Peng et al., 2006). Numerous pits occurred for the 4.5wt% Cr-nanocrystalline Ni although, with an increase in Cr addition, the formation of pits was effectively prevented. High Cr content (10.9 wt%) increased pitting corrosion resistance with fewer pits present on the coating surface. This is due to the rapid formation of a dense protective Cr-oxide-rich passive film 	  	    38 during the polarization test on the entire surface of the nanostructured Ni deposits. The results indicate that there exists a critical co-deposition content of Cr nanoparticles where, in this case, the critical value is presumed to be close to 11 wt% Cr. The corrosion behavior of electrodeposited nanocrystalline Cu alloyed with Ni were analyzed in 3% NaCl solution and compared to cast Monel-400 alloy (Ghosh et al., 2006). From both optical and SEM tests of the corroded surface, a severe pitting type of attack for Monel-400 was observed unlike the uniform and homogeneous corrosion of the nanocrystalline Cu-Ni alloy. Ni alloying was seen to significantly enhance the surface morphology of the deposited Cu alloy. Furthermore, the corrosion current density of nanocrystalline Cu-Ni alloy was superior as compared to Monel-400 which indicated better corrosion performance of the nanocrystalline alloy. The fine grain size of Cu-Ni deposits proved to be the main reason in increasing the corrosion performance of these alloys in NaCl solutions. In another study, the corrosion of an electrodeposited Ni-Co alloy with different amounts of Co in a 3.5% NaCl solution was investigated (Srivastava et al., 2006). The authors reported both the crystal structure and surface morphology of the deposits significantly changed while varying the amounts of Co. A single FCC crystal structure was present for pure Ni deposits while a mixed structure (FCC + HCP) was observed for deposits with cobalt content of 50 wt.%. Further increase in cobalt attributed to the change of phase structure to full HCP phase. The surface morphology of Ni–Co alloys changed from regular polyhedral crystallites to spherical cluster for a cobalt content of 50 wt% and further increase resulted 	  	    39 in increase in cluster size. Finally for cobalt content of 80 wt.% and beyond there was a change to acicular crystallite morphology. The highest corrosion resistance was obtained from the Ni-20 wt.% Co alloy as compared to other Ni-Co alloys, pure Ni and plain cobalt as observed from the polarization studies. The better corrosion performance of this alloy was due to the presence of both FCC and HCP crystal structure.  Figure 2. 10 Cathodic polarization curves of Co-W electrodeposition from electrolytes with different additives (Wei et al., 2007) Corrosion of nanocrystalline alloys was also seen to be affected by organic additives used during the electroplating of these nanostructured deposits. Different types of organic additives (saccharin (SAC), o-toluene sulphonamide (oTOL) and methacrylate reagent) on the cathodic process of electrodeposited Co-W thin films were studied and it was found that 	  	    40 the shape of the potentiodynamic polarization curves were similar for all curves, although a shift to more negative potential and decrease in current density were observed, depending on the presence and nature of the organic additives as shown in figure 2.10 (Wei et al., 2007). From the same figure we can see that the negative shift on the potential increased in the following order: oTOL, SAC, dibutyl methacrylate, coumarin, and ethyl-methacrylate. 2.5 Other Properties of Nanomaterials In nanocrystalline materials, the intercrystalline volume fraction is found to comprise as much as 50 percent of the total crystal volume (Tjong and Chen, 2004). These solids are assumed to have a different kind of atomic structure: a crystalline structure with long-range order for all the atoms far from the grain boundaries and a disordered structure with some short-range order at the interfacial region. Hence, the mechanical properties of these nanocrystalline materials are expected to be different as compared to their equal polycrystalline material.  It is well known that with a decrease in grain size of a material, the mechanical properties such as hardness will significantly increase, which holds true from the Hall-Petch relation as below: 𝜎? = 𝜎? + 𝑘?𝑑??/? 	  	    41 where 𝜎? is the (true) yield stess, d is the average grain diameter, and 𝜎?and 𝑘?  are emphirical constants. The formation of smaller crystals brings with it adequate increase in the volume fraction of interfaces between those crystals, and these interfaces act as barriers to dislocation glide. Thus, finer-grained materials possess more barriers to the dislocation motion and are stronger than chemically-similar, coarser-grained materials. This can be seen from several studies conducted that have reported on improved mechanical properties in nanostructured materials. An increase in hardness of the nanocrystalline Ni-Co alloy was reported by Li et al. (2008). The hardness of the electrodeposited alloy increased with an increase of cobalt and saccharin in the plating bath. Nanocrystalline lead molybdate was also reported (Anandakumar and Abdul Khadar, 2009) to increase in hardness with a decrease in grain sizes, which also satisfies the Hall-Petch relationship.  It is known that ultrafine-grained materials follow the Hall-Petch grain size strengthening behaviour into the nanocrystalline regime. On the other hand, at some grain size transitions from the regular to inverse Hall-Petch has also been reported in literature. The dependency of yield strength or hardness on grain size may become weaker, and even reverse at extremely fine-grain sizes. This phenomenon is known as the “inverse Hall-Petch effect” or “softening effect” and since it was first observed in the late 1980s, its behavior has generated tremendous attention in the field (Kumar et al., 2003; Carlton and Ferreira, 2007; Pande and Cooper, 2009; Chokshi et al., 1989a). This phenomenon is shown in figure 2.11. Several attempts have been made to further describe the softening behavior in nanocrystalline materials. Suryanarayana et al., (1992b) implied that the refinement of 	  	    42 grain size results in an increase in grain boundary fraction, triple junction fraction, and change in grain boundary structure and thus the softening of the nanocrystalline material. The change in the deformation mechanism caused from a breakdown of a dislocation pile-up was also suggested as one of the reasons causing softening of these materials (Nieh and Wordsworth, 1991). Many other researchers have also made an effort to propose several models to explain this unique behavior. Based on the dislocation network model, Scattergood and Koch (1992) have tried to interpret the negative slope of the Hall-Petch relation for these fine-grain materials by assuming that dislocation line tension is size-scale dependant. Lian and Baudelet (1993) have suggested a modified Hall-Petch relation based on the model of a bow-out of dislocation that was confirmed later experimentally for a long range of grain size in nano regime. A qualitative model explaining the abnormal Hall-Petch relationship on the basis of a decrease in excess volume and interfacial excess energy when grain sizes approach the nano size has also been presented (Lu and Sui, 1993). An annealing of nanocrystalline materials have also been reported to be one of the reasons of this softening behaviour (Fougere et al., 1992). Nanocrystalline Ni and NiFe showed an increase in hardness with a decrease in grain size up to 18 nm where a deviation from the regular Hall-Petch was then observed due to softening at the smallest grain size (Cheung et al., 1995).   Similar results were also reported by Conrad et al. (2005) for nanocrystalline TiN where they concluded the grain softening was due to the changes in texture and the presence of weakening imperfections during the fabrication process of nanocrystalline TiN. Fougere et al., (1992) suggested that 	  	    43 whether nanocrystalline materials show a regular or inverse Hall-Petch relationship below a critical grain size is dependent upon the method used to vary the grain size. They found that a regular Hall-Petch behavior is observed when hardness measurements are obtained on a series of as-prepared samples.  Figure 2. 11 Schematic diagram of the variation of yield strength as a function of grain size (Kumar et al., 2003). The magnetic properties of ferromagnetic properties are known to be sensitive to microstructural features such as internal stress, texture, and grain size distribution. Hence, the change in grain size of these materials is expected to show considerable change in their magnetic properties. The ferromagnetic properties of materials are influenced by changes in the interatomic distances. Thus, the saturation magnetization, Ms and ferromagnetic 	  	    44 transition temperature of nanocrystalline rnaterials are considerably reduced with respect to the bulk rmaterials. (Gong et al., 1991) studied ultrafine particles (10 - 50 nm) of Ni, Co, and Fe, and reported maxima in coercivity, which were attributed to the existence of a critical particle size at which the transition to single-domain particles takes place. The magnetic properties of nanocrystalline materials produced by crystallization of amorphous alloys exhibit lower coercivity, higher permeability, almost zero magnetostriction, and lower core losses due to the high electrical resistivity. These characteristics suggest that nanocrystalline Fe-based alloys are the most promising candidates for soft magnetic applications. 2.6 Previous Studies on Electrodeposited CoFe Alloy As mentioned earlier, most research work on these deposited CoFe alloys have been mainly focused on their magnetic properties. Various parameters were explored during the deposition process of the alloy in order to obtain optimum performance and gain better understanding as well as improve further their properties.  Different plating solutions were employed to deposit CoFe alloys and determine their effect on magnetic behaviour. The magnetic properties of iron group thin films (Co, Ni, Fe, CoFe, NiCo, and CoNiFe) that were electrodeposited from both chloride and sulphate baths were investigated (Kim et al., 2003b). The results showed that magnetic saturation of the CoFe alloy was increased with an increase in Fe content of the deposits obtained from both solutions. An increase in coercivity with Co content in both baths was also reported. 	  	    45 However, higher current efficiency was obtained from the chloride bath due to a low H2 limiting current as compared to those from sulphate baths.   The plating behaviour of iron-cobalt alloys in a sulphate-based solution buffered with boric acid was evaluated by Bertazzoli and Pletcher (1993). It was reported that the composition of the alloy was strongly related to the composition ratio of the cobalt and iron salts and did not depend on the current density or temperature used during the deposition process. The iron was also seen to deposit preferentially to cobalt, despite the fact that cobalt has a higher reduction potential. They attributed this behaviour to the inhibiting effect of iron (II) on cobalt nucleation and growth. However, the presence of iron (II) did not affect the deposition of cobalt (II) on the cathode. On the other hand, an additive-free, sulphate-based plating solution to produce iron-cobalt deposits was done by Liu et al., (2000) to study the effect of current density on the deposits with 18-60 at%Fe. It was seen that at lower current densities, the content of iron in the deposits was much higher than they expected based on the Co:Fe mass ratio in the solution. However, at higher current densities the composition of the deposits was nearly reflected to the composition of the Co:Fe mass ratio in the plating solution. From the XRD analysis, the authors observed a BCC phase for all tested deposits regardless of composition except for deposits obtained from the use of high current density where an FCC phase was present for deposits containing less than 20 at%Fe content in the deposits. Furthermore, the coercitivity droped as low as 12 Oe, which the authors believed is due to the shift from a BCC to an FCC phase from the co-deposition of the deposited alloys. 	  	    46 Plated iron-cobalt films were also obtained from a dibasic ammonium citrate (Zhou et al., 2008). The effect of plating temperature and solution composition was investigated where a decrease in the anamolous deposition of iron during the electrodeposition of iron-cobalt deposits was observed when high-plating temperature and rich cobalt-plating solutions were used. Furthermore, a new phase, α-Mn was found present at elevated plating temperatures as compared to the only BCC phase present when plating was carried out at room temperature, while at higher Co concentration the HCP phase dominated. However, from the findings of Zhang et al., (2007), only the FCC phase was observed in deposits of pure cobalt and over the entire range of most of the composition. In another study, Zhou et al., (2009) also used a tri-basic ammonium citrate as a stabilizing agent to plate iron-cobalt deposits at pH 4.7 and reported on the formation of metastable phases which included the α-Mn phase at a low plating temperature. At a temperature of 60oC with compositions of cobalt between 37 at% Co and 81 at% Co, a phase that was identified as a dHCP (double HCP) was codeposited along with the other phases. Furthermore, Jay et al., (2001) reported that, by annealing the α-Mn phase up to 160oC the deposits would transform to a stable BCC phase. The appearance of the α-Mn phase from the iron-cobalt alloys has also been reported to be detected from the vapour deposition (Jay et al., 2001), magnetron sputtering (Simmonds et al., 1996; Specht et al., 2004; Specht, et al., 2005), and precipitation reactions (Pourroy et al., 1999;  Pourroy et al., 2002). Meanwhile, Crozier (2009) studied the stability of the dibasic ammonium citrate plating solution and reported that the stability of the solution was prone to oxidation which caused 	  	    47 iron (II) to oxide to iron (III) and hence lead to the formation of FeO(OH) and Fe3O4 precipitates. The author increased the solution stability by increasing the plating temperature together with a voltage application across the cell, which also resulted in cobalt-rich deposits. Flat and compact deposits with a grain size of about 40 nm containing both the BCC and an α-Mn phase was obtained. However, low saturation flux densities of the deposits were very low with values between 1.8T – 1.9T, which the authors attributed to the presence of precipitates in the deposits. The effect of adding phosphoric acid into the plating solution to produce CoFeB alloys from an electroless deposition was investigated (Osaka et al., 1994). They reported that inclusion of phosphoric acid in the solution with a magnetic field during plating resulted in increased permeability of the deposits and reduced their coercivities. Coercivities value lower than 1Oe and saturation flux densities of ~1.6T were obtained in deposits containing the highest amount of phosphoric acid, which is 0.06 mol/dm3. From a different study, Osaka et al., (2003) attempted to produce cobalt-iron films with high saturation flux densities. Unfortunately, at first it was not achieved due to the incorporation of Fe(OH)3 precipitates in the deposits. Two types of methods were then proposed to overcome this problem. The first method used the addition of trimethylamineborane (TMAB) to impede the iron(III) formation. This method was only able to increase the saturation flux densities up to 2.3T. Meanwhile, in the second method the authors employed a dual electroplating cell to physically separate the anode from the plating solution and connect the cathode, 	  	    48 which is immersed in a separate solution through a salt bridge. This method was successful in obtaining the cobalt-iron deposits with high saturation flux density of 2.4T.  The use of different additives in the plating solution is another approach that has been taken into account to achieve superior properties of these alloys. The iron-cobalt alloy was also successfully deposited from a sulphate plating bath with the addition of vanadium as the additive (Shou et al., 2003). They discovered that the magnetic properties were similar to that of the bulk alloy with the addition of vanadium, although the permeability of the films were reduced. Furthermore, crack-free deposits were obtained with grain sizes of 200 nm. In order to reduce the oxygen content of the films, acetic acid was added into the plating bath with the intention of complexing the iron (II) ions (Bonhote et al., 2002; Bonhote et al., 2004). They also substituted saccharin with the organic sulphur- bearing additive (SBA) and successfully reduced the incorporated oxygen in the deposits to 0.1 at%. Saturation flux densities obtained were between 2.35 – 2.45T which was similar to the bulk alloy. An increase in plating temperature was also found to increase the grain size and resulted in increased coercivities while reducing the internal stress and electrical resistivities. Furthermore, an increase in cobalt content and magnetic annealing resulted in improved coercivities. However, internal stresses of the film alloy were also increased.   Jinnie et al. (2008) had investigated the effect of sulphur incorporation on the corrosion and magnetic properties of electrodeposited 2.4T CoFe. The authors concluded that the coercivity of the deposits had a direct correlation with the sulphur content inside the 	  	    49 deposits. An increase in sulphur content resulted in softer CoFe deposits. Higher corrosion susceptibility was also seen with higher sulphur content in deposits. A study by Ricq et al. (2001) investigated the influence of sodium saccharin on the electrochemical process as well as the corrosion of CoFe deposits. The results showed that corrosion resistance decreases in deposits obtained from solutions containing a high concentration of sodium saccharin. The authors concluded that the poor corrosion behaviour was due to the high sulphur content in the alloy coating. The effect of saccharin on the corrosion behaviour of CoFe was also investigated  by Tabakovic et al. (2006). The electrochemical properties of the electroplated CoFe were compared with sputtered CoFe and it was concluded that the electroplated CoFe had lower corrosion resistance due to the sulphide present at grain boundaries, which further enhances the anodic dissolution and hinders passivation. George et al. (2008) also conducted a study on sulphur and saccharin incorporation into electrodeposited CoFe and they too have concluded that the presence of sulphur leads to higher corrosion susceptibility. Meanwhile, deterioration of the surface morphology of electroplated CoFe alloy was observed without the use of any organic additives during the electroplating process. Kakuno et al. (1997) investigated the morphological and structural characteristic of CoxFe1-x alloys film electroplated from a sulphate bath solution without any additive addition. They observed at 70 at% Co the deposition of a HCP phase, which was not the phase predicted from the iron-cobalt phase diagram. Heterogeneous microstructures, which were prone to 	  	    50 microcracking, were also observed from SEM analysis. They concluded that deposit composition could be easily controlled through the solution chemistry. Xu et al. (2001) studied the effect of plating iron-cobalt thin films in a sulphate bath without any additive additions. They obtained dark deposits with cracks appearing on the surface after annealing at 240oC after 2 hours from a solution with pH 2.9 and a current density of 7 mA/cm2. Furthermore, the surface deposits appear to be rough with large nodules. The coercivities varied from 2.6 – 5.8 Oe with a Bs value of 2.14T. This value was significantly lower as compared to the Bs value of the bulk alloy, which is 2.4T. They concluded that the poor morphological structure of the deposits was due to the elimination of additives in the plating solution as well as incorporation of oxygen, which was confirmed by the x-ray photoelectron spectroscopy (XPS) technique. Other researchers have also analyzed other parameters that affect the CoFe alloy properties. For example, the effect of Fe3+ on the magnetic moment of electrodeposited CoFe alloys was also determined (Stanko et al., 2008) and was reported that the magnetic moment was affected by the incorporation of Fe(OH)3 into the deposits and suggested that the Fe(OH)3 nucleation, and precipitation decreases the saturation magnetic flux density of electrodeposited CoFe.  Meanwhile, the effect of oxygen incorporation towards the structure and magnetic properties of the electrodeposited CoFe alloys was studied. An increase in coercivity and a decrease in saturation magnetization was seen with oxide present at the grain boundary of 	  	    51 the deposits, which was due to the higher mobile domain wall interactions with the grain boundary (Elhalawaty et al., 2011).  Analysis using XRD and TEM indicated that coercivity is dependent on the grain size of the deposited Co88Fe12 alloy (Chang et al., 1990). Smaller grains resulted in lower film coercivities. It was suggested that both grain size and coercivities could be minimized through the co-deposition of BCC and FCC phases through the competitive grain growth. Similar findings have also been reported by other studies (Liu et al., 2000;  Mattoso et al., 2001;  Zhang et al., 2007). According to a model suggested by Herzer et al. (1990), very small grain sizes less than a few tens of nanometers, will cause an exchange of interactions between the grains that become highly important, where it prevents the alignment of domains along the crystals axesof magnetization. This model also predicts that coercivity is related to grain size by a factor of 1/D in larger grains and by a factor of D6 in very fine-grained materials.  Koza et al. (2010) investigated the influence of homogeneous magnetic fields on the properties of the electrodeposited CoFe. The superimposed magnetic field causes a magnetohydrodynamic (MHD) effect that increases the current density and deposition rate. Smoother and homogenous deposits were obtained from the superimposition magnetic field. They reported that the magnetic field did significantly affect the properties of the deposits in terms of their microstructure, roughness, internal stress state, and chemical 	  	    52 composition. However, the magnetic field superimposed perpendicular to the electrode surface does not influence significantly the electrochemical reaction. Nanowires of Co and Co90Fe10 electrodeposited in the pores of anodic alumina were produced to investigate their magnetic and structural properties of the arrays (Khan and Petrikowski, 2002). Arrays of Co and Co90Fe10 nanowires show perpendicular magnetic anisotropy and textured crystallographic behaviour. The continuous films of Co and Co90Fe10 on Cu substrates show in plane magnetic anisotropy and coercivity values between 109 and 288 Oe. Sun et al. (2005) investigated the effect of magnetic annealing on the stress and magnetic properties of iron-cobalt thin films. Their study reported that an increase in stress results from the annealing of the films at 225oC, while increasing the temperature up to 255oC reduced the stress. Film stress was observed to have no significant changes after annealing for 5 hours. Saturation magnetization was also improved by 2% through magnetic annealing. Furthermore, magnetic annealing significantly improved the coercivities of the films, although it switched the direction of easy and hard axes.   	  	    53 Chapter 3 3	   Objectives	  From the literature, it is obvious that most research work on CoFe alloys are mainly focused on their magnetic properties. Because these alloys are gaining attention in many applications, therefore it is also crucial to study their corrosion behavior in order to further expand their future usage. In light of the current review, the following problems has been identified and is of interest in this research work: 1) No studies have been reported to synthesize CoFe alloys into the nanocrystalline grain sizes and determining their various properties. 2) The refinement of grain size to the nanometer range has shown to improve the corrosion behavior of various metals and alloys. However, the effect of nanocrystalline structure of CoFe alloys on the corrosion behavior in different types of environment was not found in the literature. 3) Data reported in the literature on the effect of alloying and their concentration has presented both positive and negative effects on the corrosion behavior of nanocrystalline materials.  However, the corrosion performance of nanocrystalline CoFe alloyed with different iron concentration has not been investigated so far. 4) Using saccharin as an additive is a common practice in the electrodeposition process to obtain bright and smooth deposits. However, the exact contribution of this 	  	    54 additive on the electrochemical corrosion performance of CoFe alloys is still not fully understood.  Thus, due to the above discussion, this study aims at producing nanostructured Co and CoFe alloy coatings as well as understanding the corrosion behavior of these coatings in different solution environments by employing the DC polarization and AC EIS techniques. The objectives of this research work are as follows: 1. To synthesize nanocrystalline cobalt and cobalt-iron alloys through the electrodeposition process and study the effect of iron alloying and different iron concentration on the properties (e.g. grain size, crystal structure, and microhardness) of these deposits. 2. To analyze and further extend the understanding of the electrochemical corrosion behaviour of deposited nanocrystalline Co and CoFe alloys in highly acidic and alkaline solutions.  3. To investigate the influence of organic additives on the anodic dissolution and passivation phenomena of the Co and CoFe alloys using the electrochemical quartz crystal microbalance (EQCM) coupled with other polarization techniques. 