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Titanium corrosion in alkaline hydrogen peroxide environments Been, Jantje 1998

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TITANIUM CORROSION IN ALKALINE HYDROGEN PEROXIDE ENVIRONMENTS by  JANTJE BEEN B . A . S c , T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1986 M . A . S c . , The University o fBritish C o l u m b i a , 1989  A THESIS SUBMITTED INPARTIAL F U L F I L L M E N T O F THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in THE F A C U L T Y OFG R A D U A T E METALS A N D MATERIALS  STUDIES  ENGINEERING  W e a c c e p t t h i s t h e s i s as c o n f o r m i n g to the required standard  THE UNIVERSITY OFBRITISH October 1998 © J a n t j e B e e n , 1998  COLUMBIA  In  presenting  this  degree at the  thesis  in  partial fulfilment  of  University of  British Columbia,  1 agree  freely available for reference copying  of  department  this or  and study.  thesis for scholarly by  publication of this  his  or  her  the  requirements that the  I further agree  purposes  representatives.  may be It  thesis for financial gain shall not  is  that permission for extensive granted  by the  allowed  that without  permission.  Department of  Mpkls  CX^A M ^ W . V x U  The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  lo/\c^ /c[K  advanced  Library shall make it  understood be  for an  E^^eavvVg  head  of  my  copying  or  my written  Abstract  The corrosion o f Grade 2 titanium in alkaline hydrogen peroxide environments s t u d i e d b y w e i g h t l o s s c o r r o s i o n tests, e l e c t r o c h e m i c a l i m p e d a n c e linear polarization resistance ( L P R )  measurements  has been  spectroscopy  and potentiodynamic  (EIS),  polarography.  C a l c i u m i o n s a n d w o o d p u l p w e r e i n v e s t i g a t e d as c o r r o s i o n i n h i b i t o r s .  In  alkaline  peroxide,  the  titanium  corrosion  rate  temperature, and h y d r o g e n peroxide concentration.  increased  of  test c o u p o n s  exposed  inhibition is probably  to  increasing  The corrosion controlling  is t h o u g h t to b e the r e a c t i o n o f the o x i d e w i t h the p e r h y d r o x y l  N o evidence of thermodynamically  with  pH,  mechanism  ion.  stable c a l c i u m titanate w a s f o u n d i n the surface f i l m  calcium-inhibited  alkaline peroxide  solutions.  Calcium  t h e r e s u l t o f l o w l o c a l a l k a l i a n d p e r o x i d e c o n c e n t r a t i o n s at t h e  metal surface produced b y reaction o f adsorbed c a l c i u m w i t h h y d r o g e n peroxide.  It h a s  b e e n s h o w n that the i n h i b i t i n g effect o f c a l c i u m is t e m p o r a r y , p o s s i b l y t h r o u g h a n effect o f c a l c i u m o n the c h e m i c a l and/or p h y s i c a l stability o f the surface o x i d e .  Pulp  is an  decreased  effective the  and  corrosion  stable c o r r o s i o n rate.  The  inhibitor.  inhibiting  adsorption a n d interaction o f the p u l p fibers w i t h  effect  Raising  the p u l p  of pulp  may  H2O2,  c o n c e n t r a t i o n a n d rendering the s o l u t i o n less corrosive.  be  concentration related  to  the  thereby decreasing the peroxide The presence o f both pulp  and  c a l c i u m l e d to h i g h e r c o r r o s i o n rates than o b t a i n e d b y either o n e i n h i b i t o r alone.  Replacement investigated.  of hydrofluoric  acid with alkaline peroxide  for p i c k l i n g o f titanium  was  T i t a n i u m c o r r o s i o n rates i n a l k a l i n e p e r o x i d e e x c e e d e d those o b t a i n e d  the c o n v e n t i o n a l h y d r o f l u o r i c a c i d bath.  in  General corrosion was observed w i t h extensive  r o u g h e n i n g o f the surface g i v i n g a d u l l gray  appearance.  ii  Abstract Preferred  dissolution o f certain crystallographic  corrosion o f a titanium single crystal. was  s m a l l , the plane  was  investigated  through  the  W h e r e a s the o v e r a l l effect o n the c o r r o s i o n rate  c l o s e to a p r i s m a t i c p l a n e  corrosion resistant plane.  planes  to b e the  most  T h e p l a n e c l o s e to a b a s a l p l a n e orientation suffered  from  extensive p i t t i n g a n d r o u g h e n i n g o f the surface.  iii  orientation  appeared  Table of Contents  Abstract  ii  Table o f Contents  iv  List of Tables  viii  List of Figures  ix  Acknowledgement  xv  1  Introduction  1  2  Background  2  2.1  Oxide Covered M e t a l Electrodes  2  2.1.1  2  3  E l e c t r o n T r a n s f e r at O x i d e - C o v e r e d M e t a l E l e c t r o d e s  2.2  T i t a n i u m Crystal Structure  4  2.3  Titanium Oxide Layer  5  2.4  T i t a n i u m C o r r o s i o n i n the Presence o f H 0  2.5  Surface Cleaning  11  2.6  Hydrogen Peroxide Bleaching of Pulp W o o d Fibers  12  2.7  C a l c i u m Ion Inhibition  13  2.8  Perovskite  15  2  2  7  Thermodynamic Equilibria  17  3.1  Titanate Perovskite Formation  17  3.2  Solution Equilibria in Aqueous Calcium-Peroxide Solutions  25  iv  Table of Contents  4  Research Objectives  28  5  Experimental Apparatus and Procedures  30  5.1  Materials and Solutions  30  5.1.1  Pickle Bath Solutions  32  5.1.2  Titanium Single Crystal  33  5.2  Electrochemical Tests  34  5.3  Electrochemical Impedance Spectroscopy  37  5.3.1  42  5.4  6  V a l i d i t y o f the I m p e d a n c e D a t a  Surface Morphology  43  Results and Discussion  44  6.1  Titanium Corrosion in Alkaline Peroxide  44  6.1.1  Effects of H 0  44  6.1.2  Corrosion Mechanism  48  6.1.3  Surface Morphology  51  6.1.4  Surface Roughness  55  6.1.5  Cold Work  58  6.1.6  Pickle Bath Conditions  59  6.1.6.1  Addition of Calcium  61  6.1.6.2  Thermal Oxidation  64  6.1.7 6.2  6.3  2  2  Concentration, p H , and Temperature  Titanium Single Crystal Corrosion  66  Inhibition by C a l c i u m  71  6.2.1  Effect o f Carbonate  81  6.2.2  Surface Roughness  86  6.2.3  Surface Morphology  88  Inhibition by Pulp  91  6.3.1  V e l o c i t y effects  95  6.3.2  Surface Morphology  96  v  Table of Contents  7  Conclusions  98  8  Recommendations for Further Research  102  9  Nomenclature  104  Bibliography  107  A  Thermodynamic Data  114  A . l Determination of Thermodynamic Data  114  A . 1.1  Ti, TiH , TiO, CaTi0 , 3CaO-2Ti0 , 4CaO-3Ti0 , Ca, Ca 2  3  2  2  + 2  ,  CaH , Ca(OH) , Ca0 , OH"  114  A . 1.2  Ti  115  A . 1.3  TiO  A . 1.4  T i 0 H 0 andTi(OH)  A . 1.5  H 0  2  + 2  andTi + 2  2  2  2  2  , Ti0  2  + 3  2  + 2  , and H T i C y  2  and C a ( O H )  116 118  3  118  +  A.2 Quickbasic Computer Program C A P E R  119  A.2.1  Equilibrium Constant o f H 0  119  A.2.2  Equilibrium Constant o f Hydrogen Peroxide  120  A.2.3  The Dissolution of Calcium Hydroxide  120  A.2.4  The Formation of Calcium Peroxide  122  A.2.5  Listing of Quickbasic Computer Program C A P E R  124  A.2.6  The Solubility o f C a l c i u m Carbonate  128  A.2.7  Listing of Quickbasic Computer Program C A P E R . v 2  130  2  B  Electropolishing Titanium  134  C  Determination o f Titanium Single Crystal Orientation  136  C.l  The Back-Reflection Laue Method  136  C.2 Orientation of Titanium Single Crystal  140  vi  Table of Contents  D  Surface Analysis  151  D.l  Auger Electron Spectroscopy  151  D.2  X - r a y Photoelectron Spectroscopy ( X P S )  152  D.3  Secondary ion mass spectroscopy  154  vii  List of Tables  2.1  L a t t i c e d i m e n s i o n s o f t i t a n i u m o x i d e s rutile a n d anatase.  3.2  T h e r m o d y n a m i c data.  19  5.3  C h e m i c a l c o m p o s i t i o n o f G r a d e 2 titanium plate i n weight percent.  30  5.4  L e g e n d o f F i g u r e s 5.15, 5.16, and 5.17.  41  C.5  A n g l e s b e t w e e n i d e n t i f i e d p o l e s o f s u r f a c e 1.  147  C.6  M e a s u r e d angles b e t w e e n p o l e s o f surface 2.  148  C.7  A n g l e s b e t w e e n i d e n t i f i e d p o l e s o f surface 3.  149  viii  6  L i s t of Figures  2.1  H e x a g o n a l lattice s h o w i n g the p o s i t i o n o f slip and t w i n n i n g planes.  5  2.2  A p p r o x i m a t e l i m i t s for useful c o r r o s i o n resistance (<0.13 m m / y ) o f  14  G r a d e 2 t i t a n i u m i n a l k a l i n e s o l u t i o n s c o n t a i n i n g u p to 0.3 w t % H 0 . 2  2  2.3  Orthorhombic perovskite C a T i 0  3.4  T h e p h a s e s t a b i l i t y d i a g r a m o f the C a - H 0 s y s t e m w i t h a c a l c i u m i o n  3  structure.  15  2  20  a c t i v i t y o f 10" . 4  3.5  T h e phase stability d i a g r a m o f the T i - H 0 s y s t e m w i t h a t i t a n i u m i o n 2  21  a c t i v i t y o f 10" . 6  3.6  T h e p h a s e s t a b i l i t y d i a g r a m o f t h e T i - C a - H 0 s y s t e m at 2 5 ° C w i t h a 2  22  t i t a n i u m i o n a c t i v i t y o f 10" . 6  3.7  T h e p h a s e s t a b i l i t y d i a g r a m o f t h e T i - C a - H 0 s y s t e m at 6 0 ° C w i t h a 2  23  t i t a n i u m i o n a c t i v i t y o f 10" . 6  3.8  T h e p h a s e s t a b i l i t y d i a g r a m o f t h e T i - C a - H 0 s y s t e m at 1 0 0 ° C w i t h a 2  24  t i t a n i u m i o n a c t i v i t y o f 10" . 6  3.9  Solution Equilibria in Aqueous Calcium-Peroxide Solutions.  26  3.10  T h e effect o f temperature and the addition o f 100 p p m c a l c i u m o n the  26  perhydroxyl ion concentration. 5.11  E q u i a x e d alpha structure w i t h beta spheroids stabilized b y 0 . 1 3 % Fe.  30  5.12  S c h e m a t i c o f the c o r r o s i o n test c e l l .  32  S c h e m a t i c o f single crystal t i t a n i u m electrode illustrating the p o s i t i o n s  33  5.13  o f the three o r t h o g o n a l surfaces. 5.14  Equivalent circuit m o d e l o f a corroding titanium surface i n alkaline  38  peroxide suspension. 5.15  A s the c o r r o s i v i t y o f the s o l u t i o n increases, the charge transfer c a p a c i t a n c e increases a n d the m a x i m u m phase angle shifts to h i g h e r frequencies.  ix  39  List of Figures  5.16  B o d e plot o f electrochemical data i n various solutions o f different  40  c o r r o s i o n strengths. 5.17  N y q u i s t plots o f electrochemical data i n various solutions o f different  41  c o r r o s i o n strengths. 6.18  T h e t i t a n i u m c o r r o s i o n rate increases w i t h i n c r e a s i n g h y d r o g e n p e r o x i d e  45  concentration, temperature and p H . 6.19  A t m i l d l y c o r r o s i v e c o n d i t i o n s , at 5 0 ° C , t h e c o r r o s i o n rate d e c r e a s e s w i t h time.  6.20  46  W e i g h t l o s s c o r r o s i o n rates are i n d i c a t e d i n b o l d .  T h e d o u b l e layer capacitance i s greater i n a m o r e c o r r o s i v e  46  environment. 6.21  A n o d i z a t i o n does n o t i m p r o v e the t i t a n i u m c o r r o s i o n resistance i n 0.15 M  6.22  H 0 2  2  at p H 1 1 , 5 0 ° C .  S u r f a c e m o r p h o l o g y o f a t i t a n i u m c o u p o n after e x p o s u r e to a n a l k a l i n e peroxide solution o f p H 10,0.15M H 0 2  6.23  2  2  2  2  52  at 5 0 ° C f o r 6 . 4 h r s . 53  at 5 0 ° C f o r 3 . 2 5 h r s .  S u r f a c e m o r p h o l o g y o f a t i t a n i u m c o u p o n after e x p o s u r e t o a n a l k a l i n e peroxide solution o f p H 11, 0.25M H 0  6.25  2  S u r f a c e m o r p h o l o g y o f a t i t a n i u m c o u p o n after e x p o s u r e to a n a l k a l i n e peroxide solution o f p H 11, 0.15M H 0  6.24  48  53  at 7 0 ° C f o r 2 . 6 h r s .  A higher magnification clearly shows m a n y cusps a n d general corrosion  54  o f the g r a i n surfaces. 6.26  G r a i n b o u n d a r y c o r r o s i o n d i d n o t f o r m c r e v i c e s e x t e n d i n g b e y o n d the  54  top layer o f grains, light microscope 800x 6.27  T h e p o l a r i z a t i o n curves o f electropolished and 6 0 0 grit samples i n 0.15 M H  6.28  2  0  2  , p H 1 1 , 5 0 °C.  T h eE I S data o f electropolished and 6 0 0 grit samples i n 0.15 M H 0 , 2  pH  55  2  56  11,50°C.  6.29  T h euncorroded electropolished sample has a few cusps.  