4. To determine the effect of near-neutral solution pH and the presence of sulphide ions on the corrosion performance of cobalt-iron alloys in a neutral environments.   	  	    55 Chapter 4 4	   Experimental	  Approach	  and	  Methodology	  In this research work, two main studies were carried out in order to achieve the proposed goals. First, the synthesis of the nanocrystalline Co and CoFe alloy coatings were performed through the electrodeposition process. Second, the deposited nanocrystalline Co and CoFe alloys obtained were used to systematically study their corrosion behavior. The following describes the tools and techniques that were used to deposit the nanostructured coatings and analyze their electrochemical performance. 4.1 Electrodeposition of Nanocrystalline Co and CoFe Alloy Coatings Nanocrystalline Co and CoFe alloy deposits were synthesized from a sulphate solution. A standard three-electrode cell configuration was used to synthesize nanocrystalline Co and CoFe films as shown in figure 4.1. Titanium was used as the cathode substrate with an exposed area of 1 cm2 and the anode material was a graphite rod placed 20 cm from the cathode. The substrate was bonded to a copper wire using a conductive silver epoxy to establish an electrical connection. Prior to electrodeposition, the titanium substrate was ground with silicon carbide papers of 240, 320, 600, 800, and 1200 grit, polished with 1 µm alumina powder and then rinsed with distilled water. The substrate was then immersed in sulphuric acid for a few seconds to eliminate all contaminants. Specimens were weighed before and after the electrodeposition process.  	  	    56 The plating bath for CoFe deposits consisted of cobalt sulphate (CoSO4), iron sulphate (FeSO4), sodium chloride (NaCl), boric acid (H3BO3), and saccharin (C7H4NO3S). Boric acid was added as a pH buffer while saccharin (powder) that was purchased from Sigma Aldrich was added as a grain refiner. Iron sulphate concentration was adjusted to produce CoFe deposits with various iron contents. The chemical composition of the solution is shown in table 4.1.  Figure 4. 1 Schematic diagram of the electrodeposition process   	  	    57 Table 4. 1 Bath composition and electrodepositing parameters for Co and CoFe alloy deposits Variable Range CoSO4, g/L 250 FeSO4, g/L 10 - 80 H3BO3, g/L 30 NaCl, g/L 25 Saccharin, g/L 1.5 pH 2.5 ± 0.1 Temperature, oC 40 Peak current density, A/cm2 3  	  	    58 All chemicals were reagent grade and immersed in distilled water. The temperature of the bath was maintained at 40oC by employing a water bath and the solution was continuously stirred using a magnetic stirrer. Sulphuric acid (H2SO4) or sodium hydroxide (NaOH) was added in small drops to the electrolyte bath in order to adjust the pH of the bath solution to 2.5. During the electrodeposition process, a direct current of 3 A cm-2 was applied. The deposits were immediately taken out of the solution after the plating process and washed under running water for several minutes to remove any contaminants. All deposits were then dried with hot air and mechanically stripped from the substrate for further analysis. A number of pure nanocrystalline Co deposits were also produced from similar sulphate solution, excluding the iron sulphate, to compare their properties and characteristics with that of the nanocrystalline CoFe alloy coatings. The thicknesses of these alloys were determined by using the SEM and were measured to be about 60 – 70 µm. 4.1.1 Surface and Chemical Characterization The scanning electron microscope (SEM) Hitachi S-3000N was used to characterize the surface morphology and microstructure of the nanocrystalline Co and  CoFe alloy coatings. The chemical composition was measured using an energy dispersive X-ray spectroscopy (EDS) system attached to the SEM. The data presented regarding the chemical composition on all deposits using the EDS was an average of at least five readings. Furthermore, the Electron-probe micro-analyzer (EPMA) of the CoFe alloy were also done on a fully automated CAMECA SX-50 instrument, operating in the wavelength-dispersion mode in order to confirm the compositional readings obtained from the EDS. The EPMA was 	  	    59 carried out to compare the composition of the coatings obtained from the EDS measurement. 4.1.2 XRD X-ray diffraction (XRD) was performed using a Rigaku X-ray diffractometer using Cu Kα radiation (40 kV, 20 mA) at a scan rate of 0.04o/sec. The XRD data were recorded in a range of 10o to 70o. XRD was used to identify the preferred crystallographic orientation as well as calculate grain sizes of the electrodeposits. The average grain size of crystallites was calculated using the Scherrer formula (eq. 4.1) according to the XRD peak broadening.  d = ?.?  ?𝛽   ™? ?     (4.1) where d is the average grain diameter of the material, λ is the wavelength of the radiation used, 𝛽 is the Full Width at Half Maximum (FWHM), and ϴ is the Bragg angle (degrees). The FWHM peak broadening is measured as in radians. The grain size of the CoFe deposits was also determined using a transmission electron microscopy (TEM). Thin films were prepared from 3mm disks using the electropolishing method in an electrolyte comprising 10% perchloric acid and 90% acetic acid at a temperature of 15oC and a voltage of 20V. 	  	    60 4.1.3 TEM The grain size of the Co and CoFe alloy deposits was also determined using a transmission electron microscopy (TEM). Due to the large number of samples synthesized, only a few good samples were selected to be used for the TEM characterization. Thin films were prepared from 3mm disks that were mechanically punched from the electrodeposits. These disks were then electropolished using a Struers TenuPol-5 twin jet electropolisher. The electropolishing method consists of an electrolyte comprising 10% perchloric acid and 90% acetic acid at a temperature of -15oC and a voltage of 20V. Electropolishing was carried out for a duration of about 60 seconds. A Hitachi H7600 TEM with a 120kV tungsten filament was used to obtain bright field and dark filed images. 4.1.4 Microhardness The microhardness was measured using a microhardness tester with a pyramidal indenter. Microhardness measurements of the electrodeposits was performed by using a Vickers microhardness technique under a load of 50 g and a dwelling time of 15 seconds. Ten measurements at different locations throughout the alloy coating surface were performed and the average value was taken as the final hardness value. Samples were mounted into epoxy before the microhardness measurements were taken to ensure proper specimen support for accurate readings. The deposits were also polished and then rinsed with water prior to the hardness testing. In order to confirm that the substrate had no effect on the hardness value, the depth of indentation for all hardness measurement was less than 1µm.  	  	    61 4.2 Electrochemical Corrosion Measurements of Nanocrystalline Co and CoFe Alloy Coatings All electrochemical measurements were carried out on a Princeton Applied Research Versastat 4 potentiostat/galvanostat. The electrochemical corrosion tests were carried out using a conventional three-electrode electrochemical cell as previously shown in figure 4.1. A graphite rod and the saturated calomel electrode (SCE) were used as the counter electrode and a reference electrode, respectively. The electrodeposits were placed in the cell as a working electrode. The electrolytes and corrosion parameters used for each experiment are described in the following sections. 4.2.1 Electrolytes The corrosion experiments for the electrodeposited nanocrystalline Co and CoFe were conducted in deaerated 0.1 M H2SO4 (pH 1) and 0.1 M NaOH (pH 13). The solutions were deaerated by purging argon gas throughout the experiments. Meanwhile, the electrolyte solutions employed for the electrochemical studies in chapter 8 were 0.1 M Na2SO4 with a pH of 5, 7, 9, and 11. The pH of the solution was adjusted by adding H2SO4 or NaOH. Before each measurement, the pH of the test electrolyte was controlled by a standard pH meter. All solutions were prepared with distilled water and analytical-grade reagents. All experiments were conducted using 30 mL of solution at room temperature. The sulphide ion concentrations of 50 ppm, 125 ppm, 500 ppm, and 1000 ppm were prepared with the reagent Na2S. All solutions were prepared with distilled water and analytical-grade 	  	    62 reagents. All experiments were conducted using 30 mL of solution at room temperature. All solutions were freshly prepared before each experiment using deionized water. Each experiment was carried out at least twice to ensure reproducibility. 4.2.2 Potentiodynamic Polarization Test Prior to the electrochemical measurement, all deposited alloys were immersed in the test solution and allowed to attain a stable open circuit potential before starting the polarization scan. A potentiodynamic polarization (PDP) test was then performed with the Princeton Applied Research (PAR) VersaStat 4 potentiostat. The cathodic and anodic behaviors of the samples were obtained in the range of -0.25 V vs. OCP to 0.8 V vs. SCE with a scan rate of 0.167 mV s-1. All potentials are referred to the SCE electrode. All tests were carried out at room temperature. The PDP tests were conducted at least twice to ensure reproducibility of the results. 4.2.3 EIS The interfacial processes and surface interactions of the film/liquid interface was analyzed by electrochemical impedance spectroscopy (EIS). The EIS measurements were performed at the OCP with an alternating current (AC) disturbance signal of 10 mV, a measurement frequency range between 0.01 Hz and 20 kHz and a sampling rate of 10 points/decade. The EIS results were analyzed and fitted to equivalent circuit using ZSimpWin® v3.40. 	  	    63 4.2.4 LPR In order to extract polarization resistance (Rp) for the electrodeposits, the linear polarization resistance (LPR) tests were performed within ± 20 mV of OCP at the standard scan rate of 0.167 mV s-1. 4.2.5 Potentiostatic Polarization The potentiostatic polarization (PSP) was conducted in the test solution where preselected potential was directly applied to the working electrode and the current was measured as a function of time. The PSP was performed to prepare the electrodeposits for XPS analysis. 4.2.6 XPS X-ray photoelectron spectroscopy (XPS) was conducted using a Leybold model MAX 200 with a Mg Kα (hv = 1253.6 eV) anode X-ray source operated at 15 kV and an emission current intensity of 20 mA. Pass energies of 192 and 48 eV for wide and high resolution scans were used, respectively. The elemental Co 2p and Fe 2p peak was fit as doublet due to the spin-orbit splitting of the 2p1/2 and 2p3/2 subshell. A Shirley correction was made in the background before each curve fitting.  	  	    64 4.3 Electrochemical Quartz Crystal Microbalance  Electrochemical Quartz Crystal Microbalance (EQCM) is known as a powerful technique in studying the mechanism of deposition or monitoring corrosion phenomena of various electrochemical systems and is often combined with cyclic voltammetry (CV). This sensitive weighing device utilizes the mechanical resonance of a piezoelectric crystal, which makes it able to record instantaneously mass variation and the voltammogram of the metal electrode. EQCM measurements were performed together with the other conventional polarization tests (e.g. OCP, PDP, EIS, etc.) to determine the in situ mass changes of the working electrodes while simultaneously monitoring the electrochemical reactions occurring at the surface of the electrodeposits in chapter 7 and 8. The EQCM measurements were performed with an AT-cut, mirror-finished Ti coated on both sides of the quartz crystals with 9 MHz nominal frequency, as shown in figure 4.2. The interconnections between the EQCM, potentiostat, and computer are illustrated in figure 4.3. The Co and CoFe alloy was deposited from a solution similar to the previous solution listed in table 4.1. All chemicals were reagent-grade and immersed in distilled water. Prior to the deposition process described above, the titanium resonators/electrode were first soaked for 10 seconds in a solution composed of 1:1 volume ratio of nitric acid and H2O prior to each experiment. The Ti resonators were also subjected to electrochemical cleaning by carrying out 20 cycles at positive potentials in 0.1 M H2SO4.  	  	    65  Figure 4. 2 Schematic diagram of AT cut quartz crystal After the deposition, the CoFe alloy electrode was rinsed with distilled water. The cell was then filled with the test solution and EQCM measurements were performed. The quartz crystal microbalance QCM 922 was used in conjunction with a Princeton Applied Research Versastat 4 potentiostat. A three-electrode cell was used which consisted of a cobalt-iron (CoFe) deposited on the titanium resonator with an area of 0.2 cm2 as the working electrode. A saturated calomel electrode (SCE) and graphite was used as the reference and counter electrode, respectively. A new fresh electrode was employed for every EQCM experiment. The mass changes of the Ti resonators are detected as differences in the resonant frequency. By applying an AC voltage, resonance is excited when the frequency of the applied voltage corresponds to the natural, or resonance, fo of the crystal. Thus, resonance frequency of the crystal is directly proportional to the total mass of the crystal. From the change in resonance frequency, the changes in the mass on the electrode with a very high sensitivity could be determined using the Sauerbrey equation (4.2): 	  	    66 ∆𝑚 = − ??  ??  ?????  Δf    (4.2) where 𝑓?  is the resonant frequency of the quartz crystal resonator, 𝑝?  is the density of quartz, 2.648 g cm-3,  𝜇? is the shear modulus of quartz, 2.947 × 1011 g cm-1s-2, and  𝑛 is the harmonic number. The Sauerbrey constant and calibration was determined by performing the Cu underpotential deposition method. The underpotential deposition was carried out in a solution containing 2 mM CuSO4 and 0.1 M H2SO4 at a potential of 0.3 VSCE. After deposition the Cu was stripped anodically.  	  	    67  Figure 4. 3 Interconnections between EQCM, potentiostat, and computer   	  	    68 Chapter 5 5	   Nanocrystalline	  Cobalt-­‐Iron	  Alloy:	  Synthesis	  and	  Characterization	  In recent years, there has been tremendous interest in the development of nanocrystalline materials with ultra fine grain sizes lower than 100 nm. These materials often benefit from enhanced and sometimes novel, chemical and physical properties from the reduction in the grain size toward the sub-100 nm scale. Several synthesis techniques have been used to produce these unique materials (Nieman et al., 1989; Dimesso et al., 1999; Yu et al., 2010; Hosseini et al., 2005; Jayatissa et al., 2007; Liu et al., 1997; Manaf et al., 1993; Gloriant et al., 1998) however, electrodeposition is found to be a much simpler and economical method. Nowadays, electrodeposited cobalt-iron (CoFe) alloys are gaining much attention for applications in microelectrochemical systems (MEMS) and protective coatings due to their high saturation magnetic flux density and high Curie temperature (Tabakovic et al., 2011).  Although many research has been done on CoFe alloys in terms of their magnetic properties (Kim et al., 2003b; Khan et al., 2002; Brankovic et al., 2008) and stress (Shao et al., 2010; Tabakovic et al., 2010) very few studies have focused on the development of electrodeposited nanocrystalline CoFe alloys. Therefore, this study was conducted to synthesize nanocrystalline CoFe alloys by employing the direct current electrodeposition method in a sulphate-based solution. The effects of varying the iron concentration on the 	  	    69 properties of the nanocrystalline electrodeposits in terms of their microstructure, grain size, surface morphology and microhardness was investigated.  5.1 Effect of FeSO4 Concentration The iron content of the CoFe alloys as a function of the iron sulphate (FeSO4) concentration in the bath is shown in figure 5.1. The EDS mapping analysis corresponding to the iron content of 8 wt% Fe and 25 wt% Fe in the electrodeposits are shown in figure 5.2a and b, respectively.   Figure 5. 1 Iron content (wt%) of the CoFe alloy coatings as a function of the FeSO4 in the solution. 0	  5	  10	  15	  20	  25	  30	  35	  0	   10	   20	   30	   40	   50	   60	   70	   80	   90	  Iron	  content	  (wt%)	  	  FeSO4	  in	  the	  solution	  (g/L)	  	  	    70 As expected an increase of the iron content in the deposits was observed with an increase in the concentration of FeSO4 in the plating bath.    Figure 5. 2 EDS spectrum of CoFe alloy (a) 8 wt% Fe. (b) 25 wt% Fe. 	  	    71 The electrodeposits which contain about 5 wt% Fe was obtained from the lowest concentration of 10 g/L FeSO4 in solution. Further addition of FeSO4 of 20 – 30 g/L in the plating bath lead to a gradual increase in the iron content of the alloy coatings to approximately about 8 wt% Fe. However, a rapid increase in the iron content was observed for the deposits obtained from the solution containing higher FeSO4 (> 40 g/L) concentration. Meanwhile the content of iron in the deposits was seen to only slightly increase and was almost constant at about 25 wt% Fe between 70 – 80 g/L of FeSO4 addition into the plating solution. Coatings deposited from solutions with more than 80 g/L of FeSO4 present in the plating bath was not able to be studied as they contained multiple cracks on the coating surface and were very brittle to handle. 5.2 Current Efficiency The current efficiency (ηj) was calculated after determining the composition of cobalt and iron in the deposits using the equation (5.1) (Lowenheim, 1974) : ηj = ∆?    ?? =  ∆?  (? ™   ? ™   ?  ? ™     ? ™   )™? ™   ? ™       (5.1) where Δm is the mass of deposited alloy, mt  is the theoretical mass, xCo is the Co composition in the deposit (wt%), xFe is the Fe composition in the deposit (wt%), εFe and εCo are the electrochemical equivalents of Fe and Co, respectively, I is the current and t is the duration of the deposition process. Figure 5.3 shows the current efficiency (CE) as a function of the current density. The current efficiency was seen to increase with an increase 	  	    72 in the current density. The current efficiency was calculated to be about 70% with the highest current density of 3 A/cm2 employed in this study. The low current efficiency at lower current density has been suggested to be due to the large amount of hydrogen evolution in the electroplating process (Yu et al., 2010).   Figure 5. 3 Current efficiency (%) as a function of current density (A/cm2) 5.3 SEM and TEM Figure 5.4 shows the SEM images of deposited nanocrystallie CoFe alloy coatings with different iron content. A smooth, uniform and bright surface morphology was obtained for all deposits. An acicular morphology was obtained from the deposit without any iron content (figure 5.4a) while equiaxed grains were observed with the increase of iron content in the alloy coatings. Interestingly, it was observed that with increased iron content from 11 010203040506070800 0.5 1 1.5 2 2.5 3 3.5Current	  density	  (A/cm2)Current	  efficiency	  (%)	  	    73 to 25 wt% Fe in the alloy coating resulted in a decrease in the grain size as shown in figures 5.4c-e.   Figure 5. 4 SEM images of CoFe alloy electrodeposited with different iron contents. (a) 0 wt % Fe, (b) 5 wt% Fe (c) 11 wt% Fe (d)17 wt% Fe (e) 25 wt% Fe (f) Deposit consisting of multiple cracks. 	  	    74 From figure 5.4f, it can be seen that multiple cracks were present in the CoFe deposits containing iron content above 25 wt% Fe. This could be associated with high film stresses obtained in the alloy coatings which has been previously found to be related to the increase in iron content (Shao et al., 2010). As mentioned previously, these coatings were not analyzed further in this study due to its poor condition. In order to further confirm their fine grain structure, TEM analysis was also carried out. Figure 5.5a and (b) shows the TEM micrographs which further verify the existence of these fine grains in the CoFe deposit with an average grain size of about 18 nm.  Figure 5. 5 TEM images of CoFe alloy electrodeposited with different iron contents. (a) 17 wt% Fe, (b) 25 wt% Fe   	  	    75 5.4 XRD Analysis All CoFe deposit samples were characterized by means of XRD. Figure 5.6 shows the XRD patterns for deposited nanocrystalline Co and CoFe alloy deposits with different iron contents. CoFe alloys existed in three distinct phases namely the ε (HCP), γ (FCC) and α (BCC) phases at room temperature. It can be observed that the diffraction patterns changed considerably in peak locations and intensities with the addition of Fe in the deposits. Significant broadening of the peaks could also be observed with an increase in Fe content indicating the presence of finer grains. All deposits were seen to have similar crystallographic structure at the same peak positions of about 43o corresponding to CoFe (110) cubic structure. However, the XRD pattern for pure Co deposits exhibited the HCP crystal structure with a strong (002) texture. The diffraction pattern also indicates the presence of γ (FCC) reflections together with the ε (HCP) peaks in deposits containing low iron content (5 wt% Fe).   	  	    76  Figure 5. 6 X-ray diffraction patterns for Co and CoFe alloy coatings; (a) 100 wt%Co (b)  5 wt% Fe (c) 11 wt% Fe (d) 17 wt% Fe (e) 25 wt% Fe Deposits with 11 wt% Fe exhibit both FCC and BCC phases. Further analysis of the diffraction peaks obtained from deposits with increased iron content (18 - 25 wt% Fe) revealed a BCC crystal structure. This result was in agreement with a previous study done where considerable crystal structure change was also observed from pure Ni to NiFe deposits with different ranges of Fe present in the thin films (Abdel-Karim et al., 2011). From the phase diagram of CoFe alloys (Hansen et al., 1989), a complete substitutional solid solution between Co and Fe is formed at room temperature. In another study it was also demonstrated that as the Fe amount in the deposit increases the peak position will also shift to higher angles (Kim et al., 2003). Hence, from the result presented it can be observed that crystallographic structure or preferred orientation of phases strongly depends on the composition of the electrodeposited alloy coatings. 30	   35	   40	   45	   50	   55	   60	   65	   70	  Intensity	  (a.u.)	  2	  Theta	  (deg)	  	  (e)	  (d)	  	  (c)	  	  (b)	  	  	  	  (a)	  110	  200	  101	  100	  002	  	  	  	    77 5.5 Grain Size Decreases With Iron Content The grain size of the CoFe alloy deposits was observed to decrease with an increase in the iron content of the deposit as shown in figure 5.7. The XRD using a diffractometer with Cu Kα  radiation (λ = 0.15405) was employed and the grain sizes obtained were calculated from the line broadening of the X-ray peaks from XRD according to Scherrer’s formula (equation 5.2) as follows (Cullity et al., 1978): 𝑑 = ?.?  ?? ™? ?          (5.2) where d is the average crystallite size, λ is the wave length of the radiation, θ is the Bragg angle and β is the full width at the half maximum (FWHM). The average grain size value of the CoFe deposits decreased from 56 nm to 15 nm with an increase in the iron content of the alloy coatings. From the figure it is shown that coatings with iron content of up to 5 wt% Fe had grain sizes of about 56 nm. A further increase of 8 – 11 wt% Fe in the deposit resulted in grain sizes ranging from 40 to 31 nm, respectively. Smaller grain sizes around 15 nm were achieved through CoFe electrodeposits with the highest iron composition of 25 wt% Fe. The effect of grain refining due to the increase of Fe in the alloy deposit has also been reported in studies concerning the nanocrystalline NiFe (Abdel-karim et al., 2011; Cheung et al., 1995). 	  	    78 Figure 5. 7 Variation of grain sizes of CoFe alloy coatings as a function of iron content (wt%). 5.6 Microhardness Figure 5.8 shows the change in hardness as a function of the Fe content in the deposits. Alloy coatings between 0 - 25 wt% Fe were employed for the microhardness test. From the figure it can be seen that the hardness increased linearly from about 181 HV for pure Co to 323 HV (0 – 8 wt% Fe). A significant increase in hardness for deposits with a composition above 11 wt% Fe was obtained where the hardness was measured to be about 450 HV. The highest hardness was measured at about 620 HV for alloy coatings with 15 wt% Fe and was relatively constant thereafter. It is interesting to note that this hardness value is about three times higher than that for pure Co. The maximum in hardness for the deposits with 15 wt % 0	  10	  20	  30	  40	  50	  60	  70	  80	  0	   5	   10	   15	   20	   25	  Grain	  size	  (nm)	  Fe	  content	  (wt%)	  	  	    79 Fe is most probably the result of the presence of dual phases (FCC + BCC) as determined by XRD. However a slight decrease in hardness was later seen for deposits above 25 wt%Fe. A similar hardness trend has been reported in several studies (Cheung et al., 1995; Palumbo et al., 1990; McMahon et al., 1989; Liu et al., 1993; Sriraman et al., 2007). To the best of the authors’ knowledge, there is no systematic data that has been reported on the strength of either polycrystalline or nanocrystalline CoFe as a function of iron content. Therefore, comparisons are basically done with other electrodeposited alloy coatings reported in the literature. Although several theories have been proposed and investigated in order to further clarify and understand the reason of this deviation from the normal Hall-Petch behaviour, this unique phenomenon is still not fully understood and progressive studies are still being carried out until today. Among the factors that have been considered to be affecting this particular behaviour are diffusional creep (Chokshi et al., 1989b), decrease in interfacial excess volume (Lu et al., 1990), annealing effects (Fougere et al., 1993) and change in grain size (Brooks et al., 2001). The gradual increase obtained initially at lower iron content and the drop at higher iron content maybe be attributed to the change in the phase structure from a HCP to BCC phase structure as iron is further increased in the deposits. As can be seen, the hardness increased and reaches a maximum value of about 620 HV at around 18 wt% Fe content which coincides with the BCC phase from deposits with an average grain size of 20 nm. 	  	    80  Figure 5. 