56  6.30  T h e 6 0 0 grit surface i s m u c h rougher.  57  6.31  C o r r o s i o n o f the 6 0 0 grit surface appears to s m o o t h e n the surface.  57  x  List of  6.32  Figures  T h e c o l d w o r k e d s u r f a c e after e x p o s u r e to 0.15 M H 0 , p H 11, 5 0 °C' 2  2  58  for 3 hrs. 6.33  W i d e s c a l e p i t t i n g l e d to a general c o r r o s i o n appearance i n a h y d r o f l u o r i c  60  acid bath. 6.34  R o u g h e n i n g o f the g r a i n surface leads to a d u l l appearance i n an a l k a l i n e  60  peroxide bath. 6.35  G r a i n b o u n d a r y c o r r o s i o n i n an alkaline p e r o x i d e solution o f 2.5 M N a O H , 0.5 M  6.36  H 0 , 95 2  2  61  °C.  T h e t i t a n i u m c o r r o s i o n rate d e p e n d s l i n e a r l y o n the h y d r o g e n p e r o x i d e  62  concentration. 6.37  A t i t a n i u m c o r r o s i o n r a t e o f 4 1 5 m m / y w a s m e a s u r e d i n 0.1 M H 0 , 2 . 5 2  M 6.38  N a O H , at 9 5  63  °C.  I n t h e p r e s e n c e o f 0.1 M  CaC0  3  a n d 0.2 M H 0 2  2  ,the c o u p o n  m o r p h o l o g y a n d c o r r o s i o n r a t e r e s e m b l e t h a t o b t a i n e d i n 0.1 M 6.39  2  64 H 0 . 2  2  I n a h y d r o f l u o r i c a c i d p i c k l e bath, surface a l p h a case leads to  65  widespread cracking. 6.40  E x t e n s i v e roughening o f an alpha case titanium surface is observed i n an  66  alkaline peroxide pickle bath 6.41  C r y s t a l o r i e n t a t i o n s o f s i n g l e c r y s t a l s u r f a c e s 1, 2 , a n d 3 .  67  6.42  T h e c o r r o s i o n rate o f surface 2 is s o m e w h a t l o w e r t h a n that o f the other  68  surfaces. 6.43  S E M p h o t o g r a p h o f c o r r o d e d s i n g l e c r y s t a l S u r f a c e 1 at x 5 . 0 K .  69  6.44  S E M p h o t o g r a p h o f c o r r o d e d s i n g l e c r y s t a l S u r f a c e 2 at x 5 . 0 K .  69  6.45  F a c e t t e d s u r f a c e c o n e s o n s i n g l e c r y s t a l s u r f a c e 3 at x 1 . O K .  70  6.46  S E M p h o t o g r a p h o f c o r r o d e d s i n g l e c r y s t a l S u r f a c e 3 at x 4 . 0 K .  70  6.47  Inhibition by calcium.  72  6.48  T h e i n h i b i t i v e e f f e c t o f c a l c i u m i s s h o r t - l i v e d at m o r e c o r r o s i v e  73  conditions. 6.49  T h e effect o f c a l c i u m o n the charge transfer capacitance.  xi  74  List of Figures  6.50  C a l c i u m affects the t i t a n i u m o x i d e rendering it less c o r r o s i o n resistant.  76  6.51  In 0.15 M  77  H 0 , p H 10, 5 0 °C, the c o r r o s i o n rate appears i n d e p e n d e n t o f 2  2  p r e v i o u s e x p o s u r e to c a l c i u m . 6.52  In 0.15 M  H 0 , p H 10, 5 0 °C, w i t h 100 p p m C a , p r e v i o u s e x o s u r e to 2  2  78  c a l c i u m affected the t i t a n i u m o x i d e r e n d e r i n g it less c o r r o s i o n resistant. 6.53  C o r r o s i o n p o t e n t i a l i n 0 . 1 5 M H 0 , p H 10, 5 0 °C.  6.54  C o r r o s i o n p o t e n t i a l i n 0.15 M H 0 , p H 10, 5 0 °C,  6.55  C h a r g e transfer c a p a c i t a n c e s i n 0.15 M H 0 , p H 10, 50°C, 100 p p m C a .  80  Addition of C a C 0  82  6.56  2  2  In 0.15 M  100 p p m C a .  2  2  i n 0.15 M H 0 , p H 10, 5 0 2  2  H 0 , p H 11, 7 0 °C, the a d d i t i o n o f C a C 0 2  79  d e c r e a s e s t h e c o r r o s i o n r e s i s t a n c e at a f a s t e r r a t e  than the a d d i t i o n o f C a ( O H ) 6.57  2  2  3  79  2  2  3  °C.  virtually  83  eliminates all protection offered b y c a l c i u m in ~ 2 hours. 6.58  A t 50 C , a solution w i t h 0.15 M H 0 , 100 p p m C a i n e q u i l i b r i u m w i t h 2  air, c o n t a i n e d C a C 0  3  2  below a p H o f - 1 0 . 3 and C a 0  2  above a p H  84  of  -10.3. 6.59  A t a p H o f 10, a s o l u t i o n w i t h 0.15 M H 0 , 100 p p m C a i n e q u i l i b r i u m 2  w i t h air, contained C a C 0 68  3  2  at t e m p e r a t u r e s b e l o w 6 8 ° C a n d C a 0  2  84  above  °C.  6.60  I n c r e a s e d s u r f a c e r o u g h n e s s leads to a h i g h e r c o r r o s i o n rate.  87  6.61  N o p r o t e c t i o n i s o b s e r v e d at a l l o n 6 0 0 g r i t s a m p l e s w h e r e i n h i b i t i o n i s  88  marginal o n electropolished samples. 6.62  S m a l l pits are o b s e r v e d o n a c o u p o n c o r r o d e d i n 0.15 M H 0 , p H 10, 2  5 0 °C, 6.63  2  89  1 0 0 p p m C a f r o m C a ( O H ) , at a r a t e o f 0 . 0 1 m m / y f o r 7 h o u r s . 2  C o u p o n corroded i n 0.15 M  H 0 , p H 10, 5 0 °C, 2  2  100 p p m C a f r o m  89  100 p p m C a f r o m  90  C a C 0 , at a r a t e o f 0 . 0 3 m m / y f o r 4 h o u r s . 3  6.64  C o u p o n c o r r o d e d i n 0.25 M H 0 , p H 1 1 , 7 0 °C, 2  Ca(OH) 6.65  2  2  at a r a t e o f 6 . 6 5 m m / y f o r 2Vi  hours.  A n a c c e p t a b l e c o r r o s i o n rate is o b t a i n e d i n the presence o f 1 % p u l p .  xii  91  List of Figures  6.66  1 % p u l p i s a s e f f e c t i v e as 1 0 0 p p m o f c a l c i u m at p H 1 1 , 0 . 1 5 M H 0 , 2  and 50 6.67  2  92  °C.  T h e corrosion resistance decreases rapidly w h e n both c a l c i u m and pulp  93  are present. 6.68  T h e t i t a n i u m c o r r o s i o n rate i n a 1 % p u l p slurry is independent f r o m the  94  s a m p l e ' s c o r r o s i o n history or surface treatment. 6.69  I n c r e a s i n g v e l o c i t i e s d o not appear to affect the t i t a n i u m c o r r o s i o n rate.  95  6.70  S m a l l g r a i n b o u n d a r y p i t s a r e p r e s e n t at 5 0 ° C , p H 1 1 , 0 . 1 5 M  96  H 0 , and 2  2  0 . 2 % pulp. A . 71  E q u a t i o n A . 2 7 gives a reasonable fit o f the water e q u i l i b r i u m constant  119  as a f u n c t i o n o f i o n i c s t r e n g t h a n d t e m p e r a t u r e . A . 72  Ionization constants o f carbonic acid.  129  A.73  S o l u b i l i t y o f c a l c i u m c a r b o n a t e as a f u n c t i o n o f t e m p e r a t u r e .  129  C.74  B a c k - r e f l e c t i o n L a u e pattern o f planes o f a zone i n a crystal.  137  C.75  G r e n i n g e r ' s c h a r t g r a d u a t e d i n 2° i n t e r v a l s i n t h e s i z e f o r 3 - c m d i s t a n c e  138  f r o m s p e c i m e n to f i l m . C.76  M e a s u r e m e n t o f the zone coordinates o f a zone axis.  139  C.77  T r a n s f e r r i n g the z o n e coordinates to a stereographic p r o j e c t i o n .  140  C.78  L a u e p a t t e r n o f s u r f a c e 1.  141  C.79  S t e r e o g r a p h i c p r o j e c t i o n o f s u r f a c e 1.  142  C.80  L a u e pattern o f surface 2.  143  C.81  S t e r e o g r a p h i c p r o j e c t i o n o f surface 2.  144  C.82  L a u e p a t t e r n o f surface 3.  145  C.83  S t e r e o g r a p h i c p r o j e c t i o n o f surface 3.  146  C. 84  Single crystal surface orientation i n a stereographic triangle.  150  D. 85  A n A u g e r spectrum o f a titanium weight loss c o u p o n corroded i n 0.15  152  M  H 0 , p H 10, 5 0 °C, 2  2  100 p p m C a .  xiii  List of Figures  D.86  X P S analysis shows very similar binding energy profiles for samples  153  c o r r o d e d i n c a l c i u m free o r c a l c i u m i n h i b i t e d solutions. D.87  A c a l c i u m p r o f i l e is present i n the s a m p l e w h i c h w a s c o r r o d e d i n c a l c i u m free 0.15 M H 0 , p H 11, 70 2  D.88  2  2  156  °C.  S a m p l e w h i c h w a s c o r r o d e d i n c a l c i u m i n h i b i t e d 0.15 M H 0 , p H 10, 2  50  155  °C.  S a m p l e w h i c h w a s c o r r o d e d i n c a l c i u m i n h i b i t e d 0.15 M H 0 , p H 11, 70  D.89  2  °C.  xiv  2  156  Acknowledgement  My  sincere thanks to Professor D e s m o n d T r o m a n s for his guidance and support.  I truly  v a l u e the n u m e r o u s c o r r o s i o n d i s c u s s i o n s w e ' v e h a d a n d h o p e that t h e y m a y c o n t i n u e i n the  future.  I also  like to thank  Professor  A l e c M i t c h e l l f o r sharing  h i s extensive  knowledge on titanium and titanium alloys.  I l i k e to g i v e a s p e c i a l thanks to M a r y M a g e r for her i n v a l u a b l e assistance o n the S E M . A l s o a c k n o w l e d g e d are Serge M i l a i r e for w e l d i n g m y m a n y samples and J o a n K i t c h e n , our graduate secretary, for her help w i t h a l l the p a p e r w o r k .  I further l i k e to a c k n o w l e d g e the N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a , P A P R I C A N , a n d the U n i v e r s i t y o f B r i t i s h C o l u m b i a for their f i n a n c i a l support.  xv  Chapter 1  Introduction  Titanium  is a l o wdensity  properties.  material,  It p o s s e s s e s a t o u g h ,  4.5 g/cm , 3  with  tightly adherent  good  strength  a n d toughness  surface o x i d e w h i c h , i f damaged,  q u i c k l y r e f o r m s i n the presence o f an o x y g e n source.  Unalloyed Grade 2 titanium with a  tensile strength o f 3 4 0 M P a a n d a y i e l d strength o f 2 8 0 M P a [1] has b e e n a traditional material o f construction i n pulp and paper chlorine and chlorine d i o x i d e bleach plants based o n its superb c o r r o s i o n resistance i n h i g h l y o x i d i z i n g , l o w p H environments.  In  addition t o the b l e a c h i n g process towers themselves, t i t a n i u m has b e e n u s e d i n storage tanks,  transfer  quantities.  piping,  mixers  F o rexample,  a n d washers  a Kamyr-designed  and,  consequently,  diffusion  pulp  i s present  i n large  thickening and  washing  s y s t e m c o n t a i n s r o u g h l y 4 5 , 0 0 0 k g o f t i t a n i u m [2].  M o r e s t r i n g e n t e n v i r o n m e n t a l r e g u l a t i o n s as w e l l a s m a r k e t p r e s s u r e s t o u s e c h l o r i n e - f r e e b l e a c h i n g p r o c e s s e s h a v e i n c r e a s e d the use o f h y d r o g e n p e r o x i d e a s a b l e a c h i n g Alkaline  hydrogen peroxide  Titanium,  unfortunately,  decomposes  has o n l y  to environmentally  safe water a n d  a limited corrosion resistance to alkaline  agent.  oxygen. peroxide  w h i c h raises serious c o n c e r n for the c o m p a t i b i l i t y o f p e r o x i d e b l e a c h i n g processes w i t h existing titanium bleach plant equipment. magnesium  ions  environments understanding  were  [3,4,5,6].  found  to inhibit  Consequently,  o f titanium  R e c e n t l y , the p r e s e n c e o f c a l c i u m , silicate o r  behavior  titanium  corrosion  it i s necessary to develop  i n hydrogen  reference to the p u l p a n d paper industry.  1  peroxide  i n alkaline a more  solutions,  peroxide  fundamental  with  particular  Chapter 2  Background  2.1  Oxide Covered Metal Electrodes  O x i d e layers o n m e t a l electrodes h a v e a t r e m e n d o u s effect o n the rate o f c h a r g e transfer. W h e n the o x i d e i s s u f f i c i e n t l y t h i c k and stable, electron e x c h a n g e o c c u r s with  the oxide  film.  T h e semi-conductive  properties  o f the oxide  predominantly determine the  current/potential b e h a v i o r o f the m e t a l / m e t a l o x i d e system.  2.1.1  Electron Transfer at Oxide-Covered Metal Electrodes  R e f e r t o M o r r i s o n [ 7 ] a s w e l l as S m i c k l e r a n d S c h u l t z e [ 8 ] f o r a c o m p l e t e d i s c u s s i o n o n electrochemistry at o x i d i z e d m e t a l electrodes.  W h e n the o x i d e offers n o resistance t o  charge transfer, the electron exchange i s under k i n e t i c charge transfer control.  A t pure  k i n e t i c c o n t r o l , the surface c o n c e n t r a t i o n s o f reactants a n d p r o d u c t s e q u a l those o f the bulk solution.  