8 Variation of microhardness of CoFe alloy coatings as a function of Fe content (wt%). Another important factor affecting the behaviour of the microhardness of these nanocrystalline CoFe in this study may be attributed to the grain size effect. Initially, the increase at low Fe content in the deposits could be the result from the refinement in grain size as the Fe increases in the alloy coatings.  A study by Li and Ebrahimi, (2003) also experienced an increase in the hardness of NiFe. They suggested that the increase in strength was due to the solid solution hardening with an increase in Fe of the alloy. In another study (Chinnasamy et al., 2001), the formation of an ordered Ni3Fe in nanocrystalline NiFe due to faster grain boundary diffusion was proposed as the main factor for the increase in strength. The above findings may be associated with an increase in hardness for CoFe electrodeposits in this study as the amount of Fe content in the deposits 0	  100	  200	  300	  400	  500	  600	  700	  0	   5	   10	   15	   20	   25	  Microhardness	  HV	  (Kg/mm2)	  Fe	  content	  (wt%)	  	  	    81 is within the same range. On the other hand, the decrease in hardness at higher Fe content maybe due to several different reasons. One of the possible reason causing this deviation from the Hall-Petch equation is that the solid solution hardening is no longer effective for deposits with higher than 25 wt% Fe. Furthermore, this softening effect maybe is resulted from the refinement of the grain size due to the significant increase of the intercrystalline volume fraction associated with the fraction of triple junction (Yamasaki et al., 1998). Thus, it could be assumed in this study that below a particular fine grain size this softening effect significantly contributes in the deviation of hardness from the normal Hall-Petch behaviour.   Figure 5. 9 Variation of microhardness of CoFe alloy coatings as a function of grain size.  0	  100	  200	  300	  400	  500	  600	  700	  10	   20	   30	   40	   50	   60	   70	  Microhardness	  HV	  (Kg/mm2)	  	  Grain	  size	  (mm)	  	  	    82 Figure 5.9 shows the variation of microhardness of CoFe alloy coatings with different grain sizes. It is observed that the microhardness of the CoFe alloy coatings decreases at first with the smallest grain size of about 15 to 21 nm but was then increased at a grain size of 23 nm to 36 nm before decreasing further with larger grain sizes. This deviation from the Hall-Petch relation at the smallest grain size obtained again could be related to the reasons discussed above.  5.7 Summary Several conclusions were drawn from this study: a) Nanocrystalline CoFe alloy coatings have been successfully synthesized using direct current electrodeposition from a sulphate-based solution. Variation in the content of Fe has a very strong influence on the morphology, preferred orientation, grain size and microhardness of the nanocrystalline CoFe alloy electrodeposits.  b) An increase in FeSO4 concentration (10 – 80 g/L) in the plating bath increased the iron content of the alloy coatings (5 – 25 wt% Fe). c) Grain size was significantly reduced (15 nm – 60 nm) with an increase of iron content in the deposits.. 	  	    83 d) Different phases (HCP, FCC, and BCC) were present according to the iron composition of the alloy coatings. e) Microhardness of electrodeposited nanocrystalline CoFe alloy was increased to about three times with an increase in Fe (0-15 wt% Fe) as compared purê nanocrystalline Co. f) The deviation from the normal Hall-Petch behaviour for deposits with higher Fe content (25 wt% Fe) could be attributed to the change in crystal structure as well as the softening effect below certain grain sizes.   	  	    84 Chapter 6 6	   Electrochemical	  Properties	  of	  Electrodeposited	  Nanocrystalline	  Cobalt	  and	  Cobalt-­‐Iron	  Alloys	  in	  Acidic	  and	  Alkaline	  Solutions	  It is known that grain boundaries are high energy areas, thus the corrosion behavior of nanocrystalline materials is expected to be accelerated. Due to changes in surface area, the activity of the metallic atom determines the corrosion properties of electrodeposited nanocrystalline materials. The high volume fraction of grain boundaries results in different corrosion performance among various metals and corrosion environments. Several studies have been carried out to compare the electrochemical corrosion behaviour between nanocrystalline materials and their conventional polycrystalline counterparts (Rajeev et al., 2013; Wang et al., 2006; Zhou et al., 2007). The influence of alloying nanocrystalline materials has also been extensively studied to improve their corrosion resistance. Both superior and detrimental behaviour has been observed and was previously described in section 2.4.2. In view of the current review, this chapter focuses on the effect of alloying iron on the corrosion behavior of electrodeposited nanocrystalline CoFe alloy coatings in both acidic and alkaline environments. Potentiodynamic polarization was used to study the electrochemical corrosion behavior of these alloy deposits while electrochemical impedance spectroscopy (EIS) was employed in order to gain information on the possible 	  	    85 interactions taking place at the solid/liquid interface of the samples. The CoFe alloy coatings were compared to pure nanocrystalline Co to study the effect of iron addition.  6.1 Chemical Composition, Grain Size and Surface Morphology of CoFe Alloy Coatings Iron sulphate in the amount ranging from 10 - 80 g L-1 was used to produce CoFe alloy coatings with different iron contents. EDS and EPMA analysis showed that the Fe content of the deposits range from 5 to 25 wt%. The average grain sizes of the coatings obtained with different iron content were measured using Scherrer equation and calculated from the line broadening of the X-ray peaks from XRD. Table 6. 1 Iron composition and grain size of nanocrystalline Co and CoFe alloy Coatings Fe Content (wt%) Average Grain Size (nm) Co 0 65 Co-5Fe 5.36 56 Co-11Fe 11.42 38 Co-25Fe 25.18 15 	  	    86 Interestingly, in our earlier study it was found that the grain sizes of the deposited alloys decreased with an increase in iron concentration. (Nik Masdek and Alfantazi, 2012). The obtained chemical composition and grain size of the nanocrystalline Co and CoFe alloy coatings are listed in table 6.1. Figures 6.1a and b shows the TEM micrographs of Co-11 wt%Fe and Co-25 wt%Fe, respectively which further confirm the existence of the fine grains in the CoFe deposit. The average grain size of Co-25wt%Fe alloy coating was about 18 nm. By direct measurement from the TEM micrograph, the grain size of the Co-25 wt%Fe coating was found in the range of 10-70 nm and the calculated average grain size was about 20 ± 5 nm which was close to the value obtained from the Scherrer formula of about 18 nm. This shows that the Scherrer method is a reliable method to calculate the average grain size of the deposits.   Figure 6. 1 TEM images of CoFe alloy electrodeposited with different iron contents. (a) 11 wt% Fe and (b) 25 wt% Fe.  	  	    87 The SEM surface morphology of the nanocrystalline Co and CoFe alloy coatings are shown in figure 6.2. From the figure it can be seen that all coatings were smooth, compact and uniform. The morphology of nanocrystalline Co changed from acicular to spherical with homogeneous size distribution and smooth surface grains with the addition of Fe in the alloy coatings.   Figure 6. 2 SEM images of CoFe alloy electrodeposited with different iron contents. (a) 100 wt % Co (b) 5 wt% Fe (c) 11 wt% Fe (d) 25 wt% Fe 	  	    88 6.2 Acidic Conditions 6.2.1  Open Circuit Potential Measurements in 0.1 M H2SO4 The open circuit potential (OCP)-time behaviour was measured for nanocrystalline Co and CoFe alloys and is shown in figure 6.3. The OCP for pure Co decreased from -400 mVSCE for the first 3 minutes that could be attributed to the anodic dissolution of air formed oxide film before it stabilizes at the value of -410 mVSCE. Meanwhile, addition of iron to pure cobalt decreased the OCP value further. This indicates active dissolution of the Fe, which implies a deterioration of corrosion resistance of the alloy coating. The OCP value decreased from - 417 to - 440 mVSCE with an increase in iron content ranging from 5 – 25 wt% Fe of the nanocrystalline CoFe deposits.  Figure 6. 3 Eoc time behavior of CoFe alloys with various iron content in 0.1 M H2SO4 time	  (s )0 200 400 600 800 1000 1200 1400 1600 1800 2000E	  (mV	  vs	  SCE)-­‐445-­‐440-­‐435-­‐430-­‐425-­‐420-­‐415-­‐410-­‐405C oCo-5FeCo-11FeCo-25Fe	  	    89 6.2.2 Potentiodynamic Polarization in 0.1 M H2SO4  Figure 6.4 illustrates the potentiodynamic polarization behavior of nanocrystalline Co and CoFe alloys with different iron content in deaerated 0.1 M H2SO4 with a scan rate of 1 mVs-1. The scan was started at the cathodic potential in order to reduce air formed oxides on the electrode surface and was further continued until a high upper threshold potential of 1 x 103 mVSCE. One of the concerns about scanning to high potentials is the electrolyte conductivity and the corresponding IR drop. However, in these experiments the reference electrode was always positioned at a fixed distance of 0.5 cm from the working electrode. Also, based on the conductivity of the solution, 8.9 (mS cm-1), and the sample area of 1 cm2, the maximum contribution of the IR drop in the applied potential was calculated to be below 2.8 x10-5 and so is negligible. From the potentiodynamic polarization experiment, typical anodic and cathodic polarization curves were observed. All samples exhibit active dissolution without any distinctive transition to passivation within the applied potential range. It is also apparent that Co coating exhibited a mass transfer limit which was reached at about 0 mVSCE while CoFe alloy coatings exhibited a much lower limit. Similar results were obtained where only active dissolution was present in acidic environment (Jung and Alfantazi, 2006; Jung and Alfantazi, 2010). It can be seen that the effect of alloying iron led to an increase in the kinetics of the anodic metal dissolution which moved the corrosion potential (Ecorr) to a more negative value and increased corrosion current. The possible anodic reactions are (Heusler, 1958): 	  	    90 i	  (mA	  cm-­‐2)10-­‐5 10-­‐4 10-­‐3 10-­‐2 10-­‐1 100 101 102 103E	  (mV	  vs	  SCE)-­‐1000-­‐800-­‐600-­‐400-­‐20002004006008001000C o-­‐5F eC o-­‐11F eC o-­‐25F eC oFe + (FeOH)ads               Fe(FeOH)ads  (6.1) Fe(FeOH)ads + OH-              FeOH+ + (FeOH)ads + 2e-  (6.2) FeOH+ + H+              Fe2+ + H2O    (6.3)  Figure 6. 4 Potentiodynamic polarization curves of CoFe with various iron content in deaerated 0.1 M H2SO4 with a scan rate of 0.5 mVs-1 Electrochemical parameters obtained from Tafel extrapolation of the potentiodynamic polarization curve are listed in table 6.2. It can be observed that all nanocrystalline CoFe alloy deposits show more negative corrosion potentials as compared to purê nanocrystalline Co deposits. Nanocrystalline CoFe alloy with the least iron content of about 5 wt% Fe had i	  (mA	  cm-­‐2)10-­‐5 10-­‐4 10-­‐3 10-­‐2 10-­‐1 100 101 102 103E	  (mV	  vs	  SCE)-­‐1000-­‐800-­‐600-­‐400-­‐20002004006008001000Co-5FeCo-11FeCo-25FeCo	  	    91 a corrosion potential of -466 mVSCE while electrodeposits containing higher iron content of 11 and 25 wt% Fe showed slightly more negative corrosion potentials of -474 mVSCE and -495 mVSCE, respectively.  Table 6. 2 Potentiodynamic polarization data of Co and CoFe alloys in 0.1M H2SO4  It has been reported that further increase in Fe content reduced the grain sizes of CoFe alloy coatings (Nik Masdek and Alfantazi, 2012). This grain refinement provides a greater surface area of active sites that leads to the acceleration of the kinetics of the anodic reaction through the formation of more electrochemical corrosion cells. It is well known that grain sizes in the nanocrystalline range enhance corrosion activity due to the large Coating Ecorr (mVSCE) βa (mVdec-1) βc  (mVdec-1) Icorr (mA cm-2) Co -421 120 141 0.021 Co-5Fe -466 123 145 0.036 Co-11Fe -474 125 147 0.048 Co-25Fe -495 129 149 0.057 	  	    92 number of grain boundaries and triple junctions available for preferential attack sites in corrosive environments. Hence, it is expected that corrosion resistance decrease further with increasing Fe content due to the grain refining of the alloy deposits. This detrimental effect of high iron content on the corrosion behavior of CoFe coatings in acidic solutions could be further seen through the corrosion current density obtained from the extrapolation of the Tafel slopes as shown in table 6.2. The corrosion current density increased from 0.036 to 0.057 mA cm-2 with the addition of iron (5–11 wt%Fe) into the nanocrystalline Co coatings. Furthermore, the increase in corrosion rate for these nanocrystalline CoFe alloy coatings maybe due to the effect of the saccharin used in the plating bath during the deposition process. Several studies (Tabakovic et al., 2006; Rantschler et al., 2008; Lallemand et al., 2002; Lallemand et al., 2005) have reported that the use of saccharin in electroplating CoFe could reduce the corrosion resistance of the alloy coatings due to the presence of sulphur in the metal surface. A study by Oudar and Marcus, (1979) had also mentioned the negative effect of absorbed sulphur on the metal surface, which further accelerated the anodic dissolution due to the weakening between the metal-metal bonds. Hence, the addition of saccharin as a grain refiner in the plating solution in the present study could possibly contribute to the decrease in corrosion resistance of the CoFe alloy coatings. However, further investigations on the amount of sulphur present on the surface alloy coating have to be carried out in order to verify and confirm the effect of sulphur on the corrosion properties of these deposited CoFe alloy coatings. 	  	    93 6.2.3 Electrochemical Impedance Spectroscopy (EIS) in 0.1 M H2SO4  EIS was performed on both pure nanocrystalline Co and CoFe alloy coatings to provide more insight into the electrochemical process occurring at the sample surface and solution interface. Nyquist plots for both Co and CoFe alloys in 0.1 M H2SO4 are shown in figure 6.5. From the plots obtained it can be seen that for Co and CoFe alloy coatings, two loops could be distinguished for both types of nanocrystalline coatings. The first loop at high frequency region appears as a capacitive loop, which belongs to a double layer capacitance parallel to charge transfer resistance. At the lower frequencies, an inductive loop was observed. It has been reported that the low frequency loop was attributed to a slow reaction during metal dissolution process and surface coverage by hydrogen (Jung and Alfantazi, 2006; Keddam et al., 1981).    	  	    94  Figure 6. 5 Nyquist plots for nano Co and CoFe alloys with various iron content in deaerated 0.1 M H2SO4 The EIS results were fitted with an equivalent circuit that involves a double layer capacitance, charge transfer resistance and an element to represent adsorption. The proposed equivalent circuit is shown in figure 6.6. The electrical circuit parameters were simulated using the software package ZSimpWin. The typical Chi square values were less than 5 x 10-4 indicating a satisfactory fit. It consists of a working electrode (WE), solution resistance (Rs) in series with a double layer capacitance (Qdl), charge transfer resistance (Rct), adsorption capacitance (Qa) and the adsorption resistance (Ra).  Table 6.3 presents the values obtained by the best fit to the equivalent circuit model. It can be seen that pure Co coatings had the highest charge transfer resistance while Co 25wt% Fe 	  	    95 had the lowest charge transfer resistance of 87 and 51 Ω cm2, respectively. This indicates better corrosion resistance for pure Co as compared to CoFe alloy coating in acidic environment. This further confirms the results obtained from the polarization test discussed earlier where the corrosion current density of the CoFe alloy obtained were higher indicating lower corrosion resistance as compared to pure Co deposits.  Figure 6. 6 Equivalent circuit proposed for the electrochemical impedance response of Co and CoFe alloy coatings in 0.1 M H2SO4.   	  	    96 Table 6. 3 EIS data obtained by equivalent circuit simulation in 0.1 M H2SO4 Coating Rs (Ω cm2) Qdl-Yo (F cm-2) ndl Rct (Ω cm2) Qa-Yo (F cm-2) na Ra (Ω cm2) X2 (10-4) Co 8.965 0.0001200 0.9 87 0.0007561 0.9 109.9 5.1 Co-5Fe 9.032 0.0001451 0.9 77 0.0002919 0.8 98.0 3.2 Co-11Fe 8.655 0.0001677 0.8 58 0.0001929 0.7 75.6 4.8 Co-25Fe 8.801 0.0001554 0.8 51 0.0001065 0.8 62.3 5.3    	  	    97 6.2.4 X-ray Photoelectron Spectroscopy Analysis  XPS was carried out to determine the composition and chemical state of the nanostructured coatings. Figures 6.7a and b shows the survey (full range) XPS spectrum of as-deposited Co and CoFe alloy coating and the polarized coatings obtained after potentiostatic treatment at −300 mVSCE for 10 min, respectively. Cobalt, oxygen, carbon, sulphur and iron elements were detected on both as-deposited and polarized CoFe alloy while no Fe was detected for all Co samples as expected. The spectra obtained could be deconvoluted into sets of spin orbit doublets together with satellite peaks (Parthasarathi et al., 2012).     	  	    98  Figure 6. 7 XPS spectra (full range) of (a) as-deposited and (b) nano Co and CoFe with various iron content in 0.1 M H2SO4 after potentiostatic treatment at −300 mVSCE for 10 minutes. Figure 6.8 shows the high resolution XPS spectra of Co 2p3/2 where two peaks with binding energies of 781.3 ± 0.1 eV and 798.0 ± 0.1 eV are observed. These peaks could be attributed to the Co(OH)2 (Tan et al., 1991) according to their satellite peak positions at 787.4 and 803.3 eV, respectively. These satellite peaks occur due to the shake-up phenomenon from unpaired valence electrons that gives rise to the number of relaxed final states (Parthasarathi et al., 2012).  	  	    99    Figure 6. 8 Comparison of Co 2p peaks of  (a) as-deposited Co and Co after polarization in 0.1 M H2SO4 and (b) as-deposited and Co 25wt% Fe after polarization in 0.1 M H2SO4 B inding 	  energy	  (eV)770 780 790 800 810Intensity	  (a.u.)A s -­‐depos itedPolarized	  in0.1M	  H2S O4C o2p3/2C o2p1/2S hake-­‐upsatellitesS hake-­‐upsatellitesB inding 	  energy	  (eV)780 790 800 810Intensity	  (a.u.)A s -­‐depos itedPolarized	  in0.1M	  H2S O4S hake-­‐upsatellitesC o2p3/2C o2p1/2S hake-­‐upsatellites(a) (b) 	  	    100 Moreover, it can also be clearly seen that there is a significant increase in the ratio of cobalt oxides present for Co 25 wt%Fe alloy coatings after polarization as compared to the as-deposited coatings while the ratio of cobalt oxides for both as-deposited and polarized Co remain almost unchanged. Similar species were also reported in the literature for polarized nanocrystalline Co in H2SO4 solution (Jung and Alfantazi, 2006). Meanwhile in figure 6.9, O 1s peaks for samples polarized in 0.1 M H2SO4  were composed of three peaks at  530.1 ± 0.2 eV, 531.4 ± 0.2 eV and 532.5 ± 0.2 eV which corresponded to O2- ions, OH⁻ ions and H2O, respectively (Tsutsumi et al., 2010). On the other hand, iron peaks were only detected from the CoFe alloy coatings.   Figure 6. 9 Measured high resolution spectra of O 1s of the Co 25wt%Fe alloy coating after polarization in 0.1 M H2SO4 	  	    101 The high resolution XPS spectrum of Fe 2p is shown in figure 6.10. One strong peak was observed at a binding energy of 713.1 ± 0.1 eV and another peak at 726 eV ± 0.1 eV. The peaks was found to correspond to Fe 3+ (McIntyre and Zetaruk, 1977). It is interesting to note that the XPS spectrum of Fe was not detected from the corrosion film of Co 5 wt%Fe coating. This is probably attributed to the dissolution of the small amount of Fe in the alloy coating in the acidic medium. However, Fe peaks were detected in CoFe coatings with higher Fe contents of 11wt% and 25wt% Fe. Table 6.4 shows the atomic concentration ratio obtained from the XPS survey spectrum.   Figure 6. 10 Measured high resolution spectra of Fe 2p of the Co 25wt%Fe alloy coating after polarization in 0.1 M H2SO4 B inding 	  energy	  (eV)710 715 720 725 730Intensity	  (a.u.)F e2p3/2F e2p1/2	  	    102 Table 6. 4 Atom concentration ratio (at%) of elements in XPS analysis for as-deposited samples and polarized in 0.1 M H2SO4      As-deposited Polarized in 0.1 M H2SO4 Element Co Co-5Fe (at%) Co-11Fe (at%) Co-25Fe (at%) Co Co-5Fe (at%) Co-11Fe (at%) Co-25Fe (at%) Co 9.12 5.87 6.02 1.51 8.17 10.97 8.06 6.08 Fe - 0.43 0.60 0.82 - - 0.10 0.20 O 23.88 31.00 26.82 13.7 39.52 43.71 44.56 44.62 S 1.01 0.87 0.93 0.22 1.30 0.51 1.03 0.86 	  	    103 6.2.5 Corrosion Morphologies Figure 6.11 presents the SEM images of nanocrystalline Co and CoFe alloy coatings after potentiodynamic polarization in 0.1 M H2SO4. Different morphologies could be seen from both nanocrystalline Co and CoFe alloy deposits. A smoother and more uniform corrosion product is present for nanocrystalline Co (Fig. 6.11a). Meanwhile it can be observed that nanocrystalline CoFe coatings with 5wt% and 11 wt% Fe (Fig. 6.11b and c) exhibit an etch-pit like morphology. Larger and more enhanced pits indicating high anodic dissolution could be observed on CoFe coatings with the highest iron content as shown in figure 6.11d. This indicates a less protective oxide layer was formed on nanocrystalline CoFe alloy coatings surface in 0.1 M H2SO4 solution as compared to pure Co coating. Few studies have also observed similar conditions of reduced corrosion resistance in acidic environments for nanocrystalline Ni-P alloys, Co and Co-P (Rofagha et al., 1993b; Wang et al., 2007; Jung and Alfantazi, 2006b). It is known that in nanostructured materials, the grain boundaries are usually sites for preferential attack when in contact with a corrosive environment. The high volume fraction of grain boundaries in nanocrystalline materials could form large numbers of micro-electrochemical cells during the electrochemical process and provide more active sites to participate in corrosion reaction. Thus, the decline in corrosion resistance of nanocrystalline CoFe alloy coatings was attributed to the higher kinetics of the anodic dissolution on active sites. Since the density of grain boundaries is very high in nanocrystalline CoFe alloy, the electrochemical process is accelerated hence resulting in severe corrosion condition of the coating surface.  	  	    104  Figure 6. 11 SEM morphologies of the CoFe alloy coatings after potentiodynamic polarization test in deaerated 0.1 M H2SO4 (a) Co, (b) Co 5wt%Fe, (c) Co 11wt%Fe, (d) Co 25wt%Fe   	  	    105 6.3 Alkaline Condition 6.3.1 Open Circuit Potential Measurements in 0.1 M NaOH The open circuit potential measurements versus time for all Co and CoFe alloy coatings are shown in figure 6.12. The OCP variations in 0.1 M NaOH show similar characteristics for all samples. Co and CoFe alloy (5 wt%Fe and 25 wt%Fe) display an instantaneous shift of potential toward positive values, which most probably corresponds to the growth of a surface film on the coatings. However, for Co 11 wt%Fe the displacement of OCP towards more noble values was not immediate as compared to the other deposits but started to shift and stabilize only after a few minutes.  Figure 6. 12 Eoc time behaviour of Co and CoFe alloys with various  iron content in 0.1 M NaOH 	  	    106 6.3.2 Potentiodynamic Polarization in 0.1 M NaOH  Polarization tests were carried out on both Co and CoFe alloy coatings in the alkaline medium 0.1 M NaOH (pH 13). The scan was started from a cathodic potential range and continued in the positive direction. The potential was swept up to 1 x 103 mVSCE in order to make sure the passive region is past. Figure 6.13 shows a typical potentiodynamic polarization curve obtained from nanocrystalline Co and CoFe alloy deposits in deaerated 0.1 M NaOH with a scan rate of 1 mVs-1 at room temperature.   Figure 6. 13 Potentiodynamic polarization curves of Co and CoFe with various iron content in deaerated 0.1 M NaOH with a scan rate of 1 mVs-1 i	  (mA	  cm-­‐2)10-­‐3 10-­‐2 10-­‐1 100 101 102E	  (mV	  vs	  SCE)-­‐1500-­‐1000-­‐500050010001500Co-5FeCo-11FeCo-25FeCo	  	    107 It can be observed that the curves of all samples have generally similar behavior regardless of the iron content. A typical active-passive-transpassive behavior is obtained for all tested samples where two stages of passivation could be observed at two different electrode potentials of -700 mVSCE and 200 mVSCE, respectively. Similar corrosion behavior for Co coatings have also been observed (Jung and Alfantazi, 2010; Ives et al., 1962). The appearance of two passivation stages has been reported to be due to the formation of a duplex passive film. The duplex passivation film formed was determined to mainly consist of Co (OH)2 on the primary passivation stage while a rather complex compound of Co3O4/Co2O3 was identified on the second passivation stage. Although Fe was also present in the alloy coatings, the small amount as compared to Co content did not exhibit any obvious effect on the passivation behavior.  Table 6.5 summarizes the corrosion potential and corrosion current density obtained from the extrapolated cathodic and anodic Tafel line. It can be seen that with an increase in iron content the corrosion potential (Ecorr) moved to a more negative potential from -1080 to -1340 mVSCE. Furthermore, the corrosion current density (icorr) was also seen to decrease from 6.44 x 10-3 to 3.73 x 10-3 mA cm-2 with an increase in Fe content of the alloy coatings.  