The  c o n t r o l l i n g general kinetic equation f o r a single electron transfer  reaction is:  (2.1)  where - the current density, A / m  2  - the exchange current density, A / m  2  a  - anodic charge transfer coefficient  a  - cathodic charge transfer coefficient - overpotential, V  F  - Faraday's constant, 96,489 C / m o l  2  Chapter 2  Background  R  - G a s L a w constant, 8.3143 J / K - m o l  T  - temperature, K  W h e n the o x i d e i s s u f f i c i e n t l y t h i n (0.4 - 3 n m ) , e l e c t r o n e x c h a n g e o c c u r s b e t w e e n the redox electrolyte and the u n d e r l y i n g metal. or resonance tunneling  v i a intermediate  E l e c t r o n transfer occurs b y direct tunneling  states.  transfer i n o n e step w i t h o u t loss o f energy.  Direct tunneling  consists o f electron  A s a result o f direct tunneling, thin oxides  e x h i b i t c u r r e n t / p o t e n t i a l c h a r a c t e r i s t i c s w h i c h are s i m i l a r to t h o s e o f the b u l k m e t a l . T h e o x i d e functions as a potential energy barrier and the current decreases w i t h i n c r e a s i n g oxide thickness.  T h e probability o f direct tunneling decreases strongly w i t h increasing  o x i d e thickness. W h e r e a s a n a n o d i c overpotential raises the energy barrier for tunneling, a c a t h o d i c o v e r p o t e n t i a l l o w e r s it.  T h e anodic transfer coefficient b e c o m e s smaller w i t h  i n c r e a s i n g o x i d e t h i c k n e s s a n d the c a t h o d i c transfer c o e f f i c i e n t b e c o m e s greater.  In  resonance  tunneling,  electron transfer w i t h t h e b u l k  l o c a l i z e d i n t e r m e d i a t e states w i t h o u t the l o s s o f e n e r g y .  metal  occurs v i a short-lived  T h e s e i n t e r m e d i a t e states c o n s i s t  o f i m p u r i t i e s o r defects a n d m u s t h a v e a n energy e q u a l t o that o f the t u n n e l i n g electron. The  latter has a n energy  resonance  tunneling  at o r c l o s e t o t h e F e r m i  energy  level.  T h eprobability o f  increases w i t h the concentration o f intermediate  states a n d c a n  dominate over direct tunneling.  T h i c k e r o x i d e f i l m s b e h a v e as s e m i c o n d u c t o r s a n d e l e c t r o n t r a n s f e r o c c u r s w i t h the o x i d e layer itself.  E l e c t r o n s c a n b e e x c h a n g e d w i t h the c o n d u c t i o n b a n d o r the v a l e n c e b a n d  d e p e n d i n g o n the p r o x i m i t y o f the F e r m i energy level.  A conduction band mechanism is  characterized b y a large cathodic transfer coefficient (approaching anodic transfer coefficient (approaching characterized  b y a small  cathodic  zero) whereas  transfer  coefficient  one)and a small  a valence band a n d a large  mechanism is anodic  transfer  coefficient.  Resonance tunneling  m a y still b e important  when  the impurity  level is high.  Only  u n o c c u p i e d s t a t e s , w h i c h g e n e r a l l y a r e l o c a t e d a b o v e t h e F e r m i e n e r g y l e v e l , c a n s e r v e as  3  Chapter 2  Background  i n t e r m e d i a t e states. A t a h i g h d e n s i t y o f l o c a l i z e d i n t e r m e d i a t e states, a n i m p u r i t y m a y f o r m b y o v e r l a p p i n g orbitals o f the same energy.  band  Generally, the electron mobility i n  an impurity b a n d is m u c h l o w e r than i n the conduction or valence b a n d and o n l y a limited current c a n b e supported over a s m a l l potential range.  2.2  Titanium Crystal Structure  U n a l l o y e d o r c o m m e r c i a l l y pure A S T M Grade 2 titanium has a hexagonal close-packed (hep) s t r u c t u r e o r a - p h a s e at r o o m t e m p e r a t u r e .  W h e n h e a t e d a b o v e t h e P-transus, a b o d y  c e n t e r c u b i c ( b e c ) o r P-phase f o r m s . T h e P-transus f o r A S T M G r a d e 2 i s 9 1 3 ° C ( 1 6 7 5 °F).  A S T M  Grade 2 titanium contains a m a x i m u m o f 0 . 2 5 % oxygen, 0 . 0 3 % nitrogen,  0 . 3 % i r o n , 0 . 1 % c a r b o n a n d 1 5 0 p p m o f h y d r o g e n [1].  I r o n i s a P-stabilizer, a n d h i g h  i r o n l e v e l s m a y r e s u l t i n t h e f o r m a t i o n o f s o m e s p h e r o i d a l b e t a , p r i m a r i l y at t h e g r a i n boundaries.  T h e cc-crystal structure has a n u n u s u a l l y l o w c / a ratio. Ideally, the c / a ratio is 1.633 based o n close p a c k i n g o f identical spheres.  F o r G r a d e 2 t i t a n i u m , at r o o m  temperature,  typically [78]: a = 0.2950 n m c = 0.4683 n m c / a = 1.587 T h i s l o w ratio m a y b e responsible f o r the h i g h ductility a n d g o o d c o l d formability o f G r a d e 2 t i t a n i u m . S l i p c a n o c c u r o n t h e p r i s m , b a s a l , a n d { 1 0 l 1} p y r a m i d a l p l a n e s . T h e p r i n c i p a l t w i n n i n g p l a n e s are t h e { 1 0 1 2 } , {11 2 1 ) , a n d {11 2 2 } p l a n e s [9].  4  Chapter 2  Background  |0001} basal plane  {1121}.  {10]1|,  11 1 22J, |1012} pyramidal planes  {1010} prism plane  + Q  2  + Q •)  F i g u r e 2.1  H e x a g o n a l lattice s h o w i n g the p o s i t i o n o f slip a n d t w i n n i n g planes.  W h e n submerged i n a corrosive m e d i u m , the titanium surface i s always covered w i t h a , generally, amorphous titanium oxide film. dependent o n t h e integrity o f the oxide.  T h e o v e r a l l d i s s o l u t i o n rate i s v e r y  much  F o r thin oxides w i t h direct electron transfer w i t h  the u n d e r l y i n g m e t a l , the effect o f crystal orientation o f the m e t a l m a y b e significant a n d lead to preferred d i s s o l u t i o n o f s o m e crystal planes.  2.3  Titanium Oxide Layer  Titanium  owes its excellent corrosion resistance i n m a n y  adherent o x i d e f i l m .  B e i n g a reactive metal,  environments  E " ^ . ^ = -1.63 V  S  H  E  to its tightly  [10], without  protective oxide layer, titanium w o u l d dissolve rapidly i n most environments. the o x i d e repairs i t s e l f instantaneously i n the presence o f a n o x y g e n source. acid environments,  this  Generally, I n reducing  severe corrosion c a n b e avoided through the application o f anodic  p r o t e c t i o n w h i c h aids i n the f o r m a t i o n o f a protective surface. F o r e x a m p l e , the c o r r o s i o n rate o f a t i t a n i u m heat e x c h a n g e r i n a 4 0 % s u l f u r i c a c i d e n v i r o n m e n t c a n b e r e d u c e d  5  Chapter 2  Background  11,000 t i m e s t o a rate o f 0.005 m m / y [11]. T o m a s h o v  t h r o u g h t h e a p p l i c a t i o n o f a 2.1 V  overpotential  et al. also o b s e r v e d a n increase i n the protective nature o f the o x i d e f i l m  w i t h i n c r e a s i n g o v e r p o t e n t i a l s u p t o 1.4 V a n d s u g g e s t e d that t h i s w a s the r e s u l t o f a decreasing n u m b e r o f defects a n d decreasing ionic conductivity underlying  corroding  titanium was said to b e hampered  [12].  Access to the  b y a n i n n e r l a y e r o f anatase.  U s i n g e l e c t r o n d i f f r a c t i o n , the p o r o u s o u t e r l a y e r w a s f o u n d to h a v e the r u t i l e structure.  Rutile is themore c o m m o n  titanium dioxide, T i 0  2  , with theT i  o c t a h e d r a l l y w i t h o x y g e n a n i o n s i n a tetragonal structure [18].  4 +  cation  coordinated  I n anatase, the octahedral  structure i s a s o m e w h a t m o r e d i s t o r t e d tetragonal structure [ 1 9 ] (see T a b l e 2.1). t h o u g h anatase i s less c o m m o n , Titanium  dioxide  Even  it i s m o r e stable than rutile b y 8 - 12 k J / m o l [20].  i s used as a white pigment,  as, f o r example,  i n paints.  However,  n a t u r a l l y o c c u r r i n g f o r m s are g e n e r a l l y c o l o r e d gray or b l a c k d u e to i m p u r i t i e s .  T a b l e 2.1  L a t t i c e d i m e n s i o n s o f t i t a n i u m o x i d e s rutile a n d anatase. lattice d i m e n s i o n a, A  lattice d i m e n s i o n c, A  Anatase  3.73  9.37  Rutile  4.59  2.96  A n o d i z a t i o n to i n c r e a s i n g l y h i g h e r potentials c a n t h i c k e n the v e r y t h i n natural o x i d e f r o m about  twenty  potential. V  S H E  angstroms  to several  thousand  angstroms,  depending  o n the applied  A s the t h i c k n e s s increases, the c o l o r o f the o x i d e changes f r o m y e l l o w (~5  ) to blue ( - 2 5 V  interference.  S H E  ) t o g o l d t o p u r p l e t o green [13].  T h e c o l o r s are p r o d u c e d b y  Part o f the l i g h t s t r i k i n g the surface i s r e f l e c t e d a n d part passes t h r o u g h the  transparent o x i d e  film  t o reflect o f f the m e t a l b e l o w .  S o m e light wavelengths m a y  add,  others w i l l c a n c e l g e n e r a t i n g a c o l o r w h i c h i s c h a r a c t e r i s t i c o f that o x i d e t h i c k n e s s . similar  color  spectrum  c a nb e obtained  through  r e p r o d u c i b i l i t y is m o r e d i f f i c u l t to c o n t r o l .  6  thermal  oxidation,  however,  A  color  Chapter 2  Background  T h i c k o x i d e s w e r e t r a d i t i o n a l l y thought to increase the c o r r o s i o n resistance. used  to b e recommended  f o r heat  resistance and limit hydriding.  exchanger  tubing  to improve  Anodization  crevice  corrosion  S t u d i e s later s h o w e d that, a l t h o u g h there w a s a s l i g h t l y  h i g h e r i n i t i a l c o r r o s i o n resistance, the a n o d i z e d surfaces d i d n ' t b e h a v e m u c h better than freshly  p i c k l e d surfaces i n h y d r o c h l o r i c a c i d solutions [14,15].  F u k u z u k i [15]  the h i g h d i s s o l u t i o n o f the a n o d i z e d f i l m t o the fact that i t w a s a h y d r a t e d T i 0 . 2  attributed Ohtsuka  [16] a n d A r s o v [17] i n d e e d f o u n d that, i n 0.5 M H S 0 , at o v e r v o l t a g e s u p t o 2 0 V , the 2  surface o x i d e w a s amorphous o r hydrated.  4  A t higher overpotentials  surface f i l m c o n s i s t e d o f a m i x t u r e o f anatase a n d a m o r p h o u s  from  2 0 - 5 0 V , the  o x i d e [17].  Anatase was  formed b y recrystallization o f hydrated titanium oxide films w i t h increasing overvoltages leading to the formation  o f p r o g r e s s i v e l y larger anatase crystallites.  A t overvoltages  greater t h a n 1 2 0 V , the anatase b e g a n to t r a n s f o r m to r u t i l e [17].  In summary, a  freshly  abraded surface immediately passivates to f o r m a crystalline rutile  and/or anatase o x i d e layer.  D e p e n d i n g o n the environment, this o x i d e m a y be c o v e r e d b y  an a m o r p h o u s o r hydrated surface o x i d e , g i v i n g a t w o - l a y e r o x i d e structure. thickens the o x i d e to g i v e a c o l o r f u l spectrum o f colors.  A s the a n o d i z i n g potential i s  further increased, the surface o x i d e f i l m crystallizes t o f o r m anatase w h i c h transforms t o rutile at e v e n h i g h e r potentials. heating  o f the oxide  and is accompanied  Anodization  subsequently  T h e formation o f rutile results b y sparks  O x i d a t i o n b y heat treatment o n l y gives rutile.  and warming  from  local  o f the solution.  A p p l i c a t i o n o f thermal energy seems to  o v e r c o m e the a c t i v a t i o n energy barrier to g i v e the rutile o x i d e .  2.4  Titanium Corrosion in the Presence of H O 2  Titanium some  z  oxidation/corrosion i n thepresence o f hydrogen peroxide is encountered i n  widely  different  situations, ranging  from  applications.  7  nuclear  storage  systems  to medical  Chapter 2  Background  G r a d e 12 t i t a n i u m h a s b e e n s t u d i e d as a p o s s i b l e c a n i s t e r m a t e r i a l f o r the storage o f h i g h l e v e l n u c l e a r w a s t e s [ 2 1 ] . It w a s f o u n d that c h a n g e s i n t h e t i t a n i u m o x i d e u n d e r y r a d i a t i o n are c a u s e d b y the r a d i o l y s i s p r o d u c t s o f the b r i n e s o l u t i o n a n d not b y the d i r e c t effect o f radiation o n the m e t a l o r oxide.  T h e s e effects w e r e f o u n d t o b e the same as  those o fs m a l l concentrations o f hydrogen peroxide.  In neutral b r i n e solutions at r o o m temperature, K i m and O r i a n i [21] o b s e r v e d a shift o f the o p e n c e l l potential i n the p o s i t i v e , m o r e n o b l e d i r e c t i o n u p o n e x p o s u r e t o y radiation or small concentrations o f H 0 2  2  (~1 x 1 0 " M ) . 4  T h i s shift w a s attributed to the a v a i l a b i l i t y  o f other cathodic reactions such as h y d r o g e n peroxide reduction.  A thicker oxide layer  c o n t a i n i n g m o r e stable anatase w a s p r o d u c e d i n the presence o f H 0 2  2  leading to more  n o b l e c o r r o s i o n potentials a n d l o w e r p a s s i v e currents e v e n after the m e t a l w a s f r o m the H 0 2  2  containing brine  removed  [21].  I n the m e d i c a l f i e l d , t i t a n i u m a n d t i t a n i u m a l l o y s are i n c r e a s i n g l y u s e d i n b i o m a t e r i a l applications based o n titanium's biocompatibility and corrosion resistance.  Titanium is  hard (160 B H N for A S T M G r a d e 2) and generally offers g o o d erosion resistance to pulp slurries.  H o w e v e r , e x c e s s i v e w e a r o f t i t a n i u m p r o s t h e s e s i n a r t i c u l a t i n g j o i n t s , s u c h as i n  hips o r knees, leads t o l o o s e n i n g o f the prostheses a n d b l a c k e n i n g o f the tissue a n d s y n o v i a l f l u i d b y a titanium w e a r product  [22]. I n some  surrounding  cases,  titanium  implants have s h o w n h i g h amounts o f titanium c o m p o u n d s i n adjacent tissues as a result o f h i g h o x i d a t i o n rates.  Increased oxidation/corrosion m a y f o l l o w the production o f  hydrogen peroxide b yreactive oxygen containing metabolites  [23].  S t u d i e s c o n s i d e r i n g t h e b i o c o m p a t i b i l i t y o f t i t a n i u m as a n i m p l a n t m a t e r i a l a r e , g e n e r a l l y , c o n d u c t e d at r o o m t e m p e r a t u r e i n b u f f e r e d s a l i n e s o l u t i o n s o f n e u t r a l p H . h a v e e x a m i n e d the effects o f 0.01  t o 0.1 M H 0 2  2  In-vitro studies  o n the t i t a n i u m o x i d e l a y e r  [23,24].  U p o n a d d i t i o n o f H 0 , a n increase i n the o p e n c e l l p o t e n t i a l w a s o b s e r v e d , but w i t h a n 2  2  i n c r e a s e i n the t i t a n i u m c o r r o s i o n rate.  The presence o f a two-layer passive f i l m was  8  Chapter 2  Background  suggested.  T h e inner layer w a s b e l i e v e d to have a structure and a stoichiometry close to  T i 0 , w h i l e the outer layer w a s p r e d o m i n a t e d b y O H groups and/or b o u n d water. 2  H 0 2  2  w a s a d d e d , the outer r e g i o n b r o a d e n e d but the total f i l m t h i c k n e s s d e c r e a s e d  T h e outer layer m a y potential.  contain a titanium-complex w h i c h can lead to a higher  When [23].  electrode  T e n g v a l l et al. [25] h a v e reported the f o r m a t i o n o f a n aqueous t i t a n i u m g e l  c o n t a i n i n g relatively stable titanium-peroxy-radicals and h a v i n g o x i d i z i n g properties w i t h a r e d o x potential greater than 0.77  Scanning  tunneling  microscopy  V  S H E  .  o f titanium  surfaces  after  exposure  to a  phosphate  buffered saline ( P B S ) solution and H 0 , revealed a granular structure w i t h a 10-20 n m 2  c o r r u g a t i o n [23].  2  T h i s s e e m s t o i n d i c a t e that the d i s s o l u t i o n o c c u r s at l o c a l i z e d d e f e c t s i n  the p a s s i v e f i l m rather than u n i f o r m l y t i t a n i u m to react m o r e r e a d i l y w i t h H 0 2  2  o n the whole  surface, a n d a l l o w s  underlying  toform a titanium-peroxide complex.  I n c r e a s i n g b o t h p H a n d t e m p e r a t u r e d r a m a t i c a l l y i n c r e a s e s the t i t a n i u m c o r r o s i o n rate i n a l k a l i n e p e r o x i d e s o l u t i o n s [3].  A c c o r d i n g t o S c h u t z [3], a c c e p t a b l e c o r r o s i o n rates, less  t h a n 0 . 1 3 m m / y , a r e o n l y a c h i e v e d at p H l e v e l s l e s s t h a n 1 0 a t 7 0 ° C , a n d w e l l b e l o w p H 10 at 8 0 ° C .  The basic mechanism o f titanium corrosion i n alkaline H 0 2  2  solutions is  d e s c r i b e d b y S i g a l o v s k a y a et a l . [ 2 6 ] :  H 0 2  2  + OH" »  OOH" +  (2.2)  H 0 2  Ti + OOH" + 3 0 H " » Ti(OH) 0 2  Ti(OH) 0 2  2  + 2H 0 2  Ti(OH)  4  2  (2.3)  + H 0 + 4e 2  + H 0 2  (2.4)  2  Reaction 2.2 describes the dissociation o f hydrogen perhydroxyl ion, OOH".  peroxide  to form  the unstable  T h e latter r e a d i l y reacts w i t h t i t a n i u m to f o r m m e t a - s t a b l e h i g h l y  soluble titanium peroxide complexes, T i ( O H ) 0 . 2  2  A s the H 0 2  2  concentration decreases,  the t i t a n i u m p e r o x i d e c o m p l e x m a y h y d r o l y z e t o f o r m i n s o l u b l e t i t a n i u m h y d r o x i d e o r o x i d e p r e c i p i t a t e s [26].  T h e c o r r o s i o n rate increases w i t h i n c r e a s i n g h y d r o g e n  concentration [3,5,6,26].  9  peroxide  Chapter 2  Under  Background  more  severe  c o r r o s i o n conditions, t h e literature  is divided  o n t h e effects o f  h y d r o g e n p e r o x i d e o n the o p e n c e l l potential ( O C P ) o f pure titanium.  S i g a l o v s k a y a et a l .  [26] r e p o r t a d r o p i n p o t e n t i a l t o a less n o b l e l e v e l u p o n a d d i t i o n o f 0.3 M H 0 2  N a O H at 6 0 ° C . W y l l i e et a l . [6] f o u n d that the O C P i n c r e a s e d as H 0 2  2  2  t o 0.5  M  was added over  t h e e n t i r e p H r a n g e o f 9 t o 1 3 at 7 6 ° C .  V e r y h i g h t i t a n i u m c o r r o s i o n r a t e s c a n b e o b t a i n e d at h i g h t e m p e r a t u r e s a n d h i g h p H .  A  G r a d e 2 t i t a n i u m c o r r o s i o n rate o f 55.9 m m / y has b e e n m e a s u r e d at 6 0 °C i n a s o l u t i o n containing 2 0 g/L N a O H and 10 g / Lo f H 0 2  [27].  2  the c o r r o s i v i t y o f a l k a l i n e h y d r o g e n p e r o x i d e . an alkaline hydrogen titanium  articles.  compounds  from  Attempts have been made to utilize  A patent b y M a h o o n et a l . [28]  describes  p e r o x i d e treatment to f o r m a n adhesive receptive o x i d e layer o n  S u e [29] uses a n alkaline H 0 2  a base metal  2  solution to strip t i t a n i u m  s u c h as stainless steels o r a l l o y  steels.  surface  A n etchant  c o m p o s i t i o n o f 10 m l K O H ( 4 0 % ) , 5 m l H 0 , a n d 2 0 m l H 0 has b e e n r e c o m m e n d e d 2  2  2  for  a l p h a - b e t a t i t a n i u m a l l o y s [18].  Disadvantages  o f acidic surface treatments f o r t h e industrial c l e a n i n g a n d etching o f  t i t a n i u m i n c l u d e the t o x i c i t y o f the a c i d and subsequent d i s p o s a l p r o b l e m s and costs, as w e l l as t h e i n c r e a s e d s u s c e p t i b i l i t y o f t i t a n i u m t o h y d r o g e n e m b r i t t l e m e n t .  Under acidic  c o n d i t i o n s , h y d r o g e n p e r o x i d e c a n b e a d d e d as a n o x i d i z i n g agent, s e r v i n g as a c o r r o s i o n inhibitor as w e l l as reducing the production contain  hydrogen  peroxide  which  forms  o f hydrogen.  an oxide  Several titanium  a n d prevents  etchants  excessive  metal  d i s s o l u t i o n a n d pitting. D e s p i t e the disadvantages o f a c i d i c treatments, a l k a l i n e h y d r o g e n peroxide i s considered unsuitable for continuous operation i n surface treatments because hydrogen peroxide b e c o m e s increasingly unstable w i t h increasing temperatures and p H .  10  Chapter 2  2.5  Background  Surface Cleaning  H i g h temperature p r o c e s s i n g i n o x i d i z i n g atmospheres, as w e l l as hot r o l l i n g , f o r m i n g , extruding, f o r g i n g a n d annealing lead t o the f o r m a t i o n o f a t o u g h surface o x i d e scale.  A  layer o f o x y g e n - r i c h t i t a n i u m m e t a l o f 0.05 - 0 . 2 m m thickness d e v e l o p s beneath the scale.  B o t h , the s c a l e a n d the u n d e r l y i n g o x y g e n - r i c h layer, are h a r d a n d b r i t t l e a n d n e e d  to be r e m o v e d i n surface c l e a n i n g a n d f i n i s h i n g processes [30].  T h e outer s c a l e i s c o n d i t i o n e d / r e m o v e d b y salt b a t h c l e a n i n g , f o l l o w e d b y a c i d p i c k l i n g a n d m e c h a n i c a l m e t h o d s s u c h as belt g r i n d i n g o r abrasive b l a s t i n g [30].  T h e salt bath  salts p r o v i d e d t o the t i t a n i u m i n d u s t r y are p r o p r i e t a r y but, t h e i r g e n e r i c f o r m s c o n s i s t o f a l k a l i h y d r o x i d e s a n d o x i d i z i n g a g e n t s s u c h as s o d i u m p e r o x i d e .  T h e salt b a t h c o n v e r t s  l o w valence metal oxides to their h i g h valence, o x y g e n - r i c h analogs.  These analogs, i n  turn, react further t o f o r m a l k a l i salts, s u c h as a l k a l i titanates, w h i c h are m o r e e a s i l y soluble i n subsequent  acid p i c k l i n g operations.  T h e benefit o f salt b a t h c l e a n i n g /  d e s c a l i n g / c o n d i t i o n i n g i s that the treated surface i s o f u n i f o r m c h e m i c a l c o m p o s i t i o n a n d c h e m i c a l r e a c t i v i t y [31].  T h e effectiveness o f a salt b a t h i s q u e s t i o n a b l e w h e n subsequent m e c h a n i c a l c l e a n i n g methods  are still required.  C o n s e q u e n t l y , t h e salt b a t h i s r a r e l y u s e d i n c o m m e r c i a l  sectors o f the t i t a n i u m industry  a n d h a s been replaced b y shot b l a s t i n g o r g r i n d i n g  f o l l o w e d b y a hot, 7 0 - 8 0 °C, h y d r o f l u o r i c r e m o v e s the outer scale.  a c i d bath.  T h e shot blasting  treatment  T i t a n i u m objects are t h e n s u b m e r g e d i n the h o t a c i d f o r r o u g h l y  2 0 m i n u t e s d u r i n g w h i c h t i m e r o u g h l y 5 0 - 100 u m o f the o x y g e n - r i c h s u r f a c e l a y e r are r e m o v e d [32].  T h i s c o r r e s p o n d s to a c o r r o s i o n rate o f - 1 3 0 0 - 2 6 0 0 m m / y r .  The acid used i n acid p i c k l i n g consists typically o f a mixture o f nitric and hydrofluoric a c i d (e.g. 2 0 % H N 0 , 2 % H F ) . 3  H y d r o f l u o r i c a c i d s o l u t i o n s a s d i l u t e as 2 % o r l o w e r c a n  cause severe burns w h i c h m a y not b e i m m e d i a t e l y painful o r visible. skin  a n d attack underlying  tissues a n d bone.  11  Effects range  from  H F w i l l penetrate h y p o c a l c e m i a to  Chapter 2  pulmonary  Background  e d e m a a n d , p o s s i b l y , death [33].  Replacement  o f this hazardous  pickling  s o l u t i o n w i t h a l k a l i n e h y d r o g e n p e r o x i d e m i g h t b e f e a s i b l e i f the h i g h c o r r o s i o n rates o b s e r v e d f o r t i t a n i u m at e l e v a t e d p H a n d t e m p e r a t u r e s c a n b e e x t e n d e d t o t h e o x y g e n - r i c h titanium surface layer.  I n addition, difficulties associated w i t h the i n s t a b i l i t y o f h y d r o g e n  peroxide w i l l have to be overcome.  2.6  Hydrogen Peroxide Bleaching of Pulp Wood Fibers  Hydrogen Under  p e r o x i d e i s u s e d i n the b l e a c h i n g o f b o t h m e c h a n i c a l and c h e m i c a l pulps.  relatively m i l d conditions, hydrogen peroxide i s a n effective  b l e a c h i n g agent. chemical  pulp  lignin-preserving  U n d e r m o r e severe c o n d i t i o n s , p e r o x i d e i s u s e d i n the later stages o f bleaching  leading  to marginal  increases i n brightness  and providing  brightness stability. H e r e , h y d r o g e n p e r o x i d e p r o v i d e s a n o p p o r t u n i t y to decrease the use o f c h l o r i n e a n d c h l o r i n e d i o x i d e , w h i c h are c o n s i d e r e d to b e e n v i r o n m e n t a l l y  Pulp bleaching processes have undergone rapid development need  to produce  effluent.  