This indicates that reduction in grain size with an increase in Fe content have a significant influence on the passivation behavior of nanocrystalline CoFe coatings in alkaline solution. It is possible that a dense and homogenoues oxide layer was formed on the alloy deposits that protected further corrosion reaction. Similar observations of improved corrosion behavior for nanocrystalline materials in alkaline solutions have been reported in several 	  	    108 other studies (Youssef et al., 2004; Shriram et al., 2000; Inturi and Szklarska, 1992). This is due to the high density of nucleation sites present in the CoFe alloy coatings which lead to a rapid formation of a protective passive layer. The formation of a stable passive film increases the difficulty of ions or electrons moving toward the surface to participate in the electrochemical reaction. On the other hand, as all corrosion tests were conducted in deaerated condition, the only possible cathodic reaction is the evolution of hydrogen from water reduction as equation (6.4): 2H2O + 2e -  2OH - + H2    (6.4) Table 6. 5 Potentiodynamic polarization data of CoFe alloys in 0.1 M NaOH Coating Ecorr (mV) βa (mVdec-1) βc(mVdec-1) Icorr (mA cm-2) Co -1010 53 82 0.00883 Co-5Fe -1080 196 92 0.00644 Co-11Fe -1220 118 93 0.00637 Co-25Fe -1340 147 115 0.00373 	  	    109 6.3.2 Electrochemical Impedance Spectroscopy (EIS) in 0.1 M NaOH  The Nyquist plots for pure Co and the alloy coatings deposited with different iron contents of 5 wt%, 11 wt% and 25 wt% Fe are shown in figure 6.14 for the alkaline solution at -300 mVSCE for 10 minutes. The investigated electrodes display different frequency responses under the same conditions. The spectra observed exhibited a partially resolved semicircle at high frequencies.  Figure 6.15 shows the equivalent circuit proposed for the electrochemical impedance response. It consists of a working electrode (WE), solution resistance (Rs) in series with a double layer capacitance (Qdl), charge transfer resistance (Rct), capacitance of surface film (QF) and the surface film resistance (RF). Table 6.6 represents the impedance data for Co and CoFe alloy coatings.  The solution resistance (Rs) was nearly identical in all cases due to the similar bath chemistry and cell configuration. The surface film resistance (RF) for the pure Co and CoFe alloy coatings with lower iron content did not have much difference although for the highest iron content  alloy the resistance of passive film was significantly high. The same trend could also be observed for the charge transfer resistance (Rct) where Co 25wt%Fe results in higher Rct as compared to the other samples. It is obvious that the charge-transfer impedance increased with the increase in iron content of the deposits. This indicates that reduction in grain size with high iron content in Co 25 wt%Fe deposit results in higher corrosion resistance as compared to the other coatings. 	  	    110  Figure 6. 14 Nyquist plots for nano Co and CoFe with various iron content in deaerated 0.1 M NaOH  Figure 6. 15 Equivalent circuit proposed for the electrochemical impedance response of Co and CoFe alloy coatings in 0.1 M NaOH  	  	    111 Table 6. 6 EIS data obtained by equivalent circuit simulation in 0.1 M NaOH Coating Rs (Ω cm2) Qdl-Yo  (F cm-2) ndl Rct (Ω cm2) Qct-Yo (F cm-2) nct Rf  (Ω cm2) Co 13.37 0.0004763 0.9 67.78 0.0007561 0.9 1899 Co-5Fe 11.32 0.0001676 0.8 71.81 0.0002919 0.7 1880 Co-11Fe 10.21 0.0006391 0.9 81.01 0.0001929 0.8 1856 Co-25Fe 9.58 0.0002812 0.8 98.93 0.0001065 0.8 2923  6.3.3 X-ray Photoelectron Spectroscopy Analysis  The passive film on Co and CoFe alloy coatings were further analyzed by XPS surface analysis. Figure 6.16 shows the XPS survey spectrum of CoFe alloy coatings after being polarized in NaOH solution which consist of cobalt, oxygen, carbon, sulphur and iron for the alloy coatings. The XPS spectra for both Co (not shown) and CoFe alloy coating samples are found to be similar. Table 6.7 shows the atomic concentration ratio obtained from the XPS survey spectrum.  	  	    112  Figure 6. 16 XPS spectra (full range) of nano Co and CoFe with various iron content in 0.1 M NaOH The high resolution XPS spectra of Co 2p for Co and CoFe alloy are shown in figures 6.17a and b, respectively. Similar to the peaks observed from samples polarized in 0.1 M H2SO4, the binding energies of Co doublet peaks of all the samples were around 781.3 ± 0.1 eV and 798.0 ± 0.1 eV, which represented the Co(OH)2 2p3/2 and Co(OH)2 2p1/2 , respectively (Tan et al., 1991). Binding energy (eV)02004006008001000Intensity (a.u.)Co 2sCo 2p1/2Co 2p3/2O KLLO1sCo LMMC 1sCo 3sCo 3pCo-25FeCo-11FeCo-5FeCoFe 2p 3/2	  	    113 Table 6. 7 Atom concentration ratio (at.%) of elements in XPS analysis for as-deposited samples and polarized in 0.1 M NaOH  As-deposited Polarized in 0.1 M NaOH Element Co Co-5Fe (at%) Co-11Fe (at%) Co-25Fe (at%) Co Co-5Fe (at%) Co-11Fe (at%) Co-25Fe (at%) Co 9.12 5.87 6.02 1.51 9.67 6.92 9.23 5.18 Fe - 0.43 0.60 0.82 - 0.53 0.54 0.57 O 23.88 31.00 26.82 13.7 44.21 37.70 43.97 39.63 S 1.01 0.87 0.93 0.22 - 1.05 1.10 0.85   	  	    114     Figure 6. 17 Comparison of Co 2p peaks of  (a) as-deposited Co and Co after polarization in 0.1 M NaOH and (b) as-deposited  and Co 25wt% Fe after polarization  in 0.1 M NaOH B inding 	  energy	  (eV)770 780 790 800 810Intensity	  (a.u.)P olarized	  in0.1M	  NaOHAs -­‐depos itedC o2p3/2S hake-­‐upsatellitesC o2p1/2 S hake-­‐upsatellitesB inding 	  energy	  (eV)780 790 800 810Intensity	  (a.u.)P olarized	  in0.1M	  NaOHAs -­‐depos itedS hake-­‐up	  satellitesC o2p1/2S hake-­‐up	  satellitesC o2p3/2     (b) (a) 	  	    115 However, two other smaller peaks known as the shake-up satellite peaks were detected at binding energies of 787.4 and 803.3 eV. Furthermore, it is interesting to note that the intensity increased significantly after all samples were polarized.  Accordingly, the XPS spectra of O 1s peak of oxygen were fitted with three peaks at a binding energy of 530.1 ± 0.2 eV corresponding to O2-, 531.4 ± 0.2 eV corresponding to OH⁻ and 532.5 ± 0.2 eV corresponding to H2O, respectively as shown in Figure 6.18 (Tsutsumi et al., 2010). The water peak is from the surface contamination and the oxygen peak corresponds to oxygen in Co(OH)2. Results are similar to the XPS peaks observed for coatings studied in acidic solution as discussed earlier.   Figure 6. 18 Measured high resolution spectra of O 1s after polarization in 0.1 M NaOH B inding 	  energy	  (eV)526 528 530 532 534 536 538Intensity	  (a.u.)O1sH2OOH-­‐O2-­‐	  	    116 On the other hand, the high resolution XPS spectrum of Fe 2p is shown in figure 6.19. The spectrum shows two strong peaks of Fe 2p3/2 and Fe 2p1/2 at a binding energy of 713.1 eV and 727.3 eV, respectively which was found to correspond to the Fe3+ ion (McIntyre et al., 1977). The structure splitting of these two components of 14.2 eV is indicative of Fe 3+ ion that most probably originate from the formation of Fe2O3 on the surface of polarized coatings due to oxidation.  Figure 6. 19 Measured high resolution spectra of Fe 2p of the Co 25wt%Fe alloy coating after polarization in 0.1 M NaOH B inding 	  energy	  (eV)705 710 715 720 725 730Intensity	  (a.u.)F e2p3/2F e2p1/2	  	    117 6.3.4 Corrosion Morphologies   The SEM surface morphologies for all samples after potentiodynamic polarization in deaerated 0.1 M NaOH is shown in figure 6.20. It can be observed that all samples did not exhibit any extensive surface dissolution as was seen from the surface of the corroded samples in acidic solution. The morphologies for both nanocrystalline Co and CoFe appeared to be almost similar. However, finer grains were present in coatings that were alloyed with Fe where an increase in Fe content decreased the grain size of the CoFe alloy deposits. A more dense and smooth surface could be observed on all CoFe alloy coatings where almost no corrosion pits were present. This could be the result of a more protective passive film formed on CoFe coatings as compared to pure Co coatings. The effect of reducing the grain size of materials in the nanocrystalline range has resulted in enhanced corrosion resistance behavior as reported by Wang et al. (2007a) and Luo et al. (2010).  In the present study, this effect could also be obviously seen from the potentiodynamic polarization test and was further confirmed by EIS and XPS as well as the surface morphology of the coatings after electrochemical testing. Reduced grain size due to iron alloying in CoFe coatings promotes the formation of better passive film resulting in improved corrosion resistance. Furthermore, it is known that passivation is prone to occur on surface nanocrystalline lattice defects (Balyanov et al., 2004). Nanocrystalline CoFe with finer grain sizes as compared to nanorystalline Co is associated with a high volume fraction of intergranular defects. Hence, when nanocrystalline CoFe coatings come into contact with the aggressive alkaline solution, the rapid coverage of the passive film on the 	  	    118 alloy coating surface largely lowers the effect  of ion diffusion and thus, enhances the resistance to corrosive solution. Therefore, it can be concluded that ultra fine grain size of CoFe alloy coatings enhances the corrosion resistance in highly alkaline solution.   Figure 6. 20 SEM morphologies of the CoFe alloy coatings after potentiodynamic polarization test in deaerated 0.1 M NaOH (a) Co, (b) Co 5wt% Fe, (c) Co 11wt% Fe, (d) Co 25 wt% Fe   	  	    119 6.4 Summary Nanocrystalline Co and CoFe alloy coatings with grain sizes ranging from 15 – 60 nm were synthesized by electrodeposition in a sulphate-based solution. The corrosion behavior of nanocrystalline Co and CoFe alloy coatings have been investigated using Tafel plots in both highly acidic (pH 1) and alkaline (pH 13) solutions. Both nanocrystalline Co and CoFe alloy coatings exhibit higher anodic corrosion rates in 0.1 M H2SO4 as compared  to their electorchemical behaviour in alkaline environment. In deaerated acidic solution, both nanocrystalline Co and CoFe alloy coatings exhibit only active anodic dissolution without any transition to passivation. The high volume fraction of grain boundaries for these nanocrystalline deposits due to decrease in grain sizes serve as active surface sites for corrosion reactions to happen which results in a decrease of the corrosion resistance in low pH solution. Both nanocrystalline Co and CoFe display a typical active-passive-transpassive behavior with two passivation stages observed in 0.1 M NaOH. Iron alloying did not have any significant effect on the corrosion performance of the alloy coating in the alkaline solution. The two stages of passivation present was due to the formation of a duplex passive film.   	  	    120 CHAPTER 7 7	   An	  EQCM	  Study	  on	  the	  Influence	  of	  Saccharin	  on	  the	  Corrosion	  Properties	  of	  Nanostructured	  Cobalt	  and	  Cobalt-­‐Iron	  Alloy	  Coatings	  Saccharin is often added to the electrolyte as a grain refiner and brightener during the electroplating process. In the electrodeposition system, the addition of saccharin alters the properties of the metal deposits by changing the electrode kinetics of the deposited surface. Saccharin adsorbs on the deposited surface forming a condensed phase where the density and coverage of adsorbed saccharin depends on the potential of electrode surface and the concentration inside the solution (Buessherman, 1994; Kwon and Gewirth, 2007). Several studies have been carried out to determine the effect of the addition of saccharin as an additive during the deposition process on the characteristics and electrochemical behaviour of Co and CoFe alloy coatings. However, at present the exact contribution of sulphur resulting from the use of saccharin on the corrosion behaviour of electrodeposits is still unknown. Hence, in this study the EQCM coupled with cyclic voltammetry (CV) was employed to investigate the corrosion behavior of nanostructured Co and CoFe alloy coatings electrodeposited from a sulphate bath with the absence and presence of saccharin. The mass variation on the surface electrode during the corrosion of these electrodeposits in 0.1 M H2SO4 and 0.1 M NaOH was determined.  	  	    121 7.1 Characterization of Nanostructured Co and CoFe Alloy Coatings The SEM images in figures 7.1a-d show the effect of different saccharin concentrations on the morphology and grain size of electrodeposited nanocrystalline Co. It was observed that with the presence of saccharin in the plating solution, the morphology for all coatings changed considerably. Coarse and dark coatings with large grain sizes were obtained for Co deposits without any addition of saccharin into the electrolyte as illustrated in figure 7.1a. The use of additives significantly changed the morphology towards a smoother, bright and more uniform surface with finer spherical shape grains (figures 7.1b-d). However, it was observed that the grain sizes did not significantly decrease further for Co deposits obtained from the electrolyte with the highest saccharin concentration (figure 7.1d). This may be due to the leveling off in the overpotential or saturation of adsorption site with high concentration of saccharin thus inhibiting further grain refinement. Figure 7.2a shows a rough and inhomogeneous surface morphology for nanostructured CoFe produced from electrolytes without additives. In addition, very brittle coatings were obtained due to the high internal stress.  However, an obvious change was seen once saccharin was added into in the electrolyte where fine, smooth and dense surface coatings were obtained (figures 7.2b-d). A sequential reduction of the grain sizes appeared with the increase in saccharin concentration. 	  	    122  Figure 7. 1 SEM images of Co deposited in plating solutions with and without saccharin (a) no saccharin, (b) 1 g L-1 saccharin, (c) 3 g L-1 saccharin, and (d) 7 g L-1 saccharin In order to confirm the presence of saccharin adsorption on the deposit surface, EDS was employed. The EDS revealed the presence of cobalt, carbon, oxygen and iron (present in CoFe alloy coating) on all deposits. The chemical composition in wt% of the deposited CoFe alloy coatings is shown in table 7.1.    	  	    123 Table 7.1 Chemical composition of deposited CoFe alloy coatings in wt%     Oxygen is believed to result from the surface oxidation while sulphur originated from the addition of saccharin. As mentioned earlier, the sulphur atom is reported to be an integral part of the saccharin molecule and is often use as a tracer element to verify their existence on deposited coatings (Brankovic, 2012). It was found that no sulphur was detected on the deposits produced from the saccharin free solution. Element Co Fe S O C Content 65.53 3.02 0.83 1.91 6.71 	  	    124  Figure 7. 2 SEM images of CoFe deposited in plating solutions with and without saccharin (a) no saccharin, (b) 1 g L-1 saccharin, (c) 3 g L-1 saccharin, and (d) 7 g L-1 g saccharin The XRD patterns of nanostructured Co deposits obtained with different saccharin concentrations are shown in figure 7.3a. The measured peaks for all Co deposits exhibit a hexagonal close-packed (HCP) lattice. The position of the diffraction peak remains constant indicating that the lattice parameters are unchanged with different saccharin concentration. However, the XRD patterns show a considerable peak broadening (figure 7.3b) with an increase in saccharin concentration that could be indicative of grain refining. Several studies have also reported similar peak broadening with the use of saccharin as an additive in their plating solutions (Rashidi and Amadeh, 2010).  	  	    125     (a) (b) 	  	    126     Figure 7. 3 XRD patterns of (a) Co, (b) magnified XRD patterns of Co, (c) CoFe and (d) magnified XRD patterns of CoFe with various saccharin concentrations  (d) (c) 	  	    127 In figures 7.3c and 7.3d, illustrate the XRD patterns and magnified peaks of nanostructured CoFe alloy coatings deposited with different saccharin concentrations, respectively. A different crystal structure was obtained when Co deposits were alloyed with iron (Fe). All deposits were seen to have similar crystallographic structure at the same peak positions of about 43° corresponding to CoFe (110) of body cubic centered (BCC) structure. Furthermore, similar peak broadening were observed indicating a systematic decrease in the crystallite size upon increasing the concentration of additive saccharin. The grain sizes were calculated using the Full-Width at Half Maximum (FWHM) of the most intense diffraction peak of Co and CoFe peaks using Scherrer’s equation (Srivastava et al., 2007; Shirani et al., 2012). The grain sizes calculated from the X-ray patterns are shown in table 7.2.   Table 7.2 Variation of grain size of Co and CoFe with different saccharin concentrations Saccharin concentration  (g L-1) Grain Size (nm) Co CoFe 0 60 51 	  	    128 1 36 31 3 28 22 7 26 19  The estimated grain size of the deposits ranges from 60 to 26 nm and 51 to 19 nm for Co and CoFe alloy coatings, respectively. This is in agreement with the broadening of the diffraction peaks observed with increased saccharin concentration. The correlation between the amount of saccharin and grain size shows a decreasing inclination implying that saccharin addition has a significant effect in reducing the grain size of all deposits. The ability of saccharin to refine the grain sizes of deposited coatings was due to the effect of increasing the overpotential for Co and CoFe deposition. Saccharin is believed to adsorb onto the substrate surface as a molecular or decomposed as sulphide containing species that slows down the surface diffusion of adions towards the active sites of growth (Wu, 2002). This in return will inhibit grain growth and result in higher nucleation rate thus contributing to the production of finer grains. This broadening effect could also be attributed to the increase of crystalline imperfection such as grain boundary and micro strain as grain size decreases (Li et al., 2004). 	  	    129 7.2 Electrochemical Quartz Crystal Microbalance Study 7.2.1 CV and Mass Variation: Influence of Saccharin Concentration Figure 7.4a shows the cyclic voltammograms of nanostructured Co obtained at a sweep potential rate of 10 mV s-1 from the solution described previously. The CV seems to retain the same shape with the presence of one anodic peak associated with dissolution of Co regardless of the saccharin concentration in the coatings. Similar CV was also reported for cobalt deposition (Montes-Rojas et al., 2007; Martin et al., 2007). However, Matsushima et al., (2006) reported the presence of two anodic current peaks that was associated to the hydrogen evolution reaction that occurs simultaneously with cobalt dissolution. The difference in results obtained could be due to the types of supporting electrolyte used and pH values. It has been reported that the difference in pH alters the deposition of cobalt (Lafouresse et al., 2007). At lower pH (< 3.2) values only Co metal deposits, while at higher pH (> 3.2), Co hydroxide deposits first followed by Co metal. In the present study the pH of the plating solution was 2.5, and hence, the deposition of only Co metal occurred. Furthermore, the anodic peaks were moved to more negative potentials for coatings plated in the presence of saccharin in the plating bath. The anodic peak for Co without any saccharin was at a potential of 0.1 VSCE. Addition of 1 g L-1 and 3 g L-1 saccharin into the electrolyte significantly moved the anodic peaks to a more negative potential of -0.9 VSCE and -0.7 VSCE, respectively. Interestingly, a very small anodic peak was observed for Co with the highest saccharin concentration. It is also obvious that a large decrease in the 	  	    130 anodic peak current density is observed with increased saccharin concentration in the solution. This is indicative of the saccharin interaction with the cobalt deposit. The anodic charge has also been calculated according to the area under the peaks and was found to be 52.5 mC, 21.5 mC, 9.5 mC and 4.5 mC for coatings plated in the presence of 0, 1, 3 and 7 g L-1, respectively. It shows that those with more negative peak current and higher saccharin concentration in the plating solution correspond to less charge. Therefore, the negative shift can be related to smaller amounts of available metal to be dissolved.  This can be seen more clearly from the direct mass variation corresponding to the voltammograms in figure 7.4 b. The change in mass shows a clear correlation of the current evolution on the voltammograms. The cobalt reduction begins at -1.5 V vs SCE where there is a mass increase that indicates cobalt deposition on the electrode. The cobalt reduction continues until -0.6 V vs SCE. The oxidation process begins at -0.6 V vs SCE where the mass decreases until it becomes constant at the end of the potential sweep. A big difference in mass loss of about 20 µg cm-2 could be observed between the mass of Co deposited without and with the highest amount of saccharin concentration present in the bath.   	  	    131   	  	    132   Figure 7. 4 Cyclic voltammogram of Co (a) and CoFe (c) and mass evolution curves corresponding to the cyclic voltammetry of Co (b) and CoFe (d) with different saccharin concentrations. Scan rate: 10 mV s-1.  	  	    133 Similar CV shapes were also obtained for nanostructured CoFe alloy coatings deposited with various saccharin concentrations as shown in figure 7.4c.  Anodic peaks were also seen to move towards more negative potentials from 0 V vs SCE to -0.7 V vs SCE for CoFe alloy coatings without and with 7 g L-1 saccharin concentration. The anodic charge was also calculated according to the area under the peaks and was found to be 77.5 mC, 68.5 mC, 48.5 mC and 35.5 mC for coatings plated in the presence of 0, 1, 3 and 7 g L-1, respectively. This corresponds to similar observations previously described for Co coatings, where less charge meant less dissolution of Co from the electrode surface.  A significant difference also appeared in the anodic current density peak between coatings deposited with and without the additive. An anodic current density of 2.25 mA cm-2 was reached for the saccharin added alloy coating. This difference in comparison with the Co deposits is due to the oxidation reaction of adsorbed sulphur on the surface with the co-deposition phenomena of the alloy coatings. In figure 7. 4d the direct mass variation corresponding to the voltammogram in figure 7.4c is presented.  The continuous mass increase shows the thickening of the CoFe deposit starting at the deposition potential of -1.5 V vs SCE until it reaches a maximum mass of about 22 ug cm-2 for CoFe deposited from solution without any additive. The mass then starts to decrease during the oxidation of the deposited coating at potential -0.6 V vs SCE until no mass changes were observed at the end of the potential sweep. The mass of the deposits was seen to further decrease with increased concentration of saccharin. It has been reported that the adsorptive capacity of saccharin increases with saccharin concentration on the electrode surface thus, preventing 	  	    134 the deposition of metal ions (Kim et al., 2005). Saccharin has also been stated to suppress the reduction of copper (Xinwei and Weixing, 2008). In the present case, the decrease in deposited mass of the coatings with higher saccharin concentration in the plating bath could be explained through similar mechanisms. The presence of this additive impedes the ability of the Co and Fe ions to be reduced on the electrode and hence decreases the mass gain. For cobalt and iron dissolution, several studies (Bockris et al., 1968; Zech et al., 1999; Sasaki and Talbot, 2000) have proposed a mechanism that consisted of one electron steps with an adsorbed intermediate: Co2+ + OH- + e- ⇄  CoOHads   (7.1) CoOHads + H+ + e- ⇄ Cos + H2O    (7.2) Fe2+ + OH- + e- ⇄ FeOHads   (7.3) FeOHads + H+ + e- ⇄ Fes + H2O   (7.4) It can be seen that addition of saccharin influences the anodic current density peak for all deposits. Higher concentrations of saccharin in the electrolyte bath results in a decrease of the current density that could be attributed to the adsorption of the additives on the cathode surface whereby it lowers the rate of the electron transfer reaction. This is possible based on the electrostatic cation adsorption effect which shows that in the presence of an adsorbate, the double layer changes its structure and decreases the rate of electrochemical reaction. 	  	    135 Furthermore, the presence of saccharin does not affect the co-depositing of these species. The following reaction was proposed in addition to reactions (7.1) - (7.4) (Kim et al., 2005): Fe2+ + Co2+ + OH- + e- ⇄ (FeCoOH)2+ ads  (7.5) (FeCoOH)2+ads + H+ + e- ⇄ Fes + Co 2+ + H2O  (7.6)   7.2.2 Mass and Potential Changes: Influence of Saccharin  Figures 7.5a and b present the evolution of mass and electrode potential versus time for deposited nanostructured Co coatings with and without saccharin in 0.1 M H2SO4 during open circuit immersion. From immersion in the acidic solution, Co deposited from absence of saccharin in the plating solution showed a very small mass decrease for the first 600 s and then it decreased almost linearly beyond that point (figure 7.5a). However, the slope of ∆m with respect to time was much steeper in coatings with presence of saccharin. This could be related to the rapid Co dissolution from the surface electrode into the electrolyte. For low saccharin concentrations (1 and 3 g L-1), a substantial mass decrease of about 20 to 40 µg cm-2 was observed immediately after the immersion of the deposits.  	  	    136   Figure 7. 5 (a) Mass and (b) potential changes of nanostructured Co measured by EQCM in 0.1 M H2SO4   	  	    137 The greatest mass loss was obtained for deposits produced from the highest amount of saccharin concentration in the plating bath. The grain sizes among the deposits made in the presence of saccharin were not much difference from each other as mentioned earlier, therefore the huge difference in mass loss is most probably due to the amount of incorporated sulphur in the coating surface which acts as a catalyst in increasing the dissolution of Co. The Ecorr was also moved to more negative potentials with higher saccharin concentrations in the Co coating (figure 7.5b). This could also be an indication that presence of sulphur promotes the corrosion reaction of Co deposits in the acidic medium. In figures 7.6a and b the mass change as well as the electrode potential for nanostructured CoFe alloy coatings immersed in 0.1 M H2SO4 during open circuit is shown. Immediately after immersion of the electrodes in the acidic solution, the mass decreased rapidly for all alloy coatings. The accelerated mass loss could be due to the active dissolution of Fe into the solution. In addition, it has also been reported that the adsorbed S on the coatings surface accelerates the anodic dissolution of the metals comparing to S free coatings. This behavior was probably due to the weakening of the metal-metal bonds induced by adsorbed S, which leads to a lowering of the activation energy barrier for the passage of metal atom from the surface to the solution (Oudar and Marcus, 1979; Marcus and Oudar, 1985). Hence in this case, the adsorbed S accelerates the dissolution of iron from the electrode into the electrolyte resulting in the rapid mass loss for all samples. Thus, it could be stated that an increase of sulphur results in higher corrosion susceptibility for Co and CoFe alloy 	  	    138 coatings in 0.1 M H2SO4. The rate of mass loss or mass gain, ∆dm/dt was also evaluated in the time range of one hour by the linear regression where the results are presented in table 7.3.  In contrast, a mass increase was observed for immersion of both Co and CoFe alloy coatings in 0.1 M NaOH. From figure 7.7a the mass increase obtained from coatings without the additive was the least as compared to coatings deposited with presence of saccharin. The highest mass increase (of almost 5 µg cm-2) was obtained from Co deposits with 3 g L-1 saccharin concentration. The mass gain may be assumed to be the formation of the cobalt oxide layer. The participation of sulphur on the surface coating markedly accelerates the formation of this oxide/passive layer resulting in the mass increase of all Co deposits containing saccharin. The shift towards more positive potential (figure 7.7b) is also in agreement with the massogram results which suggest that a thick protective film is formed on the electrode surface resulting in improved corrosion resistance.  	  	    139   Figure 7. 6 (a) Mass and (b) potential changes of nanostructured CoFe measured by EQCM in 0.1 M H2SO4  	  	    140 Table 7. 3 Rate of mass change of immersion test in the absence and presence of saccharin at different concentrations in 0.1 M H2SO4 and 0.1 M NaOH Coating Saccharin  (g L-1) 0.1 M H2SO4  0.1 M NaOH ∆dm/dt (µg cm-2 s-1) ∆dm/dt (µg cm-2 s-1) Co 0 3.31  10-3 0.12  10-3 1 12.51  10-3 0.19  10-3 3 20.00  10-3 0.22  10-3 7 24.42  10-3 0.27  10-3 CoFe 0 16.63  10-3 0.48  10-3 1 18.80  10-3 0.13  10-3 3 21.10  10-3 0.09  10-3 7 23.34   10-3 0.04  10-3 	  	    141   Figure 7. 7 (a) Mass and (b) potential changes of nanostructured Co measured by EQCM in 0.1 M NaOH 	  	    142   Figure 7. 8 (a) Mass and (b) potential changes of nanostructured CoFe measured by EQCM in 0.1 M NaOH  	  	    143 Interestingly, opposite effects of the additives on the mass gain was observed for CoFe alloy coatings. The highest mass gain was obtained from the alloy coating with no additive present while the highest saccharin concentration gave the lowest mass increase (figure 7.8a). The change in potential was similar for all CoFe alloy coatings (figure 7.8b). Immediately after immersion of the electrode, the open circuit potential shifted steeply towards the cathodic direction suggesting the dissolution of the native oxide layer. Beyond the first few hours of immersion, the open circuit potential remained practically constant, and ranged between -0.81 and -0.83 V vs. SCE. This implies that presence of saccharin in the alloy deposits did not contribute to the formation of a passive/oxide layer on the surface coating.  7.2.3 Electrochemical Polarization Measurements  Figures 7.9a and b represents the potentiodynamic polarization curves recorded in 0.1 M H2SO4 for Co and CoFe alloy coatings deposited in the absence and presence of various saccharin concentrations, respectively. The potentiodynamic polarization measurements were performed from −0.25 V vs. OCP to 0.8 V vs. SCE with a scan rate of 0.167 mV s-1.  The corrosion potential and corrosion current densities of all deposits obtained are summarized in table 7.4. In the cathodic potential range, Co coatings moved slightly to lower potentials with the increase of saccharin concentration (figure 7.9a) while no 	  	    144 significant changes could be observed for CoFe alloy coatings (figure7.9b). On the other hand, in the anodic potential range all deposits exhibited only active behavior without any transition to passivation up to 0.8 V vs SCE. Ecorr values shifted towards more negative potential with an increase of saccharin for the Co deposits. The noblest coating surface with Ecorr value of -0.4 V vs SCE was observed from the deposits that were electrodeposited from the saccharin free solution. Meanwhile, Ecorr values for CoFe alloy coatings ranges from -491 to -501 mV vs. SCE. Although the difference in Ecorr values for the deposits is rather small, the addition of saccharin still shows an effect on the corrosion behavior of these coatings. This indicates that deposits obtained from an electrolyte containing saccharin as the additive are more susceptible to corrosion. The results here are in agreement with the EQCM where saccharin has shown to have a distinctive effect on the corrosion behavior of both Co and CoFe alloy deposits.    	  	    145     Figure 7. 9 Potentiodynamic polarization of nanostructured (a) Co and (b) CoFe in 0.1 M H2SO4. Scan rate: 0.167 mV s-1 	  	    146 Table 7. 4 Corrosion potential and corrosion current density of Co and CoFe with different saccharin concentrations obtained from potentiodynamic polarization curves in 0.1 M H2SO4 and 0.1 M NaOH Solution Sample Corrosion potential (mV vs SCE) Corrosion current density (µA cm-2) 0.1 M H2SO4 Co  +  0 g L-1 sacc -478 155.2 Co  +  1 g L-1 sacc -426 143.8 Co  +  3 g L-1 sacc -416 120.3 Co  +  7 g L-1 sacc -476 167.5 CoFe + 0 g L-1 sacc -496 1105.0 CoFe + 1 g L-1 sacc -491 1345.0 CoFe + 3 g L-1 sacc -501 1358.1 CoFe + 7 g L-1 sacc -481 1384.4 	  	    147 Solution Sample Corrosion potential (mV vs SCE) Corrosion current density (µA cm-2) 0.1 M NaOH Co  +  0 g L-1 sacc -600 12.4 Co  +  1 g L-1 sacc -619 5.1 Co  +  3 g L-1 sacc -835 5.2 Co  + 7 g L-1 sacc -781 4.2 CoFe + 0 g L-1 sacc -629 44.0 CoFe + 1 g L-1 sacc -583 38.0 CoFe + 3 g L-1 sacc -801 23.7 CoFe + 7 g L-1 sacc -803 20.8    	  	    148 On the other hand, a different trend was observed for the potentiodynamic polarization curves recorded for Co and CoFe alloy coatings deposited in the absence and presence of various saccharin concentrations in 0.1 M NaOH (figure 7.10). The only possible cathodic reaction was the H2 evolution from the water reduction in the cathodic regions since the electrochemical test was conducted in deaerated solution. From the anodic regions, all tested coatings showed an active to passive transition. Clearly, oxide layers have been formed in alkaline solutions reflecting in pseudo passive behavior.  Formation of oxide layer can significantly shift the potential upward. From figure 7.10a it can be observed that the most positive Ecorr value was obtained from deposits with the highest saccharin concentration while the deposit without any saccharin in the plating bath showed the most negative Ecorr value. Furthermore, the passivation current densities decreased gradually as the concentration of saccharin in the deposit is increased. The more positive corrosion potential and lower corrosion current density indicates that presence of sulphur on the Co coating surface promotes further the adsorption of hydroxide ions to form either Co(OH)2 or CoO passive films and hence results in better corrosion performance.  	  	    149     Figure 7. 10 Potentiodynamic polarization of nanostructured (a) Co and (b) CoFe in 0.1 M NaOH Scan rate: 0.167 mV s-1 i	  (A 	  cm-­‐2)10-­‐9 10-­‐8 10-­‐7 10-­‐6 10-­‐5 10-­‐4 10-­‐3 10-­‐2 10-­‐1E	  (V	  vs	  SCE)-­‐1.0-­‐0.8-­‐0.6-­‐0.4-­‐0.20.00.20.40.60.81.00 g sacc1 g sacc3 g sacc7 g sacci	  (A 	  cm-­‐2)10-­‐9 10-­‐8 10-­‐7 10-­‐6 10-­‐5 10-­‐4 10-­‐3 10-­‐2 10-­‐1 100E	  (V	  vs	  SCE)-­‐1.2-­‐1.0-­‐0.8-­‐0.6-­‐0.4-­‐0.20.00.20.40.60.81.00 g sacc1 g sacc3 g sacc7 g sacc  (a)   (b) 	  	    150 Figure 7.10b shows the polarization curves for CoFe alloy coatings in the alkaline solution. However, the passivation current densities were gradually increased as the concentration of saccharin increased, indicating the less protective nature of the passive layers formed on the CoFe alloy coatings. This behavior again points to the effect of adsorbed sulphur on the surface alloy coating that prevents the formation of a thick and protective passive film by blocking primary OH- adsorption which is known as the precursor to passive film formation. Therefore, the presence of sulphur in the CoFe alloy coatings result in a deterioration of passive film resistance by hindering the formation of a stable passive film.   7.2.4 Electrochemical Impedance Spectroscopy Figures 7.11a and b illustrate the Bode phase plots recorded in 0.1 M H2SO4 for Co and CoFe alloy coatings obtained from solutions with different saccharin concentrations, respectively. The impedance magnitude was independent of frequency from 105 to 103 Hz corresponding to the solution resistance between the working and reference electrode for Co deposits in 0.1 M H2SO4.  From the Bode plot (figure 7.11a) it can be observed that only one time constant is present. The Bode phase diagram recorded in 0.1 M H2SO4 for CoFe alloy coatings obtained from solutions with different saccharin concentration is shown in figure 7.11b. In the diagram only one maximum phase angle versus frequency plot was observed, which could be attributed to a double-layer capacitance. This behavior is due to the adsorption reaction of intermediates or surface coverage of sulphur originating from saccharin. Similar behavior has also been reported for iron in acidic solutions (Epelboin and Keddam, 1970; Epelboin and Keddam, 1972). In the case for CoFe deposited 	  	    151 without saccharin, the phase angle value increased significantly as compared to those coatings containing saccharin. Since the only difference between these alloy coatings is the presence of sulphur, this difference may be attributed to changes in electrochemical process due to increased sulphur concentration on the surface. Meanwhile, figures 7.12a and b demonstrates the Bode phase diagrams of Co and CoFe alloy coatings deposited with different saccharin concentrations in 0.1 M NaOH, respectively. In the higher frequency region, the impedance magnitude tends to become constant with the phase angle values falling rapidly with increasing frequency. This is indicative of a typical response of resistive behavior and corresponds to the solution resistance.  The phase angle against frequency curves indicates an inductive like behavior in the low frequency range. Due to certain heterogeneity of the electrode surface the measured capacitive response is not generally ideal hence, a Constant Phase Element (CPE) is introduced for fitting the spectra. The characteristic shape of Bode diagram suggests that the corrosion of both Co and CoFe alloy coatings in 0.1 M H2SO4 is a charge transfer-controlled dissolution reaction.  	  	    152   Figure 7. 11 Bode phase obtained for nanostructured Co (a) and CoFe (b) in 0.1 M H2SO4 after 1 hour immersion at OCP 	  	    153 The EIS spectra fit well to a simple equivalent circuit (figure 7.13a) that consist of an Rct (charge transfer resistance) parallel to a CPE and solution resistance in series (Rs). The CPE consists of capacitance and frequency dispersion, n a dimensionless parameter where a value close to 1 indicates CPE is most capacitive. EIS spectrum for coatings in 0.1 M NaOH presents two time constants which are not well defined. The equivalent circuit for coatings in 0.1 M NaOH is shown in figure 7.13b. In this model, addition of Rads and CPEads is added in series to the simple circuit which is attributed to an adsorbed species that contributes to the formation of the corrosion layer. CPE was used to account for the non-uniform current distribution from the capacitive behavior and the deviation resulting from surface inhomogeneity and to improve quality of the fit. The impedance of CPE is expressed by the following equation (7.7): ZCPE = [Yο (jw) n] -1   (7.7) where ZCPE is the CPE impedance (Ω cm2), Yο is the CPE admittance (Ω-1 sn cm-2) and w is the angular frequency (rad s-1). The parameter n is a constant which shows the degree of deviation from ideal capacitive behavior. When n =1, the CPE describes an ideal capacitor while a decrease in this value indicates that the surface roughness and porosity constitute to the surface behavior of a leaky capacitor (Liu et al., 2003). The values fitted to the equivalent circuit are listed in table 7.5. The typical Chi square values varied from 2.5 × 10−4 to 3.6 × 10−4, indicating a satisfactory fit 	  	    154   Figure 7. 12 Bode phase obtained for nanostructured Co (a) and CoFe (b) in 0.1 M NaOH after 1 hour immersion at OCP 	  	    155 The charge transfer resistance for Co coatings in 0.1 M H2SO4 further decreased with higher saccharin concentration in the deposits. However, charge transfer resistance for CoFe alloy coatings slightly increased with saccharin concentration. This is consistent with the EQCM and polarization results where an increase in corrosion rate was observed with further increase of the additive. The finer grain sizes of deposits with higher saccharin concentration also contributed to the accelerated dissolution of surface coatings leading to higher Rct values.    Figure 7. 13 Equivalent electrical circuit for coatings in (a) 0.1 M H2SO4  and (b) 0.1 M NaOH (b) (a) 	  	    156 Table 7. 5 Equivalent circuit fitting for the EIS data of the nanostructured Co and CoFe deposits  Element Co coating deposited with various saccharin concentrations (g L-1) in 0.1 M H2SO4  0 1 3 7 Rs (Ω cm2) 15.93 19.64 25.66 16.73 Qdl (F cm-2) 0.000923 0.003172 0.000131 0.000958 ndl 0.8843 0.8011 0.9034 0.962 Rct (Ω cm2) 1319 692.7 391.5 208.7 Chi-square (X2) 2.56 х 10-4 2.92 х 10-4 2.80 х 10-4 3.17 х 10-4 Element CoFe coating deposited with various saccharin concentrations (g L-1) in 0.1 M H2SO4  0 1 3 7 Rs (Ω cm2) 26.25 12.51 9.76 14.94 Qdl (F cm-2) 0.000042 0.000131 0.000107 0.000146 ndl 0.91497 0.8654 0.9697 0.9817 Rct (Ω cm2) 814.9 808.9 248.0 137.0 Chi-square (X2) 2.95 х 10-4 3.59 х 10-4 2.92 х 10-4 3.23 х 10-4      	  	    157 Element Co coating deposited with various saccharin concentrations (g L-1) in 0.1 M NaOH  0 1 3 7 Rs (Ω cm2) 14.64 14.05 14.24 14.39 Qdl (F cm-2) 0.007110 0.000772 0.004678 0.002576 nct 0.8100 0.7200 0.7052 0.8548 Rct (Ω cm2) 1047 1804 5870 3999 Qf (F cm-2) 0.0001677 0.0001878 0.00533 0.04501 nfilm 0.8031 0.832 0.7523 0.7321 Rfilm (Ω cm2) 8.559 6.143 4.432 3.097 Chi-square 0.65 х 10-4 0.89 х 10-4 1.32 х 10-4 1.63 х 10-4 Element CoFe coating deposited with various saccharin concentrations (g L-1) in 0.1 M NaOH  0 1 3 7 Rs (Ω cm2) 30.13 29.82 29.60 30.19 Qdl (F cm-2) 0.000505 0.000224 0.000390 0.000696 nct 0.8323 0.8248 0.8617 0.7550 Rct (Ω cm2) 2727 389.2 479.9 9033 Qf (F cm-2) 0.000412 0.000649 0.000677 0.000145 nfilm 0.5701 0.7601 0.8347 0.8625 Rfilm (Ω cm2) 4035 3998 1271 1751 Chi-square 1.71 х 10-4 1.89 х 10-4 1.32 х 10-4 1.63 х 10-4 	  	    158 Meanwhile, the result of the electrical circuit parameters for Co coatings in the alkaline medium indicates that the resistance of the passive layer formed on the surface coatings increased with higher concentration of saccharin. This can be related to the formation of a thick cobalt oxide/hydroxide layer as seen from the significant increase in mass from the EQCM result. On the other hand, for CoFe alloy coatings the value of resistance of the passive film was decreased in the presence of saccharin. This implies that the high sulphur concentration in the alloy coatings did not promote formation of cobalt-iron complexes or CoFe oxides/hydroxides passive film in the alkaline solution and thus a lower corrosion resistance is expected.  7.2.5 X-ray Photoelectron Spectroscopy (XPS) XPS studies were carried out to provide information on the elemental composition and their oxidation states on the surface coatings. The final potential values of the samples analyzed were similar to the potentials used during the potentiodynamic polarization test. The features of the spectra included Co (2p), Fe (2p), S (2p), and O (1s) which was observed for all samples. Carbon peaks were also present which probably corresponds to carbon contamination which is unavoidable in XPS studies. The XPS analyzes for all coatings also did not show the presence of sodium on any deposits. Table 7.6 presents the atomic concentration ratios (at%) obtained from the XPS survey spectra.   	  	    159 Table 7. 6 Surface film composition (at%) of as-deposited and polarized nanostructured CoFe alloy coating Element As-deposited Polarized in 0.1 M H2SO4 Polarized in 0.1 M NaOH Co 60.12 6.08 5.18 O 0.21 44.62 39.63 Fe 0.82 0.20 0.79 S 0.91 0.85 0.86 C 2.51 55.47 38.89     	  	    160 Figures 7.14 a-d shows the high resolution XPS spectra and peaks deconvoluted at Co 2p, O 1s, S 2p and Fe 2p for as-deposited and polarized CoFe samples in acidic and alkaline solutions. The software XPSPEAK was used for curve fitting. Spectral envelopes of Co 2p of as deposited and polarized samples show that Co could be present in different oxidation states. The two main peaks were found in every spectrum and assigned as Co 2p3/2 and Co 2p1/2 peaks and a low intensity satellite between these two main peaks (figure 7.14a). The binding energies of Co 2p doublet peaks of as-deposited and polarized samples were 781.3 ± 0.1 eV and 798.0 ± 0.1 eV. These peaks could be attributed to the Co(OH)2 according to their satellite peak positions at 787.4 and 803.3 eV, respectively (Tan et al., 1991). Similar peaks were also obtained from cobalt in a 0.1 M NaCl (pH 10) (Tabakovic et al., 2006). The other two peaks are seen to be located at binding energies at 780.5 and 796 eV with satellite peaks at 786 and 802.5 eV. The intense satellite peaks which appear at about 5.5 and 6.3 eV after the Co 2p3/2 and Co 2p1/2 peaks have been reported to be attributed to the CoO bond (Wang et al., 2011). In the present case, the measured Co 2p peaks were shifted to higher binding energies than for metallic Co at binding energies of 777.9 eV (Co 2p3/2) and 795.0 eV (Co 2p1/2)  as a result of the oxidation of the metallic Co to form cobalt hydroxide and cobalt oxide (Wagner, 1979).   	  	    161   Binding energy (eV)780 790 800 810Intensityt (a.u.)Co 2p3/2 Shake-upsatelliteCo 2p1/2 Shake-upsatelliteAs-deposited  Polarized in NaOHPolarized in H2SO4Binding energy (eV)526 528 530 532 534 536Intensity (a.u.)As -­‐depos itedPolarized	  in	  NaOHPolarized	  in	  H2S O4O2-OH-H2O (a)  (b) 	  	    162   Figure 7. 14 Deconvoluted XPS spectra for as-deposited and polarized CoFe alloy coating in acidic and alkaline solutions: (a) Co 2p, (b) O 1s, (c) S 2p, and (d) Fe 2p B inding 	  energy	  (eV)166 168 170 172Intensity	  (a.u.)S 2pB inding 	  energy	  (eV)710 715 720 725Intensity	  (a.u.)A s -­‐depos itedPolarized	  in	  NaOHPolarized	  in	  H2S O4Fe 2p3/2 Fe 2p1/2( c ) (d) 	  	    163 The corresponding O 1s spectra is shown in figure 7.14b. It is apparent that oxygen atoms are also present in the coatings in several different chemical states. O 1s peaks for all samples were composed of three contributions at 530.1 ± 0.2 eV corresponding to O2-, 531.4 ± 0.2 eV corresponding to OH- and 532.5 ± 0.2 eV corresponding to H2O, respectively (Tsutsumi et al., 2010). Apart from H2O, the O 1s peak at 532.5 eV can be also attributed to organic oxygen bonded with carbon (Bhargava et al., 2007) or to adsorbed OH species (Grosvenor et al., 2004). The shapes of the spectrum Co 2p, O 1s and S 2p (not shown here) for Co coatings before and after polarization is virtually identical to the alloy coatings. Figure 7.14c shows the S 2p core level spectra for deposits grown. As mentioned before the incorporation of S in these samples occurred without any trace of Na which strongly indicates the dissociation of saccharin and their complexation (Popov et al., 1993; Nakanishi et al., 2001; Lallemand et al., 2004a; Lallemand et al., 2004b). One strong S 2p peak could be observed for as-deposited sample at a binding energy of about 168.9 eV that could be ascribed to the surface S atoms adsorbed as SO2 molecules (Umebayashi et al., 2002; Ohno et al., 2003; Sakthivel et al., 2004).  The high resolution XPS spectrum of Fe 2p for as-deposited and polarized samples in 0.1 M H2SO4 and 0.1 M NaOH is shown in figure 7.14d. The spectrum shows two peaks of Fe 2p3/2 and Fe 2p1/2 at a binding energy of 713.1 ± 0.1 eV and 727.3 eV, respectively which was found to correspond to the Fe3+ (Mills and Sullivan, 2000).  Tabakovic et al. (2006) also reported on the existence of Fe 2p peaks on CoFe magnetic films. It can be observed 	  	    164 that the Fe intensity decreased significantly for polarized samples in 0.1 M H2SO4, as compared to the intensity of polarized samples from alkaline solution. This may be due to the active dissolution of iron into the solution during the polarization experiment. It is known that iron dissolves at a faster rate in acidic solutions as compared to alkaline solutions hence the amount of iron in the alloy coatings were significantly reduced (Schmutz and Landolt, 1999). 7.3 Summary The presence and absence of saccharin as an additive in the electrolyte changed the surface morphology as well as grain size of electrodeposited nanostructured Co and CoFe alloy coatings. The effect of various saccharin concentrations on the corrosion behavior of both nanostructured deposits was analyzed in acidic (0.1 M H2SO4) and alkaline (0.1 M NaOH) solutions. The presence of sulphur on the coating surface resulting from the use of saccharin as an additive in the plating solution accelerates the anodic reaction for all deposits in acidic medium. By employing the EQCM, it was observed that the mass decrease was significant with an increase in saccharin concentration indicating an active dissolution of Co and CoFe coatings into the solution. The opposite corrosion reaction were found for all nanostructured coatings in 0.1 M NaOH. The formation of a passive hydroxide film Co(OH)2 on the surface of the Co coatings was enhanced in the presence of saccharin that was observed through the significant mass gain. On the other hand, the presence of saccharin is observed to hinder the formation of a protective passive film on CoFe alloy coatings which results in the poor corrosion performance in alkaline solution. The anodic 	  	    165 and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results.   	  	    166 CHAPTER 8 8	   Electrochemical	  Behavior	  of	  Electrodeposited	  Nanocrystalline	  CoFe	  Alloy	  Coating	  in	  0.1	  M	  Na2SO4	  With	  Different	  pH	  and	  Sulphide	  Ion	  Concentration	  	  It is known that corrosion occurring on alloys in an aqueous solution is often affected greatly by a number of factors which include pH and the presence of aggressive ions in the surrounding environment. The corrosion of Co alloys are also dependent on the pH solution. Different electrochemical behaviors were observed in low and high pH solutions (Nik Masdek and Alfantazi, 2013). Active dissolution without any passivation was observed in Na2SO4 solutions with pH 3, 6, and 7 (Kim et al., 2003c). On the other hand, mixed observations have been reported for the passivation behavior in alkaline solution (Ismail and Badaway, 2000). This may be due to the variety of thermodynamically-stable cobalt oxides and/or cobalt hydroxides could be formed as predicted from the E–pH diagram of cobalt–water system at 25°C (Jung, 2006). The presence of sulphide ions has also been found to decrease the corrosion resistance of materials (Warraky, 2003; Abd. El Haleem, and  Abd. El Aal, 2008; Fatah  et al., 2011). For example, it has been reported that the corrosion resistance of copper decreased about eight times when sulphide with a concentration of 10 ppm was present in the environment. The corrosion rate of mild steel was also increased up to three times with 0.01 M of sulphide ion concentration in the solution (Hamdy et al., 2007). 	  	    167 This chapter focuses on the electrochemical reaction of the CoFe alloy coating in a sodium sulphate solution with different pH values and the presence of sulphide ions in a neutral electrolyte. Again, the EQCM was employed together with other conventional electrochemical techniques in order to determine the oxidation or reduction of electrodeposited CoFe alloy coatings in solutions with different pH values in the presence of sulphide ions. .  8.1 Effect of pH Solution The free corrosion potential of CoFe alloy coatings was traced for 60 minutes in stagnant 0.1 M Na2SO4 aqueous solutions of different pH covering the acidic, neutral, and basic media as shown in figure 8.1. It can be seen that exposure to the test solution with the pH varying from 5 to 11, caused a change in the potentials of the alloy coatings. The electrode potential in the low pH solution was found to shift towards less negative values and the steady state was reached in less than 10 minutes. Meanwhile, it is obvious that the potential of the electrodes took about 25 minutes before reaching a steady-state in the neutral and basic solution. The potential shift towards more negative values could correspond to the dissolution of native oxide layers. The steady state for the CoFe alloy in basic solutions took a bit longer to reach as compared to the electrode in the low pH solution. This process is partially dependant on the type of alloy and electrolyte composition. The time taken to reach a relatively stable value is related to the activation or the formation of an oxide film on the alloy surface (MacDonald et al., 1993; Tait and Handrich, 1994). This indicates that the corrosion resistance of the alloy changes with time and eventually reaches a stable 	  	    168 value. Meanwhile, the electrode potential in neutral solutions was found to be almost constant with a potential value of -0.63 VSCE for the whole hour of immersion. This shows that the electrode did not experience any significant electrochemical changes. The electrode immersed in basic solution of pH 11 was found to exhibit the noblest potential of -0.51 VSCE. By decreasing the pH value to 9, the electrode potential slightly shifted to a more negative value (-0.57 VSCE). Meanwhile, exposure to the acid solution resulted in the most negative potential of about -0.67 VSCE.  