These  temperatures  higher goals  brightness  pulp  lead towards  while  higher  over the past years w i t h a  closing water circuits  pulp  undesirable.  a n d decreasing  consistencies a n dhigher  operating  [34].  H y d r o g e n p e r o x i d e b l e a c h i n g o f m e c h a n i c a l p u l p is t y p i c a l l y d o n e at 1 0 % c o n s i s t e n c y at - 5 0 °C o v e r a p e r i o d o f 2 - 3 hours.  The H 0 2  2  concentration varies  from  0.06 - 0.15 M .  T h e o p t i m u m p H i s - 1 0 . 5 w i t h a h i g h e r p H o f 1 0 . 8 - 1 1 at t h e o n s e t o f b l e a c h i n g a n d a final p H o f - 9 . 5 .  T h e p e r o x i d e stage o f c h e m i c a l p u l p b l e a c h i n g takes p l a c e at s l i g h t l y  h i g h e r temperatures, 7 0 - 8 0 °C, o v e r a p e r i o d o f 2 - 6 hours.  U p s e t conditions m a y result  i n higher p H levels, higher peroxide concentrations, and higher temperatures.  Titanium  superbly  withstands  the corrosive  bleaches.  A s the latter are b e i n g d i s p l a c e d b y h y d r o g e n p e r o x i d e , t i t a n i u m m u s t also b e  12  effects  o f chlorine  a n d chlorine-based  Chapter 2  Background  c o r r o s i o n resistant to h y d r o g e n peroxide bleaching solutions.  N o tonly  should it b e  c o r r o s i o n resistant d u r i n g regular operating conditions, but also d u r i n g plant upsets.  Calcium Ion Inhibition  2.7  Laboratory corrosion studies i n alkaline peroxide s h o w e d m u c h higher titanium corrosion r a t e s t h a n f i e l d d a t a f r o m p u l p a n d p a p e r m i l l s [4]. the l a b data a p p r o a c h e d  t h e f i e l d data.  i n h i b i t e d the c o r r o s i o n o f titanium. ppm of M g  2 +  and C a  2 +  W h e n the plant w a s h w a t e r w a s used,  T h e w a s h water contained a species w h i c h  A s o l u t i o n analysis i n d i c a t e d the presence o f 5 - 12  ions.  C l e r b o i s a n d P l u m e t [35] s t u d i e d the i n h i b i t i o n b y v a r i o u s i o n s at 8 0 °C, p H - 1 0 a n d  -0.1  M  very  H 0 . 2  2  Additions o f 200  - 3 0 0p p m  o f calcium, strontium, and barium were  e f f e c t i v e i n h i b i t o r s r e g a r d l e s s o f the a n i o n o f the a d d e d salt. e f f e c t i v e c o r r o s i o n i n h i b i t i o n b y as l i t t l e as 1 p p m o f C a silicate  additions  were  less  effective  c o n c e n t r a t i o n s w e r e r e q u i r e d [3].  as c o r r o s i o n  2 +  Schutz and X i a o  at 7 0 a n d 8 0 °C [3].  inhibitors  Mg  [35] a n d m u c h  claim 2 +  higher  Whereas an increase i n temperature had a relative small  i n f l u e n c e o n t h e c o r r o s i o n rate i n t h e presence o f c a l c i u m , c a l c i u m d i d b e c o m e e f f e c t i v e as a n i n h i b i t o r w i t h i n c r e a s i n g p H a n d t e m p e r a t u r e (see F i g u r e 2.2)  T h e m e c h a n i s m for the c o r r o s i o n i n h i b i t i o n b y C a understood.  2 +  less  [36].  has not been investigated and i s not  C a l c i u m has been s h o w n to b e present i n o x i d e layers f o r m e d o n titanium  surfaces i n c o n t a c t w i t h s i m u l a t e d b o d y f l u i d [24]. neutral 0.01  and  o r 0.1 M H 0 2  2  A pretreatment w i t h slightly acidic to  solutions resulted i n a n increased oxide thickness w i t h a n  increased a m o u n t o f c a l c i u m throughout the o x i d e .  The lower H 0 2  2  concentration was  m o r e effective due t o s i m u l t a n e o u s c o r r o s i o n and o x i d a t i o n o f the surface i n 100 m M H 0 2  2  solutions  and,  consequently,  lower  net oxide  growth  during  t h e pretreatment.  C a l c i u m enhances protein adsorption onto titanium surfaces and, thus, plays a significant r o l e i n the i n t e g r a t i o n o f a t i t a n i u m i m p l a n t w i t h s u r r o u n d i n g tissues [24].  13  Chapter 2  Background  tooo T3  <D 3  100  cr 10  O  CO  O E  Q. a  9  9.6  10  11  10.6  11.6  12  12.5  13  Solution pH F i g u r e 2.2  A p p r o x i m a t e limits for useful corrosion resistance (<0.13 m m / y )  o f Grade 2  t i t a n i u m i n a l k a l i n e s o l u t i o n s c o n t a i n i n g u p to 0.3 w t % H 0 . [ f r o m ref. 3 6 ] 2  2  It h a s b e e n f o u n d i n m o d e l e x p e r i m e n t s t h a t t i t a n i u m w e a r c a n b e r e d u c e d d r a s t i c a l l y b y a n i o n i m p l a n t a t i o n treatment [22]. the  surface  produces  o f the metal  surface  I n the process, a b e a m o f i o n s i s i m p l a n t e d just b e l o w  substrate under  hardening  within  a high  the substrate  accelerating voltage. b y producing  T h e process  a surface  residual  c o m p r e s s i v e stress. M o s t l y c a r b o n a n d n i t r o g e n are u s e d i n this p r o c e s s , h o w e v e r , data o n i m p l a n t e d b a r i u m i o n s s h o w v e r y p o s i t i v e results [22].  T h e large b a r i u m  are t h o u g h t t o b e l o c a t e d p r e f e r e n t i a l l y at g r a i n b o u n d a r i e s a n d d i s l o c a t i o n s . form a perovskite phase B a T i 0  3  quite clear advantages were i m p r o v e d and corrosion.  fretting  atoms  There they  w i t h the t i t a n i u m m a t r i x and d i f f u s i n g o x y g e n .  perovskite thus b l o c k s these d i f f u s i o n paths and reduces further o x i d a t i o n .  some  The  S o m e o f the  and fatigue behavior and reduced oxidation  I n its i o n i c f o r m b a r i u m i s toxic.  Its t o x i c i t y i n t h e i m p l a n t e d f o r m i s n o t  clear.  I f c a l c i u m i o n s i n the presence o f a l k a l i n e p e r o x i d e lead t o the f o r m a t i o n o f p e r o v s k i t e phase o f C a T i 0  3  at the surface, the decrease i n the c o r r o s i o n rate m a y v e r y w e l l b e the  14  Chapter 2  Background  result o f b l o c k e d d i f f u s i o n paths.  I n other w o r d s , t h e effect o f c a l c i u m ions m a y b e  s i m i l a r to the effect o f i m p l a n t e d b a r i u m ions.  2.8  Perovskite  Ideally, the p e r o v s k i t e f a m i l y has a c u b i c structure w i t h the t i t a n i u m a n d larger m e t a l cations o c c u p y i n g the octahedral holes created b y the o x y g e n anions.  I n this structure,  the t i t a n i u m is c o o r d i n a t e d to s i x o x y g e n atoms w h e r e a s the larger m e t a l i s c o o r d i n a t e d to twelve o x y g e n atoms.  BaTi0  c o n f o r m s c l o s e l y t o this structure at r o o m  3  temperature.  C a T i 0 , h o w e v e r , i s slightly distorted to a n orthorhombic structure w i t h the 3  as i n F i g u r e 2.3 [37].  The C a  2 +  A as c o m p a r e d to 0 . 6 8 A f o r T i  i o n i s larger than T i 4 +  .  4 +  dimensions  w i t h a P a u l i n g i o n i c radius o f 0.99  B o t h are s m a l l e r t h a n O " w h i c h h a s a r a d i u s o f 1.40 2  A  [20].  The  titanium  corrosion  rate  c a n be impeded  b y alloying  additions  through  their  enhancement o f the cathode kinetics, w h i c h raises the freely c o r r o d i n g potential into the passive region.  T h e addition o f Pd,  for example,  15  increases the o p e n cell  potential  Chapter 2  inducing Active  Background  spontaneous  anodic  passivation i n otherwise corrosive reducing  dissolution can b e reduced b y strong  acid  environments.  covalent metal to metal  between titanium and alloying additions i n solid solution alloys.  bonds  I n the case o f c a l c i u m  i n h i b i t i o n o f t i t a n i u m , i t i s m o r e l i k e l y that the c o n t r o l l i n g m e c h a n i s m i s r e l a t e d t o o x i d e formation.  It m a y  b e that t h e t i t a n i u m c o r r o s i o n r e s i s t a n c e i s i m p r o v e d t h r o u g h t h e  f o r m a t i o n o f a s t a b l e o x i d e , s u c h as C a T i 0 , w h i c h m a y e x h i b i t e n h a n c e d 3  and kinetic stability i nalkaline peroxide.  16  thermodynamic  Chapter 3  Thermodynamic Equilibria  3.1  Titanate Perovskite Formation  The  formation  o f a m o r e stable titanate p e r o v s k i t e i n the presence o f C a - c o n t a i n i n g 2 +  solutions m a y o c c u r t h r o u g h the reaction o f C a  2 +  with a titanium peroxide  complex:  T i + O O F T + 3 0 I T <=> T i C y 2 H 0 + 4 e  (2.3)  2  TiCy2H 0 + Ca 2  In  the absence  2 +  + 2e o  CaTi0  3  +  2H 0  (3-5)  2  o f hydrogen peroxide,  theperovskite  may  form  through reaction o f  titanium oxides or hydroxides with calcium:  Ti0 H 0 2  2  + Ca  HTi0 " + Ca 3  2 +  2 +  + 20FT »  + OH'  CaTi0  CaTi0  3  +  3  +2H 0  (3-6)  2  H 0  (3.7)  2  T o investigate the p o s s i b i l i t y o f a t h e r m o d y n a m i c a l l y  stable titanate perovskite,  phase  s t a b i l i t y d i a g r a m s w e r e c o n s t r u c t e d f o r t h e T i - C a - w a t e r s y s t e m at 2 5 , 6 0 a n d 1 0 0 ° C . A l l phase stability d i a g r a m s w e r e generated u s i n g the t h e r m o d y n a m i c  data o f T a b l e 3.2  see A p p e n d i x A ) a n d c o m p u t e r p r o g r a m S y s t e m o f the C S I R O - M o n a s h System  (also  Thermochemistry  [38].  F i g u r e 3.4 s h o w s the phase stability d i a g r a m o f the C a - H 0 system. 2  is increased f r o m 2 5 ° C t o 100 °C, C a ( O H ) T h e stable C a ( O H )  +  2  A s the  b e c o m e s m o r e stable at l o w e r p H values.  area w i d e n s a n d shifts also to l o w e r p H values.  T h e stable C a 0  extends to s l i g h t l y l o w e r p o t e n t i a l v a l u e s w i t h i n c r e a s i n g temperatures.  17  temperature  2  area  Chapter 3  Figure  Thermodynamic  3.5 shows  passivation  Equilibria  the phase  i s present  stability  i n t h e stable  diagram  Ti0 H 0 2  for the T i - H 0 area.  2  A t higher  dominates i n a n area w h i c h i s characterized b y corrosion. the area o f c o r r o s i o n extends t o l o w e r p H values.  system.  2  p H values,  W i t h increasing  3  S H E  , the unstable peroxide T i 0 - 2 H 0 3  forms.  2  f o r m e d b y the reaction o f h y d r o g e n peroxide o n T i 0 H 0 2  acidic solutions to f o r m T i 0 Ti0  4  2  "  and/or  HTi0 "  2  2 +  [40].  3  2  2 +  .  Therefore,  [40].  2  A t potentials  Ti0 -2H 0 3  2  dissolves i n  and i n alkaline solutions w i t h the formation o f H T i 0 " 4  Unfortunately,  there  i s little  thestability o f titanium peroxides  stability diagrams is o n l y  temperatures,  T h i s peroxide can also b e  information  c o m p l e x e s w i t h t i t a n i u m i n the + 6 o x i d a t i o n state; t h e r m o d y n a m i c for T i 0  on  and  titanium  data are o n l y a v a i l a b l e  as represented  i n the  phase  approximate.  F i g u r e s 3 . 6 t o 3 . 8 s h o w the presence o f stable c a l c i u m titanate i n a T i - C a - H 0 2  T h r e e different c a l c i u m titanates w e r e c o n s i d e r e d w i t h a C a O / T i 0 1.5.  HTiGy  T h e c o r r o s i o n k i n e t i c s are s l o w w i t h i n  the H T i 0 " a r e a as i n d i c a t e d b y l o w m e a s u r e d t i t a n i u m c o r r o s i o n rates [39]. b e t w e e n 1.5 t o 2 V  Titanium  T h e titanate stability decreased w i t h increasing C a O / T i 0  2  system.  r a t i o o f 1 , 1.3, a n d  ratio and o n l y  CaTi0 ,  w i t h the p e r o v s k i t e structure, has b e e n i n c l u d e d i n the phase stability diagrams.  Even in  the presence o f just a trace o f C a  2 +  3  , the p e r o v s k i t e covers m o s t o f the aqueous H T i 0 " area 3  and extends w e l l into the T i 0 H 0 2  2  2  area.  W i t h increasing temperatures,  p e r o v s k i t e s t a b i l i t y area shifts d o w n to l o w e r potentials.  t h e titanate  T h e perovskite stability extends  to l o w e r p H v a l u e s w i t h i n c r e a s i n g temperatures, but b e c o m e s also m o r e s e n s i t i v e t o the calcium ion activity. CaTi0  3  i n t h e p r e s e n c e o f a c a l c i u m i o n a c t i v i t y o f 10" .  10" , C a T i 0 10  F i g u r e 3.8 s h o w s a r e l a t i v e l y l a r g e t h e r m o d y n a m i c 9  3  is n o longer t h e r m o d y n a m i c a l l y  18  stable.  