The corrosion rates were obtained from the LPR measurements using the following equations (8.1) and (8.2): 𝑖™?? = ?™      (8.1) 𝐵 = ??.? ?? ™ ? ?? ™?              (8.2) where 𝑖™??  stand for the corrosion current, B (V/dec) is a constant defined by equation (8.2), Rp (Ω cm) is the polarization resistance, and 𝛽™  and 𝛽™?  are the oxidation and reduction Tafel slopes, respectively. Figure 8.2 illustrates the LPR of the CoFe alloy coatings immersed in different pH solutions. In between the low pH solution and high pH solution, an overall increase in polarization resistance, Rp was observed. It is known that corrosion current density, icorr is 	  	    169 inversely related to Rp, implicating that corrosion activity decreases from low to high pH solution.  Figure 8. 1 Potential changes of CoFe alloy coating measured in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11 time (s)0 1000 2000 3000E (V vs SCE)-0.70-0.65-0.60-0.55-0.50-0.45-0.40pH 11pH 9pH 7pH 5	  	    170  Figure 8. 2 Rp of CoFe alloy coating after immersion for 1 hour in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11 8.2 EQCM Measurements Figure 8.3 show the mass changes measurement of the deposited CoFe alloy coating after 1 hour of immersion in different pH solutions. It was observed that the frequency changes (not shown here) with respect to pH solution during the OCP measurement of CoFe alloy coatings. The immersion of the electrode in the pH 5 solution leads to an increase of measured resonant frequency which according to the Sauerbrey equation corresponds to a decrease in mass of the EQCM electrode which can be seen through the mass loss (figure 8.3).  pH5 7 9 11R p (ohm cm2 )012345	  	    171 The increase in frequency and loss in mass is due to the dissolution of the CoFe alloy coating. On the other hand, the opposite trend was observed for the electrodes in neutral and basic solutions where resonant frequency was decreased indicating a mass increase. However, a mass loss was observed for the electrode in the pH 11 solution at the beginning of the immersion before a mass increase. This mass loss could be related to the dissolution of an oxide layer formed in air prior to the immersion of the electrode in the solution while the mass increase is likely due to alloy oxidation by water and accumulation of corrosion products (oxides/hydroxides) on the surface alloy.  Figure 8. 3  Mass changes of CoFe alloy coating after 1 hour OCP measured by EQCM in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11. time (s)0 1000 2000 3000Mass (ug cm-2)-6-4-20246pH 5pH 7pH 9pH 11	  	    172 8.3 Electrochemical Polarization Measurements  The potentiodynamic polarization of deposited CoFe alloy coatings in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11 is presented in figure 8.4. Samples were immersed in solution for 1 hour to ensure the electrode approached a quasi-steady-state before polarization measurements. A scan rate of 0.167 mV/s was used for all measurements. Similar polarization curves were obtained for all alloy coatings in different pH solutions. An active dissolution behavior without any transition to passivation was observed for all pH values. Basically, the two reactions: Fe → Fe2+ + 2e− and Co → Co2+ + 2e− are considered to be anodic reactions for these alloy coatings. The preferential dissolution of iron from this alloy coating is expected where similar corrosion behavior was reported (Zabinski et al., 2006). The corrosion parameters i.e. corrosion potential, Ecorr, corrosion current density, icorr, and Tafel anodic and cathodic slopes (βa and βc), were calculated from polarization data and presented in table 8.1.  It can be observed that a decrease in pH solution moves the corrosion potential in the cathodic direction by hundreds of mV. The decrease in corrosion potential with decreasing pH could be explained by oxide layer formation which are more readily formed at higher pH and thus increased the corrosion potential. A corrosion current decrease was also observed although the extent of this decrease is small ranging from 8.7 to 17.7 µA/cm2.    	  	    173 Table 8. 1 Corrosion parameters of CoFe alloys after 60 minutes of electrode immersion in stagnant 0.1 M Na2SO4 aqueous solutions of pH 5, 7, 9, and 11, at 25°C pH value Ecorr (mV vs. SCE) Icorr (µA/cm2) βa (mV/dec) βc (mV/dec) 5 -697 17.7 71 174 7 -678 14.3 79 367 9 -553 6.1 50 210 11 -406 8.7 115 252   Figure 8. 4 Potentiodynamic polarization of CoFe alloy coating in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11. Scan rate: 0.167 mV/s I (A cm-2)10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1E (V vs SCE)-1.2-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.0                          pH 11              pH 9     pH 5              pH 7	  	    174 8.4 EIS The study of corrosion by EIS has been successfully applied for over 30 years and has proven to be a powerful and accurate method for measuring corrosion rates for coatings and thin films. The expression for impedance is composed of a real and imaginary part known as the Nyquist plot. figure 8.5a demonstrates the Nyquist plot of the alloy electrode in different pH solutions. The Nyquist plot for CoFe alloy coatings immersed in different pH solutions showed that the impedance spectra increased from low to high pH solution indicating an enhanced capacitive behavior for the solid/liquid interface. The Nyquist plots showed that the diameter of the capacitive loop of CoFe alloy coatings in acidic solution was smaller than that in the neutral and basic solution. Moreover, no induction effects were observed at lower frequencies which suggest the absence of significant adsorption processes occurring outside the double layer. However, one of the major shortcomings of the Nyquist plot is the incapability to tell the frequency of each data point obtained. Hence, the Bode plots are important to make an accurate interpretation as it explicitly shows frequency information. It is a useful alternative to the Nyquist plot as it can avoid possible errors in the fitting of the Nyquist semicircle and the measurement times associated with low frequency.  	  	    175   .  .  	  	    176   Figure 8. 5 (a) Nyquist (b) Bode phase EIS and (c) Physical model and electrical equivalent circuit after 1 hour OCP in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11. Figure 8.5b presents the EIS results in the form of Bode plots during immersion in the test solution for 1 hour at Ecorr. In the Bode plot, the high frequency relaxation process is related to the corrosion product formed on the metal surface and the low frequency process is attributed to the relaxation process obtained by adsorption species (Kissi et al., 2006). In the higher-frequency region, the phase-angle values were seen to move toward 0° with increasing frequency. This is a response typical of resistive behavior and corresponds to the solution resistance, Rs. In the medium-frequency range, a phase-angle maximum of less than −90° are observed for both samples, indicating that the passive films were not fully capacitive. The physical model and equivalent electrical circuit (EEC) used, as shown in figures 8.5c and d, was chosen to simulate the experimental impedance data. A good fit with the experimental data is achieved when modelled with the equivalent electrical circuit as seen from table 8.2. The typical Chi square values varied from 5.16 × 10−5 to 	  	    177 8.34 × 10−4, indicating a satisfactory fit. In this model, Rs is the solution resistance between the working electrode and the reference; Rct and Qdl represents the charge transfer resistance and the double layer capacitance, while the Rf and Qf were introduced to account for the presence of the surface film. Because the measured capacitive response is not generally ideal, due to a certain heterogeneity of the electrode surface (Nyikos and Pajkossy, 1985), a constant phase element (CPE) is introduced for fitting the spectra, instead of an ideal capacitance element. The impedance of CPE is presented in equation (8.3): ZCPE = 1/ Q (jw)n        (8.3) where Q is the general admittance function, w is the angular frequency and n is the coefficient related to the dispersive behavior. CPE can describe several behaviors: n = 1, represents a perfect capacitance, n = 0.5 represents a Warburg element and n = 0 represents a resistance. Charge transfer resistance decreased with a decrease in the pH solution indicating an increased corrosion rate of the alloy coating, following well the LPR and polarization results presented. The value of Qdl from 3.21 x 10-4 to 8.88 x 10-4 increases with higher pH value. In higher pH solution, the accumulation of corrosion products or formation of an oxide film on the electrode surface as attested by the EQCM results where increase in mass thus gives rise to the double layer capacitance .  	  	    178 Table 8. 2 Equivalent circuit parameters of CoFe alloy in 0.1 M Na2SO4 solutions at pH of 5, 7, 9, and 11, after 1 hour of OCP.  pH Rs (Ω cm2) Qdl (S sn x 10-4) ndl Rct (Ω cm2) Qf (S sn x 10-4) nf Rf (Ω cm2) Chi square (X2) 5 11.38 3.207 0.85 1139 1.272 0.81 2039 8.34 x 10-4 7 11.52 4.424 0.91 1957 2.305 0.93 2925 7.91 x 10-4 9 11.17 5.768 0.89 1684 3.035 0.95 2312 5.16 x 10-5 11 10.21 8.876 0.96 2014 3.193 0.89 3820 6.17 x 10-4     	  	    179 8.5 Effect of Sulphide Ion The variation of open circuit potential as a function of time of immersion of CoFe alloy deposit in different sulphide concentrations is presented in figure 8.6. The curves show in general that the steady state potential is shifted to more negative direction due to the presence of sulphide. This shift increases with the increase in sulphide concentration. The OCP value for the highest sulphide ion concentration was the least noble. The negative shift of the potential due to the presence of sulphide ions indicates that the corrosion of the alloy is under anodic control. The anodic dissolution of these alloy coatings is accelerated by the action of sulphide as well as the decrease of the cathodic reaction.  Figure 8. 6 Potential changes of CoFe alloy coating measured by EQCM in 0.1 M NaSO4 time (s)0 1000 2000 3000E (V vs SCE)-0.60-0.55-0.50-0.45-0.40-0.35-0.300 ppm50 ppm125 ppm500 ppm1000 ppm	  	    180 8.6 EQCM Measurements Figure 8.7 represent the recorded changes of mass of the working electrode during OCP measurements with various sulphide ion concentrations. The mass change of these CoFe alloy electrode in the test solution immediately decreased in the first 200 seconds and remained fairly constant thereafter. This again corresponds to the elimination of mass from the electrode surface during immersion of the electrode in the solution with different sulphide ion concentration. The rate of mass loss dm/dt is evaluated for different solutions for the time of 10 minutes by a linear regression, and the results are presented in figure 8.8. It is obvious that the dissolution rate was two times faster when 1000 ppm S2− was added.  Figure 8. 7 Mass changes of CoFe alloy coating measured by EQCM after 1 hour OCP time (s)0 1000 2000 3000Mass (µg cm-2)-60-40-2000 ppm50 ppm125 ppm500 ppm1000 ppm	  	    181  Figure 8. 8 Rate of mass change of CoFe alloy coating in different sulphide ion concentration 8.7 Potentiodynamic Polarization The effect of sulphide ions on the polarization curve of CoFe alloy coatings in the neutral solution at the potential sweep rate of 0.167 mV/s is illustrated in figure 8.9. The corrosion parameters i.e. corrosion potential, Ecorr, corrosion current density, icorr, and Tafel anodic and cathodic slopes (βa and βc), were calculated from polarization data and presented in table 8.3. The corrosion potential shifted to less noble values as well as increased in corrosion current density with an increase in sulphide ion concentration. The sulphide ions are seen to have a strong detrimental effect on the corrosion resistance of the alloy coatings.  Sulphide	  ion	  concentration	  (ppm)	  dm/dt	  (μg	  cm-­‐2	  	  s-­‐1)	  	  	    182  Figure 8. 9 Potentiodynamic polarization of CoFe alloy coating in 0.1 M Na2SO4 solutions with different sulphide ion concentrations. Scan rate: 0.167 mV/s. A thick non-protective porous layer is believed to form on the alloy electrodes resulting in the absence of the classical passivation curve. This indicates that the presence of sufficient sulphide ions in the solution prevents the establishment of passivity. The corrosive effect of these ions induces a shift of the corrosion potential towards more negative values and an increase of the anodic current. The increase in the current may be attributed to the anodic oxidation of the sulphide ions.  This process of anodic oxidation is a complex process that involves the formation of polysulphide, thiosulphate, or elemental sulphur according to the following reactions (8.4-8.6) (Valensi et al., 1974): I (A cm-2)10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1E (V vs SCE)-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01000 ppm500 ppm125 ppm50 ppm0 ppm	  	    183 3HS-    S32- + 3H+ + 4e-   (8.4) HS-                 S + H+ + 2e-   (8.5) 2HS- + 3H2O       S2O32- + 8H+ + 8e-   (8.6) Furthermore, it is believed that the corrosion rate increases due to the changes in the nature of cathode reaction based on reactions (8.7) and (8.8) (Rahmouni et al., 2005; Betova et al., 2010): Na2S + H2O      2Na+ + HS- + OH-         (8.7) 2HS- + 2e-    2S2-(ads) + 2H (ads)  (8.8) In addition, the anodic dissolution of the CoFe alloy electrode in the solution in the presence of the sulphide ion could be related to similar reactions proposed in the H2S medium (Betova et al., 2010 ; Abelev et al., 2009) where the possible reaction that might occur is described as in the following reactions (8.9) – (8.13): 2H2O  O2 + 4H+ + 4e-   (8.9) 2H+ + S2- ↔  H+ + HS- ↔ H2S    (8.10) Fe + H2S +H2O ↔ FeSH-(ads) + H3O+  (8.11) FeSH- (ads) ↔  Fe(SH)(ads) + e-    (8.12) 	  	    184 Fe(SH)(ads) ↔  FeSH+ + e-    (8.13)  Table 8. 3 Corrosion parameters of CoFe alloy coating in 0.1 M Na2SO4 solutions with different sulphide ion concentrations. Sulphide ion concentration (ppm) Ecorr (mV vs. SCE) Icorr (µA/cm2) βa (mV/dec) βc (mV/dec) 0 -432 4.4 755 760 50 -491 8.3 552 415 125 -550 11.7 231 333 500 -570 9.2 223 247 1000 -594 15.7 165 147    	  	    185 8.8 EIS  EIS measurements were also carried out for all deposited alloy coatings in different sulphide ion concentrations after 1 hour of OCP. Figure 8.10a shows the Nyquist plots recorded for the electrodes after one hour of immersion in the test solution in the presence of sulphide ions. These plots demonstrate the induced effects of the presence of sulphide ions in the test solution. As the sulphide ion concentration increased, both real and imaginary parts of the impedance decreased. It can be seen in this figure that the low frequency limit of the impedance decreased when sulphide ions were added in 0.1 M Na2SO4. The overall response with a general decrease in impedance and polarization resistance during immersion, coupled with a decrease in the alloy coating capacitance indicates the degradation of the CoFe alloy coating on the substrate. The phase angle Bode plot is also presented in figure 8.10b. The decrease in the phase angle, indicate the deterioration of the coating with presence of sulphide ions. To fit the chronological impedance spectra of the alloy coatings in solutions with different sulphide ion concentration, the physical model and corresponding EEC used to simulate the experimental impedance data is presented in figure 8.10c. The resultant EIS parameters are given in table 8.4.     	  	    186 Table 8. 4 Equivalent circuit parameters of CoFe alloy in 0.1 M Na2SO4 solutions with different sulphide ion concentrations after 1 hour OCP. Sulphide ion concentration (ppm) Rs (Ω cm2) Qf (S sn x 10-4) nf Rf (Ωcm2) Qdl (S sn x 10-4) ndl Rct (Ω cm2) Chi square (X2) 0 259 6.27 0.81 339 3.27 0.81 5.62 x 104 9.8 x 10-4 50 284 4.31 0.93 202 2.11 0.71 4.28 x 104 6.5 x 10-4 125 291 2.04 0.95 176 1.72 0.75 4.01 x 104 8.9 x 10-4 500 288 1.23 0.89 180 2.03 0.70 2.15 x 104 7.9 x 10-4 1000 281 1.87 0.82 102 1.27 0.68 1.91 x 105 9.6 x 10-4  A simple Randles circuit was unable to be used to fit the obtained EIS curve in this study. The Chi square values obtained from the simple circuit was unsatisfactory. Therefore, the circuit chosen was based on the ability to obtain a reasonable fitting to the experimental data. The typical Chi square values varied from 6.5 × 10−4 to 9.8 × 10−4, indicating a satisfactory fit. In this circuit, the Rs corresponds to the solution resistance, CPE corresponds to the capacitance of the porous film and electric double layer, Rct corresponds to the film and charge transfer resistance, and Rf  represents the Faradic impedance that reveals as a capacitive loop. The electrolyte resistance was independent of sulphide concentration and valued around 288 Ω cm2. The Rct value of the sample in the Na2SO4 solution without any addition of sulphide ions was higher than that in the presence of the 	  	    187 sulphide ion. The sulphide ions also increased the corrosion current density. The sulphide concentration change from 0 to 1000 ppm caused a decrease in Qdl from 3.27 x 10-4 – 1.27 x 10-4 µF cm−2. Meanwhile, RF is determined by both the anodic process corresponding to the metal dissolution and the cathodic process. The decrease of RF with increasing sulphide concentrations may indicate the acceleration of the corrosion rate and also the increase of reactivity of surface corrosion products. The presence of a sulphide ion, S2-, accelerates both Co and Fe ion, to leave the lattice of the alloy. As the sulphide is consumed, Rct was further decreased. It can be seen that the Rct is dependent on the sulphide concentration which indicates that the corrosion rate of the alloy coating increases with the sulphide concentration. The acceleration of the corrosion rate could be attributed to the presence of sulphide ion which inhibits the formation of a protective oxide film and thus resulted in the anodic process proceeding unabated.   	  	    188   Figure 8. 10 (a) Nyquist (b) Bode phase EIS and (c) Physical model and electrical equivalent circuit after 1 hour OCP in 0.1 M Na2SO4 solutions with different sulphide ion concentrations 	  	    189 8.9 Summary The electrochemical behavior of deposited CoFe alloy coatings in 0.1 M Na2SO4 with a different pH and the presence of sulphide ions has been examined using electrochemical polarization together with the EQCM method. Based on the current electrochemical study, it was demonstrated that the deposited CoFe alloy coating exhibit only an active behavior without any passivation in 0.1 M Na2SO4 at pH 5, 7, 9, and 11. Increasing the solution pH shifted the entire polarization curve to more anodic potential. Significant mass loss of the electrode was also observed through the EQCM measurement in acidic solutions. Interestingly, mass increase in neutral and basic solution was obtained, indicating accumulation of corrosion products or formation of a protective film on the surface electrode.  The presence of the sulphide ion significantly decreased the corrosion resistance of deposited CoFe alloy coatings. By quartz crystal microbalance, it was found that the mass decreased from the initial time for all sulphide concentrations. The presence of sulphide accelerates markedly the anodic reaction. The dissolution rate was two times faster when S2− was added. The corrosion current density determined by the polarization curves also showed a decrease in the presence of sulphide ions.   	  	    190 Chapter 9 9	   Conclusions	  This study aimed at producing nanocrystalline Co and CoFe alloy coatings through the electrodeposition process and analyzes the effect of various iron concentrations on the grain size, crystal structure, and current efficiency as well as microhardness of the nanocrystalline deposits. Furthermore, electrochemical tests of these nanostructured coatings in an aqueous solution with a pH ranging from acidic to alkaline was also investigated to gain in-depth understanding of their corrosion behaviour. In general, it can be concluded that effect of grain size, alloying and the use of saccharin as an additive on the corrosion behaviour of these deposits depend largely on the condition of their surrounding environment.  From this study, several conclusions were drawn: •   The nanocrystalline Co and CoFe alloy cotings were successfully produced through the electrodeposition process while controlling several plating parameters such as bath composition, pH, organic additive, time, and temperature.  •    An increase in FeSO4 concentration in the plating bath increased the iron content of the nanocrystalline CoFe alloy coatings. Grain size was also significantly reduced with an increase of iron content in the alloy deposits. 	  	    191 •   Different phases (HCP, FCC, and BCC) were present according to the iron composition of the alloy coatings.  •   Microhardness of the electrodeposited nanocrystalline CoFe alloy coatings was increased to about three times with an increase in iron content of the deposits.  The deviation from the normal Hall-Petch behaviour for deposits with higher Fe content of 18 wt% could be attributed to the change in crystal structure as well as the softening effect below certain grain sizes. •   Nanocrystalline Co and CoFe exhibit higher corrosion rates in 0.1 M H2SO4 than in an alkaline environment. In a deaerated acidic solution, both nanocrystalline Co and CoFe alloy coatings exhibit only active anodic dissolution without any passivation. The high volume of grain boundaries for nanocrystalline CoFe due to a decrease in grain sizes serve as active surface sites for corrosion reaction which decreases the corrosion resistance in low pH solutions. Both nanocrystalline Co and CoFe display a typical active-passive-transpassive behavior with two passivation stages observed in 0.1 M NaOH. Iron alloying did not have any significant effect on the corrosion performance of the alloy coating in the alkaline solution. The two stages of passivation present are due to the formation of a duplex passive film. •   The presence and absence of saccharin as an additive in the electrolyte changed the surface morphology as well as grain size of the electrodeposited nanostructured Co and CoFe alloy coatings. The presence of sulphur on the coating surface resulting 	  	    192 from the use of saccharin as an additive in the plating solution accelerates the anodic reaction for all deposits in 0.1 M H2SO4.  •   From the EQCM measurements, loss of mass from both Co and CoFe alloy electrodes was accelerated with an increase in saccharin concentration in the electroplating solution, indicating an active dissolution of the deposited coatings.  •   The formation of a passive hydroxide film Co(OH)2 on the surface of the Co coatings was enhanced in the presence of saccharin that was observed through the significant mass gain from the EQCM test.  •   The presence of saccharin is observed to hinder the formation of a protective passive film on CoFe alloy coatings, which resulted in the poor corrosion performance in alkaline solutions. The anodic and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results. •   The use of saccharin as an additive in the electroplating bath is important in obtaining bright and smooth deposits. However, their presence can be both detrimental and beneficial depending on their surrounding pH environment. •   Deposited CoFe alloy coatings exhibit only an active behavior without any passivation in 0.1 M Na2SO4 at pH 5, 7, 9, and 11. Increasing the solution pH shifted the entire polarization curve to more anodic potential.  	  	    193 •   Significant mass decrease of the electrode was observed through the EQCM measurement in acidic solutions. Active dissolution of the coatings were further accelerated in lower pH solutions. •   Mass increase in neutral and basic solution was obtained, indicating accumulation of corrosion products or formation of a protective film on the surface electrode.  •   The presence of the sulphide ion significantly decreased the corrosion resistance of deposited CoFe alloy coatings. From the quartz crystal microbalance, it was found that the mass decreased immediately after immersion for all sulphide concentrations. The presence of sulphide accelerates markedly the anodic reaction. The dissolution rate was two times faster when S2− was added. The corrosion current density determined by the polarization curves also showed a decrease in the presence of sulphide ions.    	  	    194 Chapter 10 10	   Recommendations	  and	  Future	  Work	  In terms of the electrodeposition process in producing the nanocrystalline CoFe alloy coatings, modification of the bath chemistry as well as the deposition parameters is highly recommended to accommodate these electrodeposits in having higher iron contents without the high internal stress that resulted in a very brittle coating. This would be beneficial in expanding this nanostructured alloy in future applications. It is known that annealing at different temperatures and time will considerably change the grain sizes of the deposited coatings by inducing grain growth. Therefore, annealing the nanocrystalline deposits would be an interesting study in helping to accurately determine the effect of various grain sizes ranging from microcrystalline to nanocrystalline on the corrosion response of these deposited alloys. Moreover, this would also help to gain better understanding on deciding whether grain size refinement or composition of the alloying element that manly controls the corrosion behavior of these alloy deposits.  As mentioned previously, nanocrystalline CoFe is a suitable candidates to replace the FeNi alloy in magnetic and corrosion-resistant applications. Therefore, it would be beneficial for the nanocrystalline CoFe alloy coatings containing high Fe concentrations to be studied in a wide variety of enviroments to gain further understanding of the effect of high Fe content on the corrosion behavior of these nanocrystalline alloy coatings. 	  	    