stability area o f  W h e n the a c t i v i t y i s r e d u c e d t o  O ro ro  ro vd  i-  o  vo  a -§  ro  t- O O -I <  o  in CS  3  m  o  V  Os  O co  Ti-  ex  in rs  m  VO VO m  vo  >/-!  CN  in  VO os  O t-ro  rs ro vo m o O  © oo VO rOS in  o  ro ro rsl O  in © in rs  oo O O Ov oo ro rs  oo VO cs ro  vo  o o  in  in  rs >n ro  O m  OS in  VO rro  ro  in cs  ON  ON  in m  <n  o  ro  in  o  VO  in  in cs  in  ro  Ov  CN  oo  OS  <  ti U <  OO  00  O  00  ©'  o\  IN  o  O r"  I  CS  ©  CN  VO  CS  ©  ro  ro  CS  CS  ©  © m ro  cs ©' in © ro  rOs CS  cs in ro  Os  in  in od Os m © ©  ro ro  "cr  " 3 ro oo as cs ro ro CS  vq ro oo Os CS ro  ©  oo  in m  5 5  os vd oo m ro rin ro  oo co  o u  a,  O H  Os  co co oo oo  ro in  ro ro oo oo  00  1  (N  O H  o  O  o a  E  -1  I CO  6 u  H rs  6 ta U co  U  VO \Os  oo  S" iT  1  O  5  cs  S*  s  o S  r-;  OS vq  vd ro  os cs  cs  © co  t/3  o  O O  00  M  cs  CO  a  ro  O  vq  OS  ~o  00 ON OO  r© in VO  S*  1  1  a a oo oo . ^ ca U  CN CN  Chapter 3  Thermodynamic  Equilibria  T h e phase stability d i a g r a m s s h o w the p o s s i b i l i t y o f stable c a l c i u m titanate  formation  w h i c h m a y b e r e s p o n s i b l e for the reported c a l c i u m i n h i b i t i o n o f t i t a n i u m c o r r o s i o n i n aqueous alkaline hydrogen peroxide environments.  T h e diagrams d o not, h o w e v e r ,  give  i n f o r m a t i o n about the kinetics o f other p o s s i b l y c o m p e t i n g o x i d e formation and corrosion processes.  A l s o , the presence  o f hydrogen  peroxide,  which  unstable, places the corroding system i n a thermodynamically  is thermodynamically u n s t a b l e state a n d t h e  phase stability diagrams m a y have only limited applicability.  3.2  Solution Equilibria in Aqueous Calcium-Peroxide Solutions  I n a l l e x p e r i m e n t s , t h e p H w a s e v a l u a t e d at the test t e m p e r a t u r e .  Not  o n l y does a n  increase i n temperature affect the p H directly, but it also increases the d i s s o c i a t i o n o f h y d r o g e n p e r o x i d e w h i c h , subsequently, further affects the p H .  T h e effects o f temperature a n d i o n i c strength o n p H a n d e q u i l i b r i u m concentrations i n the p r e s e n c e o f h y d r o g e n p e r o x i d e w e r e c a l c u l a t e d b y c o m p u t e r u s i n g p r o g r a m C A P E R (see Appendix A). values  W h e r e a s h y d r o g e n p e r o x i d e d o e s s e r v e as a b u f f e r t o s o m e e x t e n t , t h e p H  at t h e a c t u a l test t e m p e r a t u r e  temperature  (seeFigure  3.9).  are significantly lower  A s the p H decreases,  than  t h e p H at r o o m  so do the perhydroxyl i o n  c o n c e n t r a t i o n a n d the t i t a n i u m c o r r o s i o n rates.  A d d i t i o n o f c a l c i u m h y d r o x i d e affects the c o n c e n t r a t i o n o f N a O H r e q u i r e d t o o b t a i n the r e q u i r e d p H at r o o m t e m p e r a t u r e , b u t h a s l i t t l e a d d i t i o n a l e f f e c t o n p H w i t h a n i n c r e a s e i n temperature. ion  A decreasing p H w i t h increasing temperature also affects the p e r h y d r o x y l  concentration  (seeFigure  3.10).  T h e latter  t e m p e r a t u r e r a n g e o f 2 5 to 9 0 °C.  25  decreases b y - 3 0 - 4 0 %  over the  Chapter 3  Thermodynamic  Equilibria  NaOH only with 0.05 M H,0,  12.0 11.5 11.0 10.5 I  Q.  10.0 9.5 9.0 8.5 20  j  30  i  i  i  40  i_  50  60  70  80  90  Temperature, C F i g u r e 3.9  Figure 3.10  T h e effect o f temperature o n p H i n the presence o f 0.05 M H 0 2  T h e effect o f temperature  and the addition o f 100 p p m  perhydroxyl ion concentration.  26  2  calcium o n the  Chapter 3  The  Thermodynamic  perhydroxyl  Equilibria  i o n concentration  calcium peroxide, C a 0 . 2  The C a 0  the presence o f h y d r o g e n p e r o x i d e .  i s affected b y c a l c i u m through is thermodynamically  2  The C a  2 +  2  5xl0"  7  M a t p H 11 a n d 6 x l 0 '  9  2 +  m o r e stable than C a ( O H )  ion concentration i n equilibrium w i t h  is v e r y s m a l l . A t 25 °C, i ne q u i l i b r i u m w i t h 0.05 M H 0 o f 100 p p m o f d i s s o l v e d C a , the C a  the formation  2  of 2  in  Ca0  2  and a total initial concentration  concentration decreases  from  M at p H 1 2 . A t p H 10, the C a  2 +  7xl0"  5  M at p H 1 0 t o  concentration decreases  w i t h i n c r e a s i n g temperature through the f o r m a t i o n o f C a ( O H ) , l e a d i n g t o a n additional +  decrease i n b o t h p H a n d the p e r h y d r o x y l i o n concentration.  T h e effect o f temperature i s  l e s s p r o n o u n c e d at p H 11 a n d 1 2 a s s m a l l c h a n g e s i n t h e c a l c i u m i o n c o n c e n t r a t i o n s  have  an i n s i g n i f i c a n t effect o n the greater h y d r o x y l i o n concentration.  Generally, peroxide  as more will  form  calcium is added  to a hydrogen  and the perhydroxyl  peroxide  solution, more  i o n concentration, w h i c h  calcium  is considered the  corrosive ion, decreases:  Ca  2 +  + HOO" + OH" «• C a 0  2  + H 0  (3.8)  2  A l s o , i n a c c o r d a n c e w i t h r e a c t i o n 3.8, the p H decreases.  W h e n a n excess o f calcium is  a d d e d , o n l y a t r a c e a m o u n t , o f t h e o r d e r o f IO" t o IO" M , o f t h e p e r h y d r o x y l i o n r e m a i n s . 6  A t a hydrogen  7  p e r o x i d e concentration o f 0.05 M , a n excess o f c a l c i u m corresponds to  additions greater than ~ 2 0 0 0  ppm.  It b e c o m e s c l e a r that the e f f e c t o f c a l c i u m o n the  p e r h y d r o x y l i o n c o n c e n t r a t i o n m a y p l a y a n important role i n the i n h i b i t i o n m e c h a n i s m .  27  Chapter  4  Research Objectives  T h e r e s e a r c h o f this thesis w a s u n d e r t a k e n to meet the f o l l o w i n g objectives:  1.  T o o b t a i n a greater understanding o f the c o r r o s i o n b e h a v i o r o f G r a d e 2 t i t a n i u m i n alkaline peroxide solutions b y using weight loss measurements and a combination  of  electrochemical techniques including electrochemical impedance spectroscopy (EIS), linear  polarization  polarography.  resistance  (LPR)  measurements  and  potentiodynamic  T h e use o f E I S and L P R permit the study o f the titanium corrosion  b e h a v i o r i n a l k a l i n e p e r o x i d e e n v i r o n m e n t s as a f u n c t i o n o f t i m e f o r t h e f i r s t t i m e .  2.  T o investigate the effects o f temperature, p H , and p e r o x i d e c o n c e n t r a t i o n o n the free c o r r o d i n g p o t e n t i a l , the c h a r g e transfer c a p a c i t a n c e , a n d the t i t a n i u m c o r r o s i o n rate.  3.  To  re-examine  the  currently  accepted corrosion  mechanism,  as  outlined  in  the  l i t e r a t u r e b y S i g a l o v s k a y a et a l . [ 2 6 ] a n d a c c e p t e d at f a c e v a l u e b y m a n y r e s e a r c h e r s .  4.  T o i n v e s t i g a t e the effect o f c r y s t a l l o g r a p h i c o r i e n t a t i o n o n the t i t a n i u m c o r r o s i o n rate and surface m o r p h o l o g y i n alkaline peroxide solutions b y studying a titanium single crystal.  5.  To  study  the  corrosion  behavior  of  titanium  under  extreme  temperatures  and  p e r h y d r o x y l i o n concentrations i n order to determine w h e t h e r a n a l k a l i n e p e r o x i d e environment  could possibly replace hydrofluoric  acid i n titanium surface cleaning.  E x t r e m e l y h i g h t i t a n i u m d i s s o l u t i o n r a t e s c a n b e o b t a i n e d at e l e v a t e d  temperatures  and  An  under  high  hydrogen  peroxide  and  hydroxide  peroxide pickle bath could eliminate environmental  28  concentrations.  alkaline  and safety concerns associated  Chapter 4  Research  Objectives  w i t h h y d r o f l u o r i c a c i d p i c k l i n g as w e l l a s h y d r o g e n p i c k u p c o n c e r n s w h i c h c a n l e a d to e m b r i t t l i n g h y d r i d e  6.  formation.  T o e x p l o r e the i n h i b i t i o n o f t i t a n i u m c o r r o s i o n b y c a l c i u m under a l k a l i n e p e r o x i d e c o n d i t i o n s , m o n i t o r i n g the c o r r o s i o n rate o v e r t h e d u r a t i o n o f the test u s i n g L P R E I S , a n d m e a s u r i n g the p H at the test t e m p e r a t u r e .  and  T o study the extent o f c o r r o s i o n  i n h i b i t i o n obtained b y c a l c i u m and w o r k towards a nunderstanding o f the inhibition m e c h a n i s m i n p u l p b l e a c h i n g solutions. M u c h o f the literature w o r k d o n e i n this f i e l d r e p o r t s p H v a l u e s w h i c h w e r e m e a s u r e d at r o o m t e m p e r a t u r e temperatures without corrections.  a n d u s e d at e l e v a t e d  A l s o , m o s t literature data has b e e n obtained  over  short r u n s w i t h o u t m e a s u r i n g the c o r r o s i o n p r o f i l e as a f u n c t i o n o f t i m e .  7.  T o explore the inhibition o f titanium corrosion b y pulp conditions.  under  alkaline  A l t h o u g h the t i t a n i u m c o r r o s i o n i n h y d r o g e n p e r o x i d e p u l p  peroxide bleaching  solutions has b e e n d i s c u s s e d and studied before, n o i n f o r m a t i o n r e g a r d i n g the effect o f p u l p , w h i c h is an essential constituent o f these b l e a c h i n g solutions, is available.  8.  T o study the interactive effects b e t w e e n p u l p and c a l c i u m .  9.  T o study the effect o f v e l o c i t y o n the t i t a n i u m c o r r o s i o n rate i n a l k a l i n e p e r o x i d e p u l p slurries.  29  Chapter 5 Experimental Apparatus and Procedures  5.1  Materials and Solutions  Commercially titanium from  pure A S T M  samples  ~3  Grade  were  m m thick  sectioned  plate.  composition is summarized 5.3.  The  2  The  i n Table  s a m p l e s w e r e a n n e a l e d at  7 6 0 °C f o r 6 m i n u t e s i n a i r to r e l i e v e a n y p o s s i b l e surface stresses. 5.11  illustrates  microstructure , equiaxed wire the  the  which  , ,  consisted A  resistance spot  back  o f the  sample  .  alpha grains.  was  Figure  F i g u r e 5.11  a  600-grit  finish,  2  0.13% Fe.  to  structure  spheroids  5% HN0 , 3  stabilized  (Etchant:  10%  by HF,  x500).  The assembly w a s then sealed and mounted  w o r k i n g area exposed.  T h i s face was mechanically polished to  (seeAppendix  B),  rinsed  i n distilled water a n d  T h e interface b e t w e e n the t i t a n i u m a n d e p o x y w a s c o a t e d w i t h a  s i l i c o n e sealant to prevent any c r e v i c e c o r r o s i o n effects.  T a b l e 5.3  with  electrode  electropolished  methanol, and dried.  alpha  Chromel  s a m p l e a n d the w i r e sheathed i n a glass tube. i n e p o x y to l e a v e ~ 1 c m  Equiaxed beta  welded  working  of  _  C h e m i c a l c o m p o s i t i o n o f G r a d e 2 titanium plate i n w e i g h t percent.  0  N  H  C  Al  Fe  Nb  Si  Ti  0.141  0.008  0.0047  0.007  0.016  0.132  0.01  0.06  remainder  30  Chapter 5  Experimental Apparatus and Procedures  E a c h w e i g h t loss sample had all faces and sides polished to a 600-grit finish f o l l o w e d b y an electropolish bath for 15 m i n . the e l e c t r o p o l i s h i n g treatment. of -125  BHN.  T h e h y d r o g e n content o f the s a m p l e w a s unaffected b y  Electropolished and 600-grit samples had hardness values  T h i s i s e q u a l t o o r s o m e w h a t b e l o w the average hardness o f G r a d e 2  t i t a n i u m , i n d i c a t i n g that a n y o x i d e f o r m e d d u r i n g the short t e r m a n n e a l h a d b e e n i n the subsequent p o l i s h i n g treatment. dia.) d r i l l e d i n one corner.  removed  E a c h weight loss sample had a small hole ( - 2 m m  T h e s a m p l e s w e r e s u s p e n d e d i n the test s o l u t i o n b y a n y l o n  w i r e attached t o the s a m p l e b y a loose l o o p t h r o u g h the corner hole.  