195 The presence of oxygen is known to have a detrimental effect on the corrosion behaviour of materials. As the current study utilizes deaerated solutions, it is therefore recommended to carry out corrosion tests under environments containing oxygen to better understand the electrochemical properties of these nanocrystalline alloy coatings in oxygenated aqueous solutions. Another type of measuremnet that would be beneficial in studying the surface of these nanostructured deposits is by employing the advanced imaging technique, Scanning Electrochemical Microscopy (SECM). This is strongly recommended, as it would be able to further measure the local chemical and electrochemical behaviour and surface reactions of the nanocrystalline electrodeposited coatings. Identifying and understanding the functional aspects of the surface structures would be possible through this SECM study.   	  	    196 11	   References	  Abdel-Karim, R., Reda, Y., Muhammed, M., El-Raghy, S., Shoeib, M.,  and Ahmed, H. (2011), Electrodeposition and Characterization of Nanocrystalline Ni-Fe Alloys, Journal of Nanomaterials. 2011.  Abd. El Haleem, S.M.,  Abd. El Aal, E.E. (2008).  Electrochemical behaviour of iron in alkaline sulphide solutions. Corrosion Engineering Science and Technology, vol. 43, pp. 173-178. Abelev, E., Sellberg, J., Ramanarayanan T.A., and Bernasek, S.L. (2009) Effect of H2S on Fe corrosion in CO2-saturated brine, Journal Material Science, vol. 44, pp. 6167-6181. Adamson, A.W., and Gast, A.P. (1997), in: Physical Chemistry of Surfaces, 6th ed., John Wiley & Sons, New York, USA. Afshari, V., Dehghanian, C. (2009), Effects of grain size on the electrochemical corrosion behaviour of electrodeposited nanocrystalline Fe coatings in alkaline solution, Corrosion Science, vol. 51, Issue 8, pp. 1844-1849. Alagiri, M., Muthamizhchelvan, C., Ponnusamy, S. (2011), Structural and magnetic properties of iron, cobalt and nickel nanoparticles, Synthetic Metals, vol. 161, issues 15–16, pp. 1776–1780.  Alfantazi, A.M., and Erb, U. (1996), Corrosion Properties Of Pulse Plated Zn-Ni Alloy Coatings, Corrosion, Vo1. 52, No. 11, pp. 880-888. Alfantazi, A.M., and Erb, U. (1996), Microhardness And Thermal Stability Of Pulse-Plated Zn-Ni Alloys Coatings, Materials Science & Engineering A, vol. 212, pp. 123-129.  	  	    197 Alfantazi, A.M., and Erb, U. (1996), Synthesis Of Nanocrystalline Zn-Ni Alloy By An Electrochemical Method, Journal of Materials Science Letters, Vol. 15, pp. 1361-1363.  Alfantazi, A.M., and Erb, U. (1997), The Role Of Substrate Materials On Zn-Ni Alloy Pulse Electrodeposition, Surface and Coatings Technology, Vol. 89, No. 3, pp. 239-245.  Alfantazi, A.M., Page, J., and Erb, U. (1996), Pulse Plating Of Zn-Ni Alloys, Journal of Applied Electrochemistry, Vol. 26, No. 12, pp. l125-1I 34. Anandakumar, V.M., and Abdul Khadar, M.,  (2009), Microhardness studies of nanocrystalline lead molybdate, Materials Science and Engineering: A, Vol. 519, Issues 1–2, pp. 141-146.  Andricacos, P.C., and Robertson, N. (1998), IBM Journal of Research and Development, vol. 42, pp. 671-680. doi: 10.1147/rd.425.0671 Aust, K., Erb, U.,and Palumbo, G., (1994), Interface Control for Resistance to Intergranular Cracking,  Material Science and Engineering A, vol. 176, pp. 329-334. Balyanov, A., Kutnyakova, J., Amirkhanova, N., Stolyarov, V., Valiev, R., Liao, X., Zhao, Y., Jiang, Y., Xu, H., Lowe, T. (2004), Corrosion resistance of ultra fine-grained Ti. Scr. Mater, vol. 51, pp. 225-229.  Baker, H., (1992), ASM Handbook Volume 3 Alloy Phase Diagrams, ASM International: Materials Park, OH. 	  	    198 Barbuci, A., Farne, G., Matteazzi, P., Riccieri, R., Cerisola, G. (1999). Corrosion behaviour of nanocrystalline Cu90Ni10 alloy in neutral solution containing chlorides, Corrosion Science, vol. 41, pp. 463-475.  Ben Yu, MSc. Thesis, University of Toronto, 2007. Bertazzoli, M. and Pletcher, D. (1993),  Studies of the mechanism for the electrodeposition of Fe Co alloys, Electrochemica Acta, vol. 38, issue 5, pp. 671-676.  Betteridge, W., (1982), Cobalt and Its Alloys, Ellis Horwood:Chichester. Betova, I. Bojinov, M., Hyokyvirta, O., Saario,  T. (2010), Effect of sulphide on the corrosion behaviour of AISI 316L stainless steel and its constituent elements in simulated Kraft digester conditions, Corrosion Sci, vol. 52, issue 4, pp.1499-1507. Bhargava, G., Gouzman, I., Chun, C, M., Ramanarayanan, T, A., Bernasek, S, L. (2007), Studies of the oxidation of iron by water vapour using X-ray photoelectron spectroscopy and QUASES™ Appl Surf Sci, vol. 253, pp. 4322-4329. Bockris, J. O., Drazic, D., Despic, A, R. (1968), The electrode kinetics of the deposition and dissolution of iron, Electrochim Acta, vol. 4, pp. 325-361. Bonhote, C., Lam, J.W., and Last, M. (2004), Properties of DC Plated 2.4 Tesla CoFe Alloys, Electrochemical Society Proceedings, no. 23, pp. 363-373.  Bonhote, C., Xu, H., Cooper, E.I., and Romankiw, L.T. (2002) Electroplated 2.4 Tesla CoFe Films, Electrochemical Society Proceedings PV 2000-27, Section III,  pp. 319 Brankovic, S, R., Vasiljevic, N., Klemmer, T, J., Johns, E, C. (2005), Influence of Additive Adsorption on Properties of Pulse Deposited CoFeNi Alloys, J Electrochem Soc, vol. 152, pp. C196–C202. 	  	    199 Brankovic, S.R. (2012), Saccharin effect on properties of 2.4 T CoFe films, Electrochim Acta, vol. 84, pp. 139-144. Brankovic, S.R., Bae, S.E., Litvinov, D. (2008), The effect of Fe3+ on magnetic moment of electrodeposited CoFe alloys—Experimental study and analytical model, Electrochimica Acta, vol. 53, issue 20, pp. 5934-5940.  Brooks, I. (2012), PhD Thesis, University of Toronto. Synthesis and mechanical properties of bulk quantities of electrodeposited nanocrystalline materials. Brooks, I., Erb, U. (2001), Hardness of electrodeposited microcrystalline and nanocrystalline γ-phase Zn-Ni alloys, Scr. Mater, vol. 44, pp. 853-858.  Brown, I.J., Sotiropoulos, S. (2001),  Electrodeposition of Ni from a high internal  phase emulsion (HIPE) template, Electrochim. Acta, vol. 46, pp. 2711-2720.  Buess-Herman, C. (1994), Self-assembled monolayers at electrode metal surfaces. Prog. Surf. Sci, vol. 46, pp. 335–375. Budevski, E., Staikov, G., Lorenz, W.J., (1996), Electrochemical phase transformation and growth, Weinheim, VCH. Buravikhin, V.A., Litvintsev, V.V., Didovich, Yu. N., Kazakov, V.G., and Ushakov, A.I. (1974),  Magnetic and electrical properties of Co-rich cobalt-iron films, Journal of Physics, vol. 24, issue 6, pp. 636-641.  Carlton, C.E., Ferreira, P.J. (2007) What is behind the inverse Hall–Petch effect in nanocrystalline materials?, Acta Mater, vol. 55, pp. 3749-3756. 	  	    200 Cavallotti, P.L., Nobili, L., and Vicenzo, A., (2005) Phase structure of electrodeposited alloys, Electrochemica Acta, vol. 50, pp. 4557-4565. Chang, J.W., Andricacos, B. P., and Romankiw, L.T. (1990) Proceedings of the Symposium on Magnetic Materials, Processes and Devices, PV90-8, pp. 360. Chang, J.W., Andricacos, P.C., Petek, B., and Romankiw, L.T. (1990), Proceedings of the Symposium of Magnetic Materials, Process and Devices, PV 90-8, The Electrochemical Society, Pennington, NJ, 361. Cheng, D., Tellkamp, V.L., Lavernia, C.J., Lavernia E.J. (2001). Corrosion Properties of Nanocrystalline Co–Cr Coatings, Annals of Biomedical Engineering, vol. 29, pp. 803-809. Cheung, C., Djuanda, F., Erb, U., and Palumbo, G. (1995), Electrodeposition of nanocrystalline Ni-Fe alloys, NanoStructured Materials, vol. 5, No. 5, pp. 513-523. Chinnasamy, C.N., Narayanasamy, A., Ponpandian, N., Chattopadhay, K., Saravanakumar, M. (2001), Mater. Sci. Eng. A, vol. 304-306, pp. 408-412. Chokshi, A.H., Rosen, A., Karch, J., Gleiter, H. (1989), Scripta Mater., vol. 25, pp. 641. Chokshi, A.H., Rosen, A., Karch, J., Gleiter, H. (1989), On the validity of the hall-petch relationship in nanocrystalline materials, Scripta Metallurgica. vol. 23, pp. 1679-1863. Choo, R.T.C., Toguri, J.M., El-Sherik, A.M., Erb, U. (1995), Mass transfer and electrocrystallization analyses of nanocrystalline nickel production by pulse plating, Journal of Applied Electrochemistry, vol. 25, issue 4, pp. 384-403.  	  	    201 Conrad, H., Narayan, J., Jung, K. (2005), Grain size softening in nanocrystalline TiN, International Journal of Refractory Metals and Hard Materials, Vol. 23, Issues 4–6, pp. 301-305.  Crozier, B.M. (2009), Electrodeposition of iron-cobalt alloys from a dibasic ammonium citrate stabilized plating solutionM.Sc Thesis, University of Alberta.  Cullity, B.D. (1978), Elements of X-Ray Diffraction , 2nd ed., Addison-Wesley Publishing Company Inc.  Davis, J.R. (2001), Surface engineering for corrosion and wear resistance, ASM International, Materials Park, OH, USA, pp. 2-10. Detor, A.J., and Schuh, C.A. (2007), Tailoring and patterning the grain size of nanocrystalline alloy, Acta Mater., vol. 55, pp. 371–379. Dimesso, L., Heider, L., Hahn,  H. (1999), Synthesis of nanocrystalline Mn-oxides by gas condensation, Solid State Ionics, vol. 123, pp. 39-46.  Dulal, S.M.S.I., Yun, H. J., Shin, C. B., Kim, C. K. (2007). Electrodeposition of CoWP film. III. Effect of pH and temperature, Electrochimica Acta, vol. 53, no. 2, pp. 934-943.  Elhalawaty, S., Carpenter, R.W., George, J., and Brankovic, S.R. (2011), Oxygen Incorporation into Electrodeposited CoFe Films: Consequences for Structure and Magnetic Properties, J. Electrochem Society, Vol. 158, Issue 11, pp. D641-D646.  Eliaz, N., Sridhar, T.M., Gileadi, E. (2005), Synthesis and characterization of nickel tungsten alloys by electrodeposition, Electrochim. Acta, vol. 50, pp. 2893-2904.  	  	    202 Elkedim, O., Chao, H., Guay, D., (2002). Preparation and corrosion behavior of nanocrystalline iron gradient materials produced by powder processing. Journal of Materials Processing Technology, vol. 121, pp. 383-389. Elsukov, E. P., Yu., Vorobev, NTrubachev, A.V., Barinov, V. A. (1990), Structure and Magnetic Properties of Fe-P Electrodeposited Alloys, Physica status solidi (a), vol. 117, Issue 1, pp. 291–298. Epelboin, I., and Keddam, M. (1970), Faradaic Impedances: Diffusion Impedance and Reaction Impedance, J Electrochem Soc, vol. 117, pp. 1052-1056. Epelboin, I., and Keddam, M. (1972), Kinetics of formation of primary and secondary passivity in sulphuric aqueous media, Electrochim Acta, vol. 17, pp. 177-186. El-Sherik, A.M., Erb, U., Palumbo, G., Aust, K.T., (1992), Deviation from Hall-Petch Behavior in As-Prepared Nanocrystalline Nickel, Scripta Metall, Mater. vol. 27, pp. 1185. Fan, C., and Piron D. (1996), Study of anamolous nickel-cobalt electrodeposition with different electrolytes and current densities, Electrochimica Acta, vol. 41, no. 10, pp.1713-1719. Farzaneh, M.A., Raeissi, K., Golozar M.A.,  (2010), Effect of current density on deposition process and properties of nanocrystalline Ni-Co-W alloy coating, Journal of Alloys and Compounds, vol. 489, issue 2, pp. 488-492. Fatah, M. C., Ismail, M.C., Ari-Wahjoedi, B.,  Kurnia, K.A. (2011).  Effects of sulphide ion on the corrosion behaviour of X52 steel in a carbon dioxide environment at temperature 40oC, Materials Chemistry and Physics, vol. 127, issue 1-2, pp. 347-352. 	  	    203 Fougere, G. E., Weertman, J. R., Siegel, J. R., and Kim, S. (1992), Grain-size dependent hardening and softening of nanocrystalline Cu and Pd, Scripta Metallurgica et Materialia, vol. 26, issue 12, pp. 1879-1983.  Fougere, G.E., Weertman, J.R., Siegel, R.W. (1993), On the hardening and softeningof nanocrystalline materials, Nanostructured Materials, vol. 3, pp. 379-384.  George, J., Rantschler, J., Bae, S.E., Litvinov, D., and Brankovic, S.R. (2008), Sulfur and Saccharin Incorporation into Electrodeposited CoFe Alloys: Consequences for Magnetic and Corrosion Properties, Journal of the Electrochemical Society, vol. 155, issue 9, pp. D589-D594.  Ghosh, S.K., Dey, G.K., Dusane, R.O., and Grover, A.K.,  (2006), Journal of Alloys and Compounds, vol. 426, pp. 235-243  Gleiter, H., (1981) Materials With Ultra-Fine Grain Sizes, Proceedings of the 2nd Riso International Symposium on Metallurgy and Materials Science (edited by Hansen N. et al), Roskilde, pp. 15.  Gleiter, H., (1989), Nanocrystalline Materiais, Progress in Materials Science, Vol. 33, pp. 223-315. Gloriant, T., and Greer, A.L. (1998), Al-based nanocrystalline composites by rapid solidification of Al-Ni-Sm alloys, Nanostructured Materials. 10 (1998) 389-396.  Gomes, M.I. da Silva Pereira, (2006), Pulsed electrodeposition of Zn in the presence of surfactants, Electrochim. Acta,  vol. 51, pp. 1342-1350. Gong, W., Li, H., Zhao. Z., and Chen, J. (1991), Ultrafine particles of Fe, Co, and Ni ferromagnetic metals, Journal of Applied Physics, vol. 69, pp. 5119-5121. 	  	    204 Gonzalez, F., Brennenstuhl A.M., Palumbo G., Erb U. and Lichtenberger P.C. (1996), Electrodeposited nanostructured nickel for in-situ nuclear steam generator repair, Mater. Sci. For., Vol. 225–227, pp. 831–836. Grosvenor, A, P., Kobe, B, A., McIntyre, N, S. (2004), Studies of the oxidation of iron by water vapour using X-ray photoelectron spectroscopy and QUASES™, Surf Sci, vol. 572, pp. 217-227. Guoying, W., Hongliang, G., Xiao, Z., Qiong, W., Junying, Y., Baoyan, W. (2007), Effect of organic additives on characterization of electrodeposited Co-W thin films. Applied Surface Science, vol. 253, no. 18, pp. 7461-7466 Hamdy, A.S., Sa’eh, A.G., Shoehib, M.A., and Barakat, Y. (2007). Electrochim. Acta, vol. 52, pp. 7068. Hansen, K.A.M. (1989), Constitution of Binary Alloys, 2nd ed., Genium Publishing Corporation, Schenectady, New York.  Hansen, N., Horsewell, k., Leffers T., Lilholt, H. (1981) International Symposium on Metallurgy and Materials Science, 14- 18 September 1981, pp. 15-21. Heim, U., and Schwitzgebel, G. (1999). Nanostructured Materials, vol. 12, pp. 19-22.  Helfand, M.A., Clayton, C.R., Diegel, R.B., and Sorenson, N.R. (1992), J. Electrochem. Soc., vol. 139, no. 8, pp. 2121.  Heusler, K.E. (1958), Z Elektrochem, vol. 62, pp. 582. 	  	    205 Herzer, G., (1990), Grain size dependence of coercitivity and permeability in nanocrystalline ferromagnets, IEEE Transactions on Magnetics, vol. 26, issue, 5, pp.1397-1402. Herzer, G., (1992), Nanocrystalline soft magnetic materials, J. Magn. Mater, vol. 112, issue, 1-3, pp. 258-262. Hongliang, G., Qiong, W., Guoying, W., Xinyan, W., and Qiaoying, Z. (2007), Effect of bath temperature on electrodeposited permanent magnetic Co-Pt-W(W) films. Bulletin Korean Chemistry Society, vol. 28, no. 12, pp. 2214-2218.  Inturi, R., Szklarska-Smialowska, Z. (1992), Localized corrosion of nanocrystalline 304 type stainless steel films. Corrosion, vol. 48, pp. 398-403.  Ismail, K. M., and Badaway, W.A. (2000). Electrochemical behaviour of cobalt in aqueous solution at diferente pH,  J. Appl. Electrochem., vol. 30 , pp. 693-704. Ives, D.J.G., and Rawson, A.E. (1962), Copper Corrosion III . Electrochemical Theory of General Corrosion. Journal Chemical Society, vol. 109, pp. 447-466.  Jay, J.P., Jurca, I-S., Pourroy, G., Viart, N., Meny. C., and Pannisod, P. (2001), Co NMR study in Co-Fe alloys/Co magnetite composites, Solid State Sciences, vol. 3, issue 3, pp. 301-308.  Jayatissa, A.H., Guo, K., Jayasuriya, A.C., Gupta, T. (2007), Fabrication of nanocrystalline cobalt oxide via sol–gel coating, Materials Science and Engineering, B, vol. 144, pp. 69-72.  Jinnie, G., James R., Sang, E.B., Dmitri, L., and Stanko, B. (2008), Sulfur and Saccharin Incorporation into Electrodeposited CoFe Alloys, Consequenceces for Magnetic and 	  	    206 Corrosion Properties, Journal of the Electrochemical Society, vol. 155 no, 9,  pp. D598-D594. Judy, J.W., Muller, R.S., Zappe, H.H. (1995), Magnetic microactuation of polysilicon flexure structures, J. Microelectromech. Syst., vol.4, pp. 162-169.  Jung, H., Alfantazi A. M. (2010), Corrosion properties of electrodeposited cobalt in sulfate solutions containing chloride ions, Electrochim. Acta, vol. 55, pp. 865-869.  Jung, H., Alfantazi, A. M. (2006), An electrochemical impedance spectroscopy and polarization study of nanocrystalline Co and Co–P alloy in 0.1 MH2 SO4 solution. Electrochim. Acta, vol. 51, pp. 1806-1814.  Jung, H., and Alfantazi, A. (2007), Phosphorous alloying and annealing effects on the corrosion properties of nanocrystalline Co–P alloys in acidic solution, Corrosion, vol. 63, pp. 159-170. Jung, H., and Alfantazi, A.M. (2010), Corrosion Behavior of Nanocrystalline Co and Co-P Alloys in NaOH Solution, Corrosion, Vol. 66, No. 3, pp. 1-12. Jung, H., (2006), Ph.D. Thesis, Corrosion behaviour of nanocrystallien Co and Co-P alloy, The University British Columbia, Vancouver, Canada. Rahmouni, K., Keddam, M., Srhiri A., and Takenouti, H. (2005), Corrosion of copper in 3% NaCl solution polluted by sulphide ions, Corrosion Sci, vol. 47, pp. 3249-3266. Kakuno, E.M., Mosca, D.H., Mazzaro, I., Mattoso, N., Shreiner, W.H., Gomes, M.A.B. (1997), Structure, Composition, and Morphology of Electrodeposited Co x Fe1  −  x Alloys, Journal of the Electrochemical Society, vol. 144, issue 9, pp. 3222-3226. 	  	    207 Karakus, C., and Chin, D.T. (1994), Metal distribution in jet plating, Journal of the Electrochemical Society, vol. 141, no 3, pp. 691-697. Keddam, M., Mottos, OR., Takenouti, H. (1981), Reaction model for iron dissolution studied by electrode impedance I. Experimental results and reaction model, J. Electrochem. Soc, vol. 128, pp. 257-266.  Kedim, O.E., Paris, S., Phigini, C., Bernard, F., Gaffet, E., Munir, Z.A. (2004). Electrochemical behavior of nanocrystalline iron aluminide obtained by mechanically activated field activated pressure assisted synthesis, Material Science and Engineering, vol. A369, pp. 49-55. Khan, H.R. and Petrikowski, K. (2002), Magnetic and structural properties of the electrochemically deposited arrays of Co and CoFe nanowires, J Magn Magn Mater, vol. 249, pp. 458-461.  Kim, D., Park, D., Yoo, B.Y., Sumodjo, P.T.A., Myung, N.V. (2003), Magnetic properties of nanocrystalline iron group thin film alloys electrodeposited from sulfate and chloride baths, Electrochim. Acta, vol. 48, pp. 819-830.  Kim, S, H., Sohn, H, J., Joo, Y, C. (2005), Effect of saccharin addition on the microstructure of electrodeposited Fe–36 wt.% Ni alloy, Surf Coat Technol, vol. 199, pp. 43-48. Kim, S.H., Aust, K.T., Erb, U., and Gonzalez, F. (2002), The Corrosion Behaviour of Nanocrystalline Electrodeposits, Proc. AESF SUR/FIN, AESF,  Kim, S.H., Aust, K.T., Erb, U., Gonzales, F., Palumbo, G. (2003), A comparison of the corrosion behavior of polycrystalline and nanocrystalline Co, Scripta Materiallia, vol.48, pp. 1379-1384. 	  	    208 Kim, S.H.,  Franken, T.,  Hibbard, G.D., Erb, U., Aust, K.T.,  Palumbo, G. (2003), Journal of Metastable and Nanocrystalline Materials, Proceedings of the 9th International Symposium on Metastable Mechanically Alloyed and Nanocrystalline, vol. 15-16, pp. 643. Kissi, M., Bouklah, M., Hammouti, B., Benkaddour, M. (2006), Establishment of equivalent circuits from electrochemical impedance spectroscopy study of corrosion inhibition of steel by pyrazine in sulphuric acidic solution, Appl. Surf. Sci, vol. 252, issue 12, pp. 4190-4197. Koch, C.C., Scattergood, R.O., Youssef, K.M., Chan, E., Zhu, Y.T.(2010), Nanostructured materials by mechanical alloying: new results on property enhancement. J. Mater. Sci, vol. 45, no. 17, pp. 4725-4732. Kohn, A., Eizenberg, M., Shacham-Diamand, Y., and Sverdlov, Y. (2001), Characterization of electroless deposited Co(W,P) thin films for encapsulation of copper metallization, Material and Science Engineering A, vol. 302, pp 18-25. Kouncheva, M., Raichevski, G., and Vitkova, St. (1987), The effect of sulphur and carbon inclusions on the corrosion resistance of electrodeposited Ni-Fe alloy coatings, Surf and Coat Technol, vol. 31, pp. 137–142. Koza, J.A., Karnbach, F., Uhlemann,  M., McCord, J., Mickel, C., Gebert, A., Baunack, S., Schultz, L. (2010), Electrocrystallisation of CoFe alloys under the influence of external homogeneous magnetic fields—Properties of deposited thin films, Electrochim. Acta, vol. 55, issue 3, pp. 819-831.  Krstic, V., Erb, U., Palumbo, G. (1993), Effect of porosity on Young's modulus of nanocrystalline materials, Scripta Metall. Mater, vol. 29, pp. 1501-1504. 	  	    209 Kumar, K.S., Van Swygenhoven, H., Suresh, S. (2003), Mechanical behavior of nanocrystalline metals and alloys, Acta Mater, vol. 51, pp. 5743-5744.  Kwon, H., and Gewirth., A.A. (2007), SERS Examination of Saccharin Adsorption on Ni Electrodes, J Electrochem Soc, vol. 154, issue 11, pp. D577-D583. Lafouresse, M., Medvedev, A., Kutuso, K., Schwarzacher, W., Masliy, A. (2007), pH dependence of the composition of electrodeposited Co films in aqueous sulfate solutions, Russ J Electrochem, vol. 43, pp. 856-858. Lai, Y., Lin Z., Chen, Z., Huang, J., and Lin, C. (2010), Fabrication of patterned CdS/TiO2 heterojunction by wettability template-assisted electrodeposition, Materials Letters, vol. 64, no. 11, pp. 1309-1312. Lallemand, F., Comte, D., Ricq, L., Renaux, P., Pagetti, J., Dieppedale, C., Gaud, P. (2004), Effects of organic additives on electroplated soft magnetic CoFeCr films, Appl Surf Sci, vol. 225, pp. 59-71. Lallemand, F., Ricq, L., Deschaseaux, E., De Vettor, L., Berçot, P. (2005), Electrodeposition of cobalt-iron alloys in pulsed current from electrolytes containing organic additives, Surface and Coatings Technology, vol. 197, pp.10-17.  Lallemand, F., Ricq, L., Wery, M., Berçot, P., Pagetti, J. (2002), Effect s of the structure of organic additives in the electrochemical preparation and characterization of CoFe film, Electrochim. Acta, vol. 47, pp. 4149-4156.  Lallemand, F., Ricq, L., Wery, M., Berçot, P., Pagetti, J. (2004), Kinetic and morphological investigation of CoFe alloy electrodeposition in the presence of organic additives, Surface and Coatings Technology, vol.179, PP. 314-323.  	  	    210 Li, H., and Ebrahimi, F. (2003), Synthesis and characterization of electrodeposited nanocrystalline nickel–iron alloys, Materials Science and Engineering A, vol. 347, pp. 93-101. Li-yuan, Q., Jian-she, L., and Qing, J. (2010), Effect of grain size on corrosion behavior of electrodeposited bulk nanocrystalline Ni, Transactions of Nonferrous Metals Society, vol. 20, issue 1, pp. 82-89. Li, Y., Jiang, H., Wang, D., Ge, H. (2008), Effects of saccharin and cobalt concentration in electrolytic solution on microhardness of nanocrystalline Ni–Co alloys, Surface and Coatings Technology, Vol. 202, Issue 20, pp. 4952-4956.  Li, Y., Wang, F., Liu. G. (2004), Grain size effect on the electrochemical corrosion behavior of surface nanocrystallized low-carbon steel, J. Corrosion, vol. 60, pp. 891-896. Lian, J., and Baudelet, B., (1993), A modified Hall-Petch relationship for nanocrystalline materials, NanoStructured Materials, vol. 2, issue 4, pp. 415-419. Liang, Y., Liu, M., Chen, J., Liu, X., Zhou, Y.(2011), Electrodeposition and Characterization of Ni/Ti3Si(Al)C2 Composite Coatings, Journal of Materials Science & Technology, vol. 27, pp.1016-1024.  Liu, C., Bi, Q., Leyland, A., Matthews, A. (2003), An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part I. Establishment of equivalent circuits for EIS data modeling, Corros Sci, vol. 45, pp. 1243-1256. 	  	    211 Liu, L., Li, Y., and Wang, F. (2007). Influence of micro-structure on corrosion behavior of a Ni-based superalloy in 3.5% NaCl, Electrochimica Acta, Vol. 52, Issue 25, pp. 7193-7202. Liu, X., Evans, P., and Zangari, G. (2000) IEEE Transactions on Magnetics, vol. 36, no.5, pp. 3479. Liu, X.D., Hu, Z.Q., Ding, B.Z. (1993), Hall-Petch relation in nanocrystalline Fe-Mo-Si-B alloys, Nanostructured Materials, vol 2, issue 5, pp. 545-552.  Liu, X.Q., Tao, S.W., Shen, Y.S. (1997), Preparation and characterization of nanocrystalline α-Fe2O3 by a sol-gel process, Sensors Actuators B: Chem, vol. 40, pp. 161-165.  Lodhi, Z.F., Mol, J.M.C., Hovestad, A., Terryn, H., Wit, J.H.W.  (2007), Electrodeposition of Zn–Co and Zn–Co–Fe alloys from acidic chloride electrolytes, Surface and Coatings Technology, vol. 202, pp. 84-90. Lowenheim, F.A. (1974), Modern Electroplating 3rd ed., Wiley & Sons, New-York. Lu, A.Q., Zhang Y., Li Y., Liu, G., Zhang Q.H., Liu C.M., (2006). Effect of nanocrystalline and twin boundaries on corrosion behaviour of 316L stainless steel using SMAT, Acta Metallurgica Sinica, vol. 19, issue 3, pp. 183-189.  Lu, K., and Lu, J. (1999), Surface nanocrystallization (SNC) of metallic materials-presentation of the concept behind a new approach, Journal of Materials Science and Technology, vol. 15, no. 3, pp. 193-197. Lu, K., Sui, M.L., (1993). An explanation to the abnormal Hall-Petch relation in nanocrystalline materials, Scripta Metall Material, vol. 28, pp. 1465-1470. 	  	    212 Lu, K., Wie, W.D., Wang, J.T. (1990), Microhardness and fracture properties of nanocrystalline Ni-P alloy, Scripta Metall, vol. 24 pp. 2319-2323.  Luo, W., Xu, Y., Wang, Q., Shi, P., Yan, M. (2010), Effect of grain size on corrosion of nanocrystalline copper in NaOH solution. Corros. Sci, vol. 52, pp. 3509-3513.  Macdonald, D.D., Song, K. Makela, K. Yoshida, (1993). Macdonald, D.D., Song, K. Makela, K. Yoshida, Corrosion, Corrosion, vol. 49, no. 1,  pp. 8-16.  Madaah Hosseini, H.R., and Bahrami, A. (2005), Preparation of nanocrystalline Fe–Si–Ni soft magnetic powders by mechanical alloying, Materials Science and Engineering: B, vol. 123, pp. 74-79.  Manaf, A., Buckley, R.A., Davies, H.A. (1993) New nanocrystalline high-remanence Nd-Fe-B alloys by rapid solidification, J Magn Magn Mater, vol. 128, pp. 302-306.  Marcus, P., Oudar, J. (1985) In Hydrogen Degradation of Ferrous Alloys, Oriani RA, Hirth JP, Smialowski M, Eds.; Noyes Publications: Park Ridge, NJ, vol. 3, pp. 36-77. Martín, A, J., Chaparro, A, M., Daza, L. (2007), Electrochemical quartz crystal microbalance study of the electrodeposition of Co, Pt and Pt–Co alloy, J Power Sources, vol. 169, pp. 65-70. Matsushima, J.T., Trivinho-Strixino, F., Pereira, E.C. (2006), Investigation of cobalt deposition using the electrochemical quartz crystal microbalance, Electrochim Acta, vol. 51, pp. 1960-1966. Mattoso, N., Fernandes, V., Abbate, M., Schreiner, W.H., and Mosca, D.H. (2001). Structural and chemical characterization of Fe-Co alloys prepared by electrodeposition, Electrochemical and Solid State Letters, vol. 4, no. 4, pp. C20-C22. 	  	    213 Mauer R., Erb U., Gleiter H., (1984). Mater. Sci. Eng., 63, L13. McIntyre, N.S., and Zetaruk, D.G. (1977), X-ray photoelectron spectroscopic studies of iron oxides, Anal. Chem, vol. 49, pp. 1521-1529.  McMahon, G., and Erb, U. (1989), Bulk Amorphous and Nanocrystalline Ni-P Alloys by Electroplating, Microstructure Science, Vol. 17, pp. 447-457. Meyers, M. A., Mishra, A., Benson, D. J. 2006, Mechanical properties of nanocrystalline materials, Progress in Materials Science, vol. 51, pp. 427-556. Milan, P., and Mordechay, S. (2006). Fundamentals of Electrochemical Deposition, 2nd Ed. John Wiley & Sons, pp. 215 Mills, P., and Sullivan, J. (2000), A study of the core level electrons in iron and its three oxides by means of X-ray photoelectron spectroscopy, J Phys D: Appl Phys, vol. 16, pp. 723-732.  Mishra, R., and Balasubramaniam, R. (2004), Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel, Corrosion Science, vol. 46, issue 12, pp. 3019-3029.  Mohan, P., Suryanarayana, C., Desai, V., (2004). Corrosion Properties of MoSi2+Si3N4 Nanocomposites, In Nanomaterials: Synthesis, Characterisation, and Application, eds. S. Bandyopadhyay et al., Tata McGraw-Hill Publ. Co. Ltd., New Delhi, India, 2004, pp. 171-181. Montes-Rojas, A., Torres-Rodríguez, L.M., Nieto-Delgado, C. (2007), Electromicrogravimetric study of underpotential deposition of Co on textured gold electrode in ammonia medium, New J. Chem, vol. 3. pp. 1769-1776. 	  	    214 Myung, N.V., and Nobe, K., (2001), Electrodeposited Iron Group Thin Film Alloys, Journal of the Electrochemical Society, issue 148, Vol. 3, pp. C136-C144. Nakanishi, T., Ozaki, M., Nam, H, S., Yokihiko, T., Osaka, T. (2001), „Pulsed Electrodeposition of Nanocrystalline CoNiFe Soft Magnetic Thin Films, J Electrochem Soc, vol. 148, pp. C627-631. Nam, H.S., Yokoshima, T., Nakanishi, T., Osaka, T., Yamazaki, Y., and Lee, D.N., (2001) Microstructure of electroplated soft magnetic CoNiFe thin films, Thin Solid Films, vol. 382, issue 2, pp. 288-293. Natter, H.,  and Hempelmann, R., (1996), J. Phys. Chem, vol. 100, pp. 19525.  Natter, H., Schmelzer, M., and Hempelmann, R. (1998), Nanocrystalline Nickel and Nickel-Copper Alloys: Synthesis, Characterization, and Thermal Stability, J. Mater. Res., 13, pp. 1186-1197. Natter, J., Krajewski, T., and Hempelmann, R. (1996), Nanocrystalline Palladium by Pulsed Electrodeposition, Ber Bunsen-Ges Phys Chem, vol.100, pp. 55-64. Nieh T.G., Wadsworth, J. (1991). Hall- Petch Relation in Nanocrystalline Solids, Scripta Metallurgica et Materialia, vol. 25, no. 4, pp. 955-958. Nieman, G.W., Weertman, J.R., Siegel, R.W. (1989),  Microhardness of nanocrystalline palladium and copper produced by inert-gas condensation, Scripta Metallurgica, vol. 23, pp. 2013-2018.  Nik, Masdek, N.R., and Alfantazi, A. (2010), Review of Studies on Corrosion of Electrodeposited Nanocrystalline Metals and Alloys, ECS Transactions, vol. 28, issue 24, pp. 249-260.  	  	    215 Nik, Masdek, N.R., Alfantazi, A.M. (2012), Nanocrystalline Cobalt-Iron Alloy: Synthesis and Characterization, Materials Science and Engineering: A, vol. 550, pp. 388-394.  Nik Masdek, N.R., and Alfantazi, A.M. (2013) Electrochemical Properties of Electrodeposited Nanocrystalline Cobalt and Cobalt-Iron Alloys in Acidic and Alkaline Solutions, Journal of Applied Electrochemistry, Volume 43, Issue 7, pp. 721-734.  Nyikos, L., and Pajkossy, T. (1985), Fractal dimension and fractional power frequency-dependent impedance of blocking electrodes, Electrochim. Acta, vol. 30, pp. 1533-1540. Ohno, T., Mitsui, T., and Matsumura, M. (2003), Photocatalytic Activity of S-doped TiO2 Photocatalyst under Visible Light, Chem Lett, vol. 32, pp. 364-365. Okamoto, H., (2008). Cobalt-Iron, Journal of Phase Equilibria and Diffusion, vol. 29, pp. 383-384. Osaka, T. (2000). Electrodeposition of highly functional thin films for magnetic recording devices of the next century, Electrochimica Acta, vol. 45, pp. 3311-3321 Osaka, T., Homma, T., Kageyama, K., Matsunae, Y. (1994), Preparation of Electroless-Deposited CoFeB Soft Magnetic Films with High Saturation Magnetic Flux Density, vol. 62, issue 10, pp. 987. Osaka, T., Takai, M., Ohasi, K., Yamada, K. (1998), A soft magnetic CoNiFe film with high saturation magnetic flux density and low coercivity, Nature 392, pp. 796-798. Osaka, T., Yokoshima, T., Shiga, D., Imai, K., and Takashima, K., (2003), A High Moment CoFe Soft Magnetic Thin Film Prepared by Electrodeposition, Electrochemical and Solid-State Letters, vol. 6, issue 4, pp. 53-55. 	  	    216 Oudar, J., and Marcus, P. (1979), Role of adsorbed sulphur in the dissolution and passivation of nickel and nickel-sulphur alloys, Appl Surf Sci, vol. 3, pp. 48-67. Palumbo, G., Gonzalez, F., Brennenstuhl, A.M., Erb, U., Shmayda, W.,  and Litchenberger, P.C. (1997). In-situ nuclear steam generator repair using electrodeposited nanocrystalline nickel, Nanostr. Mat, vol. 9, pp. 737-746.  Palumbo, G., Thorpe, S.I. and Aust, K.T., (1990), On the Contribution of Triple Junctions to the Structure and Properties of Nanocrystalline Materials, Scripta Metall. Mater., vol. 24 pp. 1347-1350. Pande, C.S., Cooper, K.P. (2009) Nanomechanics of Hall-Petch relationship in nanocrystalline materials, Prog. Mater. Sci, vol. 54, pp. 689-706.  Park, D.Y., Yoo, B.Y., Kelcher, S., and Myung, N.V. (2006). Electrodeposition of low stress high magnetic moment Fe-rich FeCoNi thin films, Electrochimica Acta, vol. 51, no.12, pp. 2523-2530. Parthasarathi, B., Seenivasan, H., Rajam, K.S., Grips, V.K.W. (2012), Characterization of amorphous Co-P alloy coatings electrodeposited with pulse current using gluconate bath. Applied Surface Science, vol. 258, pp. 9544-9553.   Pellicer, E., Varea, A., Pané, S., Sivaraman, K.M., Nelson, B.J., Suriñach, S., Baró, M.D., Sort, J. (2011), A comparison between fine-grained and nanocrystalline electrodeposited Cu–Ni films. Insights on mechanical and corrosion performance, vol. 205, pp. 5285–5293. Peng, X., Zhang, Y., Zhao, J., Wang, F. (2006), Electrochemical corrosion performance in 3.5% NaCl of the electrodeposited nanocrystalline Ni films with and without 	  	    217 dispersions of Cr nanoparticles, Electrochimica Acta, Vol. 51, Issue 23, pp. 4922-4927  Popov, B, N., Yin, K, M., White, R, E. (1993), Galvanostatic Pulse and Pulse Reverse Plating of Nickel-­‐Iron Alloys from Electrolytes Containing Organic Compounds on a Rotating Disk Electrode, J Electrochem Soc, vol. 140, pp. 1321-1330. Pourroy, G., Viart N., and Läkamp, S. (1999), Magnetic characterization of composites Fe–Co alloy/Co containing magnetite, Journal of Magnetism and Magnetic Materials, vol. 203, pp. 37-40. Pourroy, G., Valles-Minquez, A., Jurca, I.S., Meny, C., Viart N., and Panissod, P. (2002), Journal of Alloys and Compounds, vol. 333(1-2), pp. 296. Rashidi, A, M., and Amadeh, A. (2010), Effect of Electroplating Parameters on Microstructure of Nanocrystalline Nickel Coatings, Technol., vol. 26, pp. 82-86. Rajeev K. G., Singh Raman, R.K., Koch, C.C., and , Murty, B.S. (2013). Effect of Nanocrystalline Structure on the Corrosion of a Fe20Cr Alloy, International Journal of Electrochemical Science, vol. 8, pp. 6791-6806. Ricq, L., Lallemand, F., Gigandet, M. P., Pagetti, J. (2001), Influence of sodium saccharin on the electrodeposition and characterization of CoFe magnetic film, Surface and Coatings Technology, vol. 138, issue 2-3, pp. 278-283.  Rofagha, R., Erb, U., Ostrander, D., Palumbo, G., Aust, K. (1993), The effects of grain size and phosphorus on the corrosion of nanocrystalline Ni-P alloys. Nanostructured materials, vol. 2, pp. 1-10.  	  	    218 Rofàgha, R., Langer, R, El-Sherik, AM., Eh, U., Palurnbo, G. and Aust, KT. (1992),  A Cornparison of the Corrosion Behaviour of Nanocrystalline and Normal Crystalline Nickel, Mat. Res. Soc. Symp. Proc., Vol. 238, pp.751-755. Rofagha, R., Langer, R., El-Sherik, A. M., Erb, U., Palumbo, G., and Aust., K. T. (1991), The corrosion behaviour of nanocrystalline nickel. Scripta Material, vol. 25, pp. 2867-2872.  Romankiw, L. T., Croll, I.,  and Hatzaki, M. (1970) Batch fabricated thin film magnetic recording heads, IEEE Trans. Magn., vol. 6, pp. 579-599. Ross, C.A. (1994), Electrodeposited multilayer thin films, Annu. Rev.Mater.Sci., vol. 24, pp. 159-188.  Saber, Kh., Koch, C.C., Fedkiw, P.S. (2003), Pulse current electrodeposition of nanocrystalline zinc, Mater. Sci. Eng. A, vol. 341, issue 1-2, pp. 174-181. Safranek, W.J., (1974), Properties of Electrodeposited Metals and Alloys., Elsevier, Chapter 13. Sahari, A., Azizi, A., Schmerber, G., Abes, M., Bucher, J.P., Dinia, A. (2006). Electrochemical nucleation and growth of Co and CoFe alloys on Pt/Si substrates, Catalysis Today, vol.113, pp. 257-262.  Sakthivel, S., Janczarek, M., and Kisch, H. (2004), Visible Light Activity and Photoelectrochemical Properties of Nitrogen-Doped TiO2, J Phys Chem B, vol. 108, pp. 19384-19387. Sasaki, K, Y., and Talbot, J, B. (2000), Electrodeposition of Iron-­‐Group Metals and Binary Alloys from Sulfate Baths. II. Modeling, J Electrochem Soc, vol. 147, pp. 189-197. 	  	    219 Scattergood, R.O., Koch, C.C. (1992), Modified Model for Hall-Petch Behavior in Nanocrystalline Materials, Scripta Metall. Mater, vol. 27, pp. 1195-1200. Schmutz, P., and Landolt, D. (1999), In-situ microgravimetric studies of passive alloys: potential sweep and potential step experiments with Fe–25Cr and Fe–17Cr–33Mo in acid and alkaline solution, Corr Sci, vol. 41, pp. 2143-2163. Shao, I., Romankiw, L.T., Bonhote, C. (2010), Stress in electrodeposited CoFe alloy films, J. Cryst. Growth, vol. 312, pp. 1262-1266.  Shirani, A., Momenzadeh, M., Sanjab, S. (2012), Surfactant effect on electrochemical behavior of Co–TiO2 nanocomposite coatings, Surf Coating Technol, vol. 206, pp. 2870-2876. Shou, I., Vereecken, P.M., Chien, C.L., Cammarata, R.C., and Searson, P.C. (2003), Electrochemical Deposition of FeCo and FeCoV Alloys, Journal of the Electrochemical Society, vol. 150, issue 3, pp. C184-C188.  Shriram, S., Mohan, S., Renganathan, N., Venkatachalam, R. (2000), Electrodeposition of nanocrystalline nickel: a brief review. Trans. Inst. Met. Finish, vol. 78, pp. 194-197. Siegel, R.W., (1994), Nanostructured materials -mind over matter- , Nanostructured Materials, vol. 4, issue1, pp. 121-138.  Simmonds, M.C., Newman, R.C., Fujimoto S., and Colligon, J.S. (1996). Synthesis of a novel phase in a Fe-Cr alloy grown by ion assisted co-sputter deposition, Thin Solid Films, vol. 279, pp. 4-6. Specht, E.D., Rack, P.D., Rar, A., Pharr, G.M., George E.P., and Hong, H. (2004), Nonequilibrium structures in codeposited Cr-Fe-Ni films, in Combinatorial and 	  	    220 Artificial Intelligence Methods in Materials Science II, edited by R.A. Potyrailo, A. Karim, Q. Wang, and T. Chikyow (Materials Research Society, Warrendale, PA, 2004) Vol. 804, pp. 45-50. Specht, E.D., Rack, P.D., Rar, A., Pharr, G.M., George, E.P., Fowlkes, J.D., Hong H., and Karapetrova, E., (2005), Metastable Phase Evolution and Grain Growth in Annealed Nanocrystalline Cr-Fe-Ni Films,Thin Solid Films, vol. 493, issue 1-2, pp. 307-312. Sriraman, K.R., Ganesh Sundara Raman, S., Seshadri, S.K. (2007), Influence of crystallite size on the hardness and fatigue life of steel samples coated with electrodeposited nanocrystalline Ni–W alloys, Mater Lett, vol. 61, pp. 715-718.  Sriraman, K.R., Raman, S.G.S., Seshadri, S.K. (2007), Corrosion behavior of electrodeposited nanocrystalline Ni–W and Ni–Fe–W alloys, Mater. Sci. Eng. A, vol. 460–461, pp. 39–45. Srivastava, M., Ezhil Selvi, V., William Grips, V.K., Rajam, K.S. (2006), Corrosion resistance and microstructure of electrodeposited nickel–cobalt alloy coatings, Surface and Coatings Technology, vol. 201, pp.3051-3060. Srivastava, M., William, Grips, V.K., Rajam, K.S. (2007), Electrochemical deposition and tribological behaviour of Ni and Ni–Co metal matrix composites with SiC nano-particles, Appl Surf Sci, vol. 253, pp. 3814-3824. Stefec, R. (1995), Magnetic properties of electrodeposited iron-nickel alloys, Czechoslovak Journal of Physics, Vol 23, no. 11, pp.1249-1262. Sun, N.X., Mehdizadeh, S., Bonhote, C.,Xiao, Q.F., and York, B., (2005), Magnetic annealing of plated high saturation magnetization soft magnetic FeCo alloy films, Journal of Applied Physiscs, vol. 97, issue 10, pp. 10N904 - 10N907. 	  	    221 Suryanarayana, C., and Froes, F.H., (1992), The Structure and Mechanical Properties of Metallic Nanocrystals, Metal. Mater. Trans, vol. 23A, pp. 1071-1081. Suryanarayana, C., Mukhopadhyay, D., Patnkar, S.N. (1992), Grain size effects in nanocrystalline materials, Journal of materials research, vol. 7, pp. 2114-2118. Tabakovic, I., Riemer, S., Jayaraju, N., Venkatasamy, V., Gong,  J. (2011), Relationship of Fe2+ concentration in solution and current efficiency in electrodeposition of CoFe films, Electrochim. Acta, vol. 58, pp. 25-32.  Tabakovic, I., Gong, J., Riemer,  S., Venkatasamy,  V., Kief, M. (2010), Stress evolution in CoxFe1−x (x = 0.33–0.87) electrodeposited films, Electrochim. Acta, vol. 55, pp. 9035-9041.  Tabakovic, I., Reimer, S., Tabakovic, K., Sun, M., and Kief, M. (2006) Mechanism of Saccharin Transformation to Metal Sulfides and Effect of Inclusions on Corrosion Susceptibility of Electroplated CoFe Magnetic Films, J Electrochem Soc, vol. 153, pp. C586-C593. Tabakovic, I., Riemer, S., Kvitek, R., Jallen, P., and Inturi, V, (2001), Origin of Inclusion of Foreign Elements During the Electrodeposition of Soft Magnetic NiFe, CoNiFe and CoNiFeO Alloys: SIMS and XPS Analysis, Magnetic Materials, Processes and Devices VI, Krongelb, S., Romankiw, L. T., Ahn, C. H., Chang, J.-W.,  and Schwazarcher, and C. H. Ahn, Eds., PV 2000-29, The Electrochemical Society Proceedings Series, Pennington, NJ, pp. 253–263. Tait, W.S.,  and Handrich, K. A. (1994), Cation Enhancement of Internally Coated Metal Container Corrosion Failure. Corrosion, Corrosion, vol. 50, pp. 373-377. 	  	    222 Tan, B, J., Klabunde, K, J., Sherwood, P, M, A. (1991), XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica, J Am Chem Soc, vol. 113, pp. 855-861.  Tang, P.T., Watanabe T., Anderson, J.E.T., Bech-Nielsen, G. (1995). Improved corrosion resistance of pulse plated nickel through crystallisation control, Journal of Apllied Electrochemistry, vol. 25, pp. 347-352. Tao, S., and Li, D.Y., (2006), Tribological, mechanical and electrochemical properties of nanocrystalline copper deposits produced by pulse electrodeposition, Nanotechnology, vol. 17, pp. 65-78. Tjong, S.C., and Chen, H. (2004), Nanocrystalline materials and coatings, J. Mater. Sci. and Eng., Vol. 45, pp. 1-88.  Tochitskii, T.A., Shadrow, V.G., Nemtsevich, L.V., and Boltushkin, A.V., (1996) On the Mechanism of Metastable Phases' Formation in Electrodeposited Co-based Films, Crystal Research and Technology, vol. 31, issue 5, pp. 583-588. Toepffer, H. W. (1899) Z. Elektrochem.,  vol. 25, no. 6, pp. 342-344. Tsutsumi, Y., Nishimura, D., Doi, H., Nomura, N., Hanawa, T. (2010), Cathodic alkaline treatment of zirconium to give the ability to form calcium phosphate. Acta Biomaterialia, vol. 6, pp. 4161-4166.  Umebayashi, T., Yamaki, T., Itoh, H., and Asai, K. (2002), Band gap narrowing of titanium dioxide by sulphur doping, Appl Phys Lett, vol. 81, pp. 454-456. 	  	    223 Valensi, G., Van Muylder, J., Pourbaix, M., in: M. Pourbaix (Ed.) (1974), Proceedings of the Atlas of Electrochemical Equilibria in Aqueous Media, NACE, Houston, TX, pp. 545. Valiev, R.Z., Islamgaliev R.K., Alexandrov, I.V. (2000), Bulk Nanostructured Materials from Severe Plastic Deformation, Prog. Mater. Sci, vol. 45, pp. 103-189.  Vasilakopoulos, D., Bouroushian, M., Spyrellis, N., Mater. J. (2006), Texture and morphology of pulse plated zinc electrodeposits Sci, vol. 41, pp. 2869-2875. Vinogradov A., Mimaki, T., Hashimoto, S., Valiev, R. (1999).On the corrosion behaviour of ultra-fine grain copper, Scripta Materialia, vol. 41, pp. 319-326. Virginia, Costa, K. (1997), Parameters influencing the electrodeposition of Ni-Fe alloys, Surface and Coatings Technology, vol. 96, pp. 135-139.  Wagner, C.D. (1979) Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corp. Wang, L., Gao, Y, Xu, T., and Xue, Q. (2006), A comparative study on the trobological behavior of nanocrystalline nickel and cobalt coatings correlated with grain size and phase structure, Materials Chemistry and Physics, vol.99, no.1, pp. 96-103. Wang, L., Zhang, J., Gao, Y., Xue, Q., Hu, L., Xu, T. (2006). Grain size effect in corrosion behavior of electrodeposited nanocrystalline Ni coatings in alkaline solution. Scripta. Mater, vol. 55, issue 7, pp. 657-660. Wang, L., Lin, Y., Zeng, Z., Liu, W., Xue, Q., Hu, L., Zhang, J. (2007), Electrochemical corrosion behavior of nanocrystalline Co coatings explained by higher grain boundary density, Electrochimica Acta, vol. 52, Issue 13, pp. 4342-4350 	  	    224 Wang, S., Zhang, B., Zhao, C., Li, S., Zhang, M., Yan, L. (2011), Valence control of cobalt oxide thin films by annealing atmosphere, Appl Surf Sci, vol.  257, pp. 3358-3362. Wang, S.G., Shen, C.B., Long, K., Yang, H.Y., Wang, F.H. (2005). Journal Physics Chemistry B, vol. 109, pp. 2499-2503. Wang, Z.B., Tao, N.R., Tong, W.P., Lu, J., Lu, K. (2003). Preparation and electrochemical corrosion behaviour of bulk nanocrystalline ingot iron in HCl acid solution, Acta Material, vol. 51, pp. 4319. Warraky, E.l.,  A. A. (2003). The effect of sulphide ions on the corrosion inhibition of copper in acidic chloride solutions, Anti-Corrosion Methods and Materials, vol. 50, pp. 40. Warcholinski, B., Gilewicz, A., Kuklinski, Z., and Myslinski, P. (2010), Hard CrCN/CrN multilayer coatings for tribological applications, Surface and Coatings Technology, vol. 204, no.14, pp. 2289-2293. Wei, L., Ping, H., Kaikai, L., Pengfei, Y., Yuxing, W., and Biao, Y. (2013) Effect of Bath Temperature on the Microstructural Properties of Electrodeposited Nanocrystalline FeCo Films. International Journal of Electrochemical Science, vol. 8, pp. 2354-2364. Wei, G., Ge, H., Zhu, X., Wu, Q., Yu, J., Wang. B. (2007), Effect of organic additives on characterization of electrodeposited Co-W thin films, Applied Surface Science, Vol. 253, Issue 18, pp. 7461-7466  Witkin, D.B., and Lavernia, E.J., (2006), Synthesis and mechanical behavior of nanostructured materials via cryomilling, Prog. Mater. Sci, vol. 51, pp. 1-60. 	  	    225 Wu, B, Y, C. (2002) Synthesis and characyerization of nanocrystalline alloys in the binary Ni-Co system. M. Sc. Thesis, University of Toronto. Xinwei, C., and Weixing, C. (2008), Saccharin Effects on Direct-Current Electroplating Nanocrystalline Ni–Cu Alloys, J Electrochem Soc, vol. 155, pp. K133-K139. Xu, H., Dinan, T.E., Cooper, E, I., Romankiw, L.T., Bonhote, C., and Miller, D. (2001), Electrochemical Proceedings, 2001-08, pp. 346.  Yamasaki, T., Schlossmacher, P., Ehrlich, K., Ogino, Y., Formation of amorphous electrodeposited Ni–W alloys and their nanocrystallization. Nanostruct Mater., 1998, vol. 10, pp. 375–88. Yamashita, M., Mimaki, S., Hashimoto, S., Miura H., (1991). Intergranular corrosion of copper and alpha-Cu–Al alloy bicrystals, Philos. Mag. A, vol. 63, pp 707. Youjun, Yu., Jiansong, Zhou., Jianmin, Chen., Huidi, Zhou., Chun, Guo., Baogang, Guo. (2010), Synthesis of nanocrystalline Ni3Al by mechanical alloying and its microstructural characterization, Journal of Alloys and Compounds, vol. 498, pp. 107-112.  Youssef, K.M.S., Koch, C., Fedkiw, P. (2004), Influence of additives and pulse electrodeposition parameters on production of nanocrystalline zinc from zinc chloride electrolytes. J. Electrochem. Soc, vol. 151, pp. 103-111.  Youssef, Kh.M.S., Koch, C.C., and Fedkiw, P.S. (2004), Improved corrosion behavior of nanocrystalline zinc produced by pulse-current electrodeposition, Corros. Sci., vol. 46, issue 1, pp. 51-64  	  	    226 Yu, J.K., Han, E.H., Lu, L., and Wei, X.J. (2005). Corrosion behaviors of nanocrystalline and conventional polycrystalline copper,	   Journal of Material Science, vol. 40, pp. 1019-1022. Yu, R.H., Basu, S., Ren, L., Zhang, Y., Parvizi-Majidi, A., Unruh, K.M., and Xiao, J. Q. (2000),  High temperature soft magnetic materials: FeCo alloys and composites, IEEE Transactions on Magnetics, vol.36, no. 5, pp. 3388-3393.  Yu, Y., Zhou, J., Chen, J., Zhou, H. Guo, C., Guo, B. (2010), Synthesis of nanocrystalline Ni3Al by mechanical alloying and its microstructural characterization, J. Alloys Compounds, vol. 498, pp.107-112.  Zabinski, P.R., Meguro, S., Asami, K., and Hashimoto, K. (2006), Electrodeposited Co-Ni-Fe-C Alloys for Hydrogen Evolution in a Hot 8 kmol.m-3 NaOH,  Materials Transactions, vol. 47, no. 11, pp. 2860-2866. Zech, N., Podlaha, E, J., and Landolt, D. (1999), Anomalous Co-deposition of Iron Group Metals: II. Mathematical Model, J Electrochem Soc, vol. 146 pp. 2892-2900. Zhang, Y., and Ivey, D.G. (2007) Characterization of Co-Fe and Co-Fe-Ni soft magnetic films electrodeposited from citrate-stabilized sulphate baths, Materials Science and Engineering B, vol. 140, no. 1-2, pp. 15-22. Zhao, H., Liu, L., Zhu, J., Tang, Y., Hu, W., (2007) Microstructure and corrosion behavior of electrodeposited nickel prepared from a sulphamate bath, Mater. Lett, vol. 61, pp. 1605-1608. Zhou, S., Liu, Q., and Ivey, D.G. (2008), IEEE International Nanoelectronics Conference, pp. 474-479. 	  	    227 Zhou, X.S., M.Sc Thesis, Electrodeposition of Nanocrystalline Co, Fe and CoFe Soft Magnetic Films from Ammonium Citrate Solutions, University of Alberta, 2009.   	  	    228 Appendix	  A	   	  	  Reproducibility of Potentiodynamic Polarization Experiments    	  	    229   Figure A.1 Reproducibility test in 0.1M H2SO4 solution for potentiodynamic polarization of deposited nanocrystalline; (a) pure Co, (b) Co 5wt%Fe, (c) Co 11wt%Fe and (d) Co 25wt%Fe. 	  	    230     	  	    231   Figure A.2 Reproducibility test in 0.1 M NaOH solution for potentiodynamic polarization of deposited nanocrystalline: (a) pure Co, (b) Co 5wt%Fe, (c) Co 11wt%Fe and (d) Co 25wt%Fe.  	  	  	    232 Appendix	  B	  Electrochemical Quartz Crystal Microbalance The EQCM is a very sensitive device that has been used in many electrochemistry applications. This device is now recognized as a powerful electro-analytical technique in in-situ mass detection and interfacial investigation. This is mainly due to its capability of detecting small mass changes on conductive surfaces. The EQCM is generally used in conjunction with other electrochemical measurements such as cyclic voltammetry, chronoamperometry, chronopotentiometry etc. Valuable information could be obtained for example the type and amount of various species that exist at the electrode/solution interface, the relationship between structure and function in the electrochemical field as well as the kinetic and thermodynamic information of the reactions. The mechanism involved for its in-situ capability is mainly attributed to the unique properties of piezoelectric materials where this material has the ability to induce a stress in the material by inducing a charge imbalance across the material surface. For this particular study, the AT cut piezoelectric quartz wafers were utilized. The piezoelectric was sputtered with conductive material on both sides of the wafer. In general this is often referred as resonators. There are many types of resonators available such as NT, DT, BT and MT cut resonators The AT cut resonators are cut at 35.25O from the optic axis of the quartz. The type of cut 	  	    233 for each resonators effect the temperature stability, mechanical stability and operable frequency range of these resonators. The EQCM measures the mass change of the resonators by measuring the change in frequency of the quartz crystal. Changes of the frequency are associated to the addition or removal of a certain mass due to dissolution or film deposition at the surface of the resonators. It is extremely useful in monitoring the rate of deposition or decay of a thin coating in an aqueous solution. The Sauerbrey equation relates the mass change per unit area at the surface resonator to the observed change in oscillation frequency of the crystal according to the equation below: Δf = -Cm Δm     (B1) where Δf is the observed frequency change in Hz, Cm is the Sauerbrey constant, and Δm is the change in mass per unit area. From this equation it can be observed that an increase in mass results in a negative frequency shift. An alternating charge is applied to the resonators during the EQCM measurements so that the material is resonating at a specific frequency. The resonant frequency of the AT cut used in this study was 9 MHz. Generally, the EQCM is coupled with a potentiostat in order to simultaneously observe the mass changes and other electrochemical measurements.  	  	    234 B 1  Determination of the Sauerbrey Constant  Valid interpretation of the Sauerbrey equation is important thus the experimental calibration of the Sauerbrey constant, Cm must be carefully carried out to ensure accurate mass-frequency correlation. Few reports have determined some practical procedures in calibrating the EQCM that employs the metal deposition. In this study, the Cu deposition was carried out to calibrate the QCM922. Voltammetric stripping of the deposited Cu layer on the titanium resonators was performed at potential more anodic than the equilibrium deposition potential of Cu. The Cu deposition was done from a 2mM CuSO4 and 0.1 M H2SO4 solution at a 0.3 V for 20 min. A blank cyclic voltammetry scan of the titanium in the 0.1 M H2SO4 was also carried out. The charge of the Cu dissolution was measured relative to the blank scan. The frequency change was then plotted against the charge associated with the anodic Cu dissolution. The Sauerbrey constant was then calculated from the slope using equation below: Δf = -106 Cm ηΔQMW                           nF    (B2) where η is the current efficiency = 1, ΔQ is the charge due to anodic Cu dissolution (C cm-2), MW is the molecular weight of Cu (g mol-1), n is the number of electrons transferred (n = 2), and F is the Faraday constant (96485 C mol-1). Factor 106 is the commonly used unit for Cm of Hz cm2 µg-1. 

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