The samples were  w e i g h e d b e f o r e a n d after the c o r r o s i o n test to a n a c c u r a c y o f 0 . 0 0 0 0 5 g . C o n s i d e r i n g that the average  c o r r o s i o n test t o o k - 3 h o u r s  a n dthe sample  a c c u r a c y o f the w e i g h t loss data is w i t h i n - 0 . 0 1 - 0 . 0 2  area w a s - 1 5 - 3 0  c m , the 2  mm/y.  A l k a l i n e hydrogen peroxide solutions were prepared  from distilled water and  grade  present  chemicals.  N o commercial  stabilizers were  p e r o x i d e s o l u t i o n a n d n o s t a b i l i z e r s s u c h as D T P A the tests.  i n the as-received 3 0 %  o r s o d i u m silicate were added  C a l c i u m w a s added as granular reagent grade C a ( O H ) . 2  great t e n d e n c y  Since Ca(OH)  t o precipitate o u t o n a n y surface, a l l contacting equipment  cleaned i n 1 M H C l after e a c h run.  reagent  during 2  was  has a acid  T h e p r e s e n c e o f p u l p w a s i n v e s t i g a t e d t h r o u g h the  addition o f f u l l y b l e a c h e d kraft p u l p , w h i c h served as inert fiber c o n s u m i n g very  little  p e r o x i d e v i a b l e a c h i n g r e a c t i o n s a n d w h i c h w a s as f r e e o f c o n t a m i n a n t s a s p o s s i b l e .  T h e apparatus i s s c h e m a t i c a l l y presented i n F i g u r e 5.12. positioned i n a hot water bath f o r temperature control.  T h e4 L Pyrex beaker  was  Fresh hydrogen peroxide  was  i n t r o d u c e d n e a r t h e m e c h a n i c a l s t i r r e r at t h e b o t t o m o f t h e b e a k e r , t o e n s u r e r a p i d m i x i n g , v i a a buret w i t h a T e f l o n tube attached to itstip.  T h e test s o l u t i o n w a s p r e p a r e d b y  i n i t i a l l y h e a t i n g d i s t i l l e d w a t e r t o t h e test t e m p e r a t u r e . was then added followed b y hydrogen peroxide.  C a l c i u m hydroxide and/or  pulp  T h e p H at t h e test t e m p e r a t u r e  was  a d j u s t e d t o t h e test p H t h r o u g h t h e a d d i t i o n o f s o d i u m h y d r o x i d e . coupons  a n dthe working  electrode were  then placed into  T h eweight  the solution.  concentration w a s measured repeatedly b y titration w i t h 1 N K M n 0 . 4  31  loss  The H 0  Samples  2  2  were  Chapter 5  Experimental Apparatus and Procedures  filtered p r i o r t o titration w h e n p u l p w a s present. was  added  as required.  Concentrated 3 0 % peroxide  T h e fluctuation i n the hydrogen peroxide  affected b y the temperature a n d p H but w a s , generally, w i t h i n ±15%  F i g u r e 5.12  5.1.1  solution  concentration  was  f r o m the m e a n .  S c h e m a t i c o f the c o r r o s i o n test c e l l .  Pickle Bath Solutions  P i c k l e bath solutions were prepared i n a s m a l l P y r e x beaker.  The m i x i n g o f nitric acid  and h y d r o f l u o r i c a c i d t o the desired concentrations o f the h y d r o f l u o r i c a c i d p i c k l e bath, increased the b a t h temperature to - 4 0 °C. T h e h y d r o g e n p e r o x i d e c o n c e n t r a t i o n i n the 95 ° C a l k a l i n e p e r o x i d e b a t h w a s m e a s u r e d repeatedly d u r i n g the short run.  Throughout  the run,  i n order to  small volumes  o f concentrated hydrogen peroxide  were  added  m a i n t a i n an approximately constant peroxide concentration. R e g u l a r 600 grit weight loss c o r r o s i o n c o u p o n s w e r e s u b m e r g e d i n b o t h types o f p i c k l e baths for 10 minutes.  32  Chapter 5  5.1.2  Experimental Apparatus and Procedures  Titanium Single Crystal  T h e t i t a n i u m s i n g l e crystal w a s p u r i f i e d and g r o w n b y D r . A i n u l A k h t a r i n the laboratory o f Prof. A l e c M i t c h e l l b y the f l o a t i n g z o n e technique u s i n g a n electron b e a m furnace  [71]. I n a properly  melting  conducted process, thesolidifying material maintains a  c o n s i s t e n t c r y s t a l l o g r a p h i c o r i e n t a t i o n as the z o n e m o v e s a n d a s i n g l e c r y s t a l i s p r o d u c e d . A f e w t w i n n i n g p l a n e s , w h e r e the c r y s t a l orientation c h a n g e d , w e r e c l e a r l y v i s i b l e o n the single crystal rod.  A 1.5 c m l e n g t h w a s c u t from t h e 7 m m d i a . t i t a n i u m r o d , t a k i n g c a r e  not t o i n c l u d e a n y t w i n n i n g p l a n e s , thereby e n s u r i n g that the s a m p l e w a s t r u l y a s i n g l e crystal.  A C h r o m e l w i r e w a s resistance spot w e l d e d t othe l o n g side o f the cylinder. T h e  w i r e w a s sheathed i n a glass tube and the entire a s s e m b l y w a s sealed a n d m o u n t e d i n epoxy. finish,  Three faces, at r o u g h l y rinsed  and dried.  right  angles t o e a c h other, w e r e p o l i s h e d t o a 600-grit  i n w a t e r a n d m e t h a n o l , e l e c t r o p o l i s h e d , rinsed a g a i n i n w a t e r a n d m e t h a n o l ,  T h e s e three s u r f a c e s w e r e r a n d o m l y l a b e l e d s u r f a c e 1, 2 , a n d 3 as i l l u s t r a t e d i n  F i g u r e 5.13.  B y s h i e l d i n g t w o surfaces w i t h s i l i c o n e sealant, c o r r o s i o n data c o u l d b e  o b t a i n e d from o n e s u r f a c e at a t i m e .  Surface 3  Chromel  Glass  F i g u r e 5.13  S c h e m a t i c o f single crystal t i t a n i u m electrode illustrating the p o s i t i o n s o f the three orthogonal surfaces.  33  Chapter 5  5.2  Experimental Apparatus and Procedures  Electrochemical Tests  Potentiodynamic  polarography,  linear  polarization  resistance  ( L P R ) tests,  and  e l e c t r o c h e m i c a l i m p e d a n c e spectroscopy (EIS) w e r e applied to a three electrode system u s i n g a Solartron 1286 electrochemical interface and a Solartron 1250 frequency response analyzer controlled b y computer using personally written data acquisition software.  A  t i t a n i u m counter electrode w a s u s e d t o prevent the i n t r o d u c t i o n o f f o r e i g n m e t a l i o n s and to  avoid  the catalytic effect o f a p l a t i n u m  decomposition.  counter  electrode o n hydrogen  peroxide  A titanium reference electrode w a s used to obtain reliable E I S and  LPR  data w i t h o u t the i n t r o d u c t i o n o f additional t i m e constants.  O p e n cell potentials were frequently measured w i t h a A g / A g C l reference electrode. T h e latter w a s p o s i t i o n e d i n a s m a l l r e s e r v o i r l o c a t e d a b o v e t h e test v e s s e l f i l l e d w i t h t h e test s o l u t i o n but, generally, c l o s e to r o o m temperature. mm  from  the face o f the w o r k i n g electrode.  to p r e v e n t the f o r m a t i o n o f o v e r b r i d g i n g  The Luggin capillary was placed ~7-8  T h e relatively large distance w a s necessary  gas bubbles.  It w a s a l s o f o u n d n e c e s s a r y t o  p l a c e a c o t t o n thread i n s i d e the L u g g i n c a p i l l a r y t o ensure electrical contact and prevent an open circuit due to formation o f bubbles o foxygen.  T h e s e b u b b l e s f o r m e d n o t o n l y at  the L u g g i n tip but also i n s i d e the c a p i l l a r y as the H 0 2  2  continued to decompose.  A t the  e n d o f t h e e x p e r i m e n t , a r i s i n g p o t e n t i a l s c a n w a s p e r f o r m e d at 1 m V / s .  In E I S e x p e r i m e n t s , a 3.5 m V r m s . A C v o l t a g e w a s a p p l i e d at frequencies b e t w e e n 1 0 0 kHz  a n d 0.1 H z .  C I R C U I T [41].  E I S data fitting was performed  using Boukamp's  EQUIVALENT  T h e L P R d a t a w a s c o l l e c t e d o v e r p o t e n t i a l i n t e r v a l s o f ±20  open cell potential.  m V from t h e  S i n c e the data were not a l w a y s quite linear over this potential range,  the data near the o p e n c e l l potential w e r e least squares c u r v e fitted t o a s e c o n d polynomial.  order  T h e resistance t o charge transfer w a s obtained f r o m the slope o f the tangent  at z e r o c u r r e n t .  34  Chapter 5  Charge  Experimental Apparatus and Procedures  transfer  resistances, equivalent  to corrosion  resistances, were  converted to  c o r r o s i o n rates u s i n g the S t e r n - G e a r y r e l a t i o n [42]:  i  =  2.303.(b  l c O T  where  i  c o r  a +  |b |).R c  c t  B /  (  A.  P  is the c o r r o s i o n current density, A m p s / m  5  9  )  -  y  J  2  b , b are the anode a n d the cathode T a f e l c o e f f i c i e n t s , r e s p e c t i v e l y , V a  R  ct  c  is the charge transfer resistance, o h m s - m  2  B i sa constant, V  U s i n g a b o f - 0 . 1 2 0 V , the T a f e l slope for h y d r o g e n e v o l u t i o n o n t i t a n i u m [43], a n d a b c  a  o f - 0 . 2 5 V , o b t a i n e d for t i t a n i u m electrodes i n a l k a l i n e solutions [44], g i v e s a B v a l u e o f 0.035 V . T h e latter w a s u s e d to calculate the c o r r o s i o n current d e n s i t y and, subsequently, the c o r r o s i o n rate i n  mm/y:  i •W R = K • —  (5.10)  p-F  where  R i s the c o r r o s i o n rate,  mm/y  K is a c o n v e r s i o n factor, 31,536 m - m m - s / ( c m y ) 2  i  c o r  W  3  is the c o r r o s i o n current density, C / ( s - m ) 2  is the equivalent weight, 11.975  p is the density, 4.5 g / c m  g/mol  3  F i s Faraday's number, 96,484.56 C / m o l  T h e r e d u c t i o n o f h y d r o g e n p e r o x i d e increases the cathode current density and, t o prevent current overload, o n l y s m a l l negative overpotentials c o u l d b e applied i n the polarization run.  T h i s m a d e i t m o r e d i f f i c u l t to o b t a i n e x p e r i m e n t a l c a t h o d e T a f e l c o e f f i c i e n t s a n d the  literature value w a s used.  The experimental Tafel coefficient is expected to b e within  2 0 % o f the literature value.  35  Chapter 5  Experimental Apparatus and Procedures  T h e effect o f m a s s transfer limitations b e c o m e apparent at s m a l l a n o d i c  overpotentials,  m a k i n g i t d i f f i c u l t t o o b t a i n a v a l i d a n o d i c T a f e l c o e f f i c i e n t f o r the c o r r o s i o n b e h a v i o r at the c o r r o s i o n potential.  H e n c e , the literature v a l u e w a s used.  T h e v a r i a t i o n i n the a n o d i c  T a f e l c o e f f i c i e n t i s e x p e c t e d t o b e w i t h i n 5 0 % o f the literature value.  I n the presence o f  c a l c i u m , c o r r o s i o n c o n d i t i o n s appear t o b e d i f f u s i o n c o n t r o l l e d (see C h a p t e r 6.2.2), w i t h b  a  approaching  The  infinity.  anticipated  variations  i n the Tafel  c o r r o s i o n rate b y r o u g h l y a factor o f t w o . a corrosion point o fview.  coefficients  could  lead  to variations  i nthe  S u c h a variation is considered acceptable f r o m  B a s e d o n w e i g h t loss data, the e l e c t r o c h e m i c a l c o r r o s i o n rates  p r e d i c t the a c t u a l c o r r o s i o n rates w e l l w i t h i n the error range o f a factor o f t w o .  The  confidence  level i n the obtained  data i s increased through t h e u s e o f multiple  corrosion measuring techniques in parallel.  W e i g h t loss data is very useful since it gives  a c o r r o s i o n rate o f t h e m a t e r i a l i n t h e test e n v i r o n m e n t disturbances, s m a l l as they m a y be.  Unfortunately,  without  a n y electrochemical  weight loss data o n l y give  average  c o r r o s i o n rates o v e r the d u r a t i o n o f the tests a n d d o n o t r e f l e c t a n y v a r i a t i o n o f these c o r r o s i o n rates o v e r t h e test p e r i o d s .  Both  L P Rand E I S give  corrosion profiles  over  the time  period  o f the test.  techniques a p p l y o n l y s m a l l perturbations to the s a m p l e m a t e r i a l , thereby m e a s u r i n g  Both the  c o r r o s i o n rate w i t h o u t a f f e c t i n g the s a m p l e surface c o n d i t i o n .  L P R data is relatively easy  to a n a l y z e ,  under  Also,  although  L P R data  Techniques  such  meaningful  needs  data m a y  to b e corrected  as positive  feedback  c o m p e n s a t i o n d u r i n g the measurement,  not b e obtained for the ohmic  a n d current  passive  resistance  interruption  conditions.  o f the solution. c a n b e used for  but both have their limitations and difficulties i n  their application.  I n the w o r k o f this thesis, the o h m i c resistance w a s o b t a i n e d f r o m the  EIS  frequencies.  data at h i g h  before  analysis.  T h i s resistance w a s then used to correct the L P R  E I S data can b e m o r e meaningful  under passive conditions and  data may  relate m o r e i n f o r m a t i o n regarding the c o r r o s i o n m e c h a n i s m and the b r e a k d o w n o f passive  36  Chapter 5  oxides  or  Experimental Apparatus and Procedures  protective  coatings.  However,  acquisition of  a  meaningful  polarization  resistance d e p e n d s o n the d e v e l o p m e n t o f a n equivalent circuit m o d e l o f the c o r r o s i o n process to w h i c h the data c a n then b e least squares c u r v e fitted. simple process and m a y  T h i s is not always a  e a s i l y l e a d to c o m p l e x or erroneous equivalent circuits,  h e n c e e r r o n e o u s c o r r o s i o n rates.  and  In addition, the v a l i d i t y criteria s h o u l d be satisfied i n  o r d e r f o r i m p e d a n c e m e a s u r e m e n t s to b e m e a n i n g f u l (see S e c t i o n 5.3.1).  Potentiodynamic  polarization  techniques  can be  used  very  successful in  evaluating  p a s s i v i t y , o r the effects o f a l l o y i n g additions or inhibitors o n the c o r r o s i o n  behavior.  C a r e f u l c o n s i d e r a t i o n s h o u l d be g i v e n to the correct s c a n rate a n d the effects o f uncompensated  ohmic  resistance w h i c h  may  lead to  erroneous  values  of  the  an  Tafel  coefficients a n d c o r r o s i o n current densities. S i n c e the potential is s c a n n e d t h r o u g h a w i d e range o f values, the surface c o n d i t i o n o f the s a m p l e m a y be altered t h r o u g h the f o r m a t i o n or reduction o f oxides. corrosion process over different  5.3  The  T h i s t e c h n i q u e s h o u l d therefore not b e u s e d to m o n i t o r time,  but  is useful  when  comparing  corrosion behaviors  the in  environments.  Electrochemical Impedance Spectroscopy  electrochemical impedance  s p e c t r o s c o p y data w e r e fitted to a o n e t i m e  constant  m o d e l , r e p r e s e n t a t i v e o f the m e t a l / o x i d e / d o u b l e l a y e r / s o l u t i o n i n t e r f a c e (see F i g u r e 5.14). represents the solution resistance, whereas C capacitance and resistance, respectively.  c t  a n d R,.  t  represent the charge transfer  G e n e r a l l y , c o r r o d i n g t i t a n i u m w i t h its tenacious  o x i d e is represented b y t w o time-constant models [19,45], where one is associated w i t h the o x i d e a n d the other w i t h the d o u b l e layer.  A  second time constant m a y have been  p r e s e n t at t h e l o w f r e q u e n c i e s , h o w e v e r , t h e i n s t a b i l i t y o f h y d r o g e n p e r o x i d e r e s u l t e d i n f l u c t u a t i n g l o w f r e q u e n c y signals m a k i n g it i m p o s s i b l e to detect a s e c o n d t i m e constant. I n a n y case, s i n c e the c o r r o s i o n rate b a s e d o n the one t i m e constant m o d e l  37  corresponded  Chapter 5  Experimental Apparatus and Procedures  so w e l l w i t h the w e i g h t loss data, the a d d i t i o n a l resistance to c o r r o s i o n c a u s e d b y  the  d o u b l e l a y e r m u s t h a v e b e e n s m a l l relative to the resistance o f the o x i d e .  Metal  F i g u r e 5.14  Oxide  Equivalent circuit m o d e l o f a corroding titanium surface i n alkaline peroxide suspension.  T h e c a p a c i t o r e l e m e n t c o n t a i n s n o n i d e a l i t i e s s u c h as s u r f a c e r o u g h n e s s , w h i c h l e d t o t h e use o f a constant p h a s e element, C P E , instead o f a p u r e c a p a c i t o r [41].  The impedance  of  the C P E is represented b y :  z =  l  (5.11)  (c M) n c t  where Z  co n  complex impedance,  frequency,  ohms.cm  2  Hz  v a r i e s f r o m -1 f o r a n i d e a l i n d u c t o r t o 1 f o r a n i d e a l c a p a c i t o r  38  Chapter 5  Experimental Apparatus and Procedures  V a l u e s for n ranged b e t w e e n 0.88 to 0.99.  The  above  equivalent  circuit model  gave  S o m e data are s h o w n i n F i g u r e s 5.15 consequently,  the procedure.  With  a good correlation w i t h experimental  data.  5.17 t o illustrate the v a l i d i t y o f the m o d e l increasing  capacitance increased and shifted to higher  corrosion  frequencies  rates,  the charge  (see F i g u r e 5.15).  and,  transfer  T h i s latter shift  is the result o f the m u c h l o w e r charge transfer resistance i n p a r a l l e l w i t h the capacitance. Consequently,  higher  corrosion  currents  fully  charge  t h e c a p a c i t o r at m u c h  higher  frequencies leading to a n open element i n the equivalent circuit a n d a total resistance c o n s i s t i n g o f the s u m o f the o h m i c a n d charge transfer resistances as illustrated i n F i g u r e 5.16.  V  A  ,  •  O  EIS data  - Model  Frequency, Hz F i g u r e 5.15  A s the c o r r o s i v i t y o f the s o l u t i o n increases, the charge transfer capacitance increases a n d the m a x i m u m phase angle shifts t o h i g h e r T a b l e 5.4 for the legend.  39  frequencies.  See  Chapter 5  Experimental Apparatus and Procedures  Frequency, Hz F i g u r e 5.16  B o d e plot o f electrochemical data i n various solutions o f different corrosion strengths.  S e e T a b l e 5.4 for the l e g e n d .  T h e data are p r e s e n t e d a g a i n i n N y q u i s t p l o t s i n F i g u r e 5.17.  T h e d r a m a t i c effect o f the  presence o f c a l c i u m i s illustrated b y a n incomplete semicircle and m u c h higher real and i m a g i n a r y resistances. T h e legend to F i g u r e s 5.15, 5.16, and 5.17 i s presented i n T a b l e 5.4.  40  Chapter 5  Figure 5.17  Experimental Apparatus and Procedures  Nyquist  plots o f electrochemical data i n various  solutions o f different  c o r r o s i o n strengths. S e e T a b l e 5.4 f o r the l e g e n d . T a b l e 5.4 Symbol  L e g e n d o f F i g u r e s 5.15, 5.16, and 5.17. M o d e l Parameters  Test Conditions Rsol Q-cm  V  n  Ret 2  Q-cm  2  pF/cm  2  407  788  58  0.95  384  201,000  16.5  0.91  A  61  141  89  0.96  1 1 . 0 . 2 5 M H A ,  14  5.6  198  0.93  p H 10, 0 . 1 5 M H O , 2  2  50 °C A  p H 10, 0.15 M  H A ,  •  p H 11, 0.15 M H  0  pH  50 °C, 100 p p m C a  50 °C  70 °C  41  Chapter 5  5.3.1  Experimental Apparatus and Procedures  V a l i d i t y of the Impedance  Impedance  measurements  Data  are o n l y m e a n i n g f u l w h e n the s y s t e m satisfies the c r i t e r i a o f  linearity, causality, a n d stability, and the readings  are c o n t i n u o u s  behavior is approximated b y applying a s m a l l amplitude A C signal.  and finite.  Linear  Causality, in w h i c h  the m e a s u r e d response is o n l y c a u s e d b y the a p p l i e d perturbation, c a n be o p t i m i z e d selecting appropriate current ranges and signal amplitudes.  by  T o b e stable, the s y s t e m must  r e t u r n t o i t s o r i g i n a l s t a t e at t h e f i n i s h o f t h e m e a s u r e m e n t .  In most corroding systems,  the i m p e d a n c e is c o n t i n u o u s and finite o v e r the entire s p e c t r u m o f A C  frequencies.  I n a n a l k a l i n e p e r o x i d e s y s t e m , s t a b i l i t y i s the m o r e d i f f i c u l t c r i t e r i o n to s a t i s f y as the peroxide  is continuously  d u r i n g the scan.  decomposing  and  small make-up  q u a n t i t i e s are a d d e d  even  A t the l o w e r frequencies, n o i s y data c a n be the result o f the continuous  generation, transport a n d c o l l a p s e o f air bubbles, v i o l a t i n g the c a u s a l i t y criteria. violations become  m o r e serious i n the m o r e corrosive environments  where  These  hydrogen  peroxide decomposition is more vigorous.  S i n c e l o w f r e q u e n c y data is less reliable, the K r a m e r s - K r o n i g t r a n s f o r m equations c a n not b e u s e d to test the v a l i d i t y o f the e x p e r i m e n t a l d a t a as t h e y r e q u i r e a n i n t e g r a t i o n o v e r the entire f r e q u e n c y r a n g e a p p r o a c h i n g a d c l i m i t [46].  H o w e v e r , i n this thesis, the electrochemical impedance spectroscopy data c o m p a r e w e l l w i t h l i n e a r p o l a r i z a t i o n d a t a as w e l l as w e i g h t l o s s data. i n its o w n right.  very  T h i s is a validation check  T h e E I S data, therefore, g i v e s a g o o d r e f l e c t i o n o f the r e a l s y s t e m as  represented b y the suggested m o d e l .  42  Chapter 5  5.4  Experimental Apparatus and Procedures  Surface  Scanning  Morphology  electron microscopy  corrosion environment  (SEM)  was  employed  o n the sample surface m o r p h o l o g y .  boundary corrosion and surface roughening surface w i t h a very electrons.  to  w a s studied.  observe  the  The S E M  fine, narrow b e a m o f electrons generating  from  the  scans the  sample  secondary, l o w  energy  The  secondary  a surface v o l u m e w i t h a depth and w i d t h equal to approximately  the diameter o f the incident b e a m . gives a large depth  of  T h e extent o f pitting, grain  T h e s e are d r a w n to the e l e c t r o n detector b y a p o s i t i v e v o l t a g e .  electrons originate  effects  T h e electron m i c r o s c o p e ' s s m a l l diameter o f the b e a m  o f f i e l d a n d the l o n g  focal length permits  a high  magnification  resulting i n the startling clear images characteristic o f S E M photos.  U n l e s s s p e c i f i e d otherwise, the i n c i d e n c e angle w a s 90 degrees. accelerating v o l t a g e v a r i e d f r o m 2 0 k e V to 5 k e V . surface i m a g e revealed clearly the sample topography.  The  incident  beam  A t l o w accelerating voltages,  T h e strong interfering effects o f  o x i d e s s o m e t i m e s n e c e s s i t a t e d t h e u s e o f h i g h e r a c c e l e r a t i n g p o t e n t i a l s at w h i c h s o m e the finer surface details m a y be lost.  43  the  of  Chapter 6  Results and Discussion  The  initial  work  was  performed  peroxide, p H and temperature  to obtain  information  o n the effects  of  hydrogen  o n t h e t i t a n i u m c o r r o s i o n rate a n d study t h e extent o f  inhibition offered b y 100 p p m o fc a l c i u m ions.  T h e objective here w a s not to construct a  c a l c i u m i n h i b i t i o n w i n d o w as h a s b e e n d o n e b y R . S c h u t z a n d M . X i a o o v e r a w i d e r a n g e o f c a l c i u m c o n c e n t r a t i o n s (see C h a p t e r 2 ) , b u t t o c o n f i r m t h e i r r e s u l t s a n d s t u d y t h e mechanism o f calcium inhibition.  During  the i n i t i a l r u n s i t b e c a m e apparent that the  results differed f r o m those d o c u m e n t e d i n m u c h o f the literature.  Unfortunately,  many  r e s e a r c h e r s m e a s u r e d t h e p H o f t h e i r t e s t s o l u t i o n s at r o o m t e m p e r a t u r e a n d t h e e f f e c t o f t i m e o n c a l c i u m i n h i b i t i o n h a d b e e n i g n o r e d [3,4,5,6]. A s d i s c u s s e d i n C h a p t e r 3, the p H at t h e a c t u a l t e s t t e m p e r a t u r e c a n b e s u b s t a n t i a l l y l o w e r t h a n t h e p H at r o o m  temperature.  A l s o , as R a s t e n [47] h a s p o i n t e d out, the i n h i b i t e d c o r r o s i o n rate i s a f f e c t e d b y the t i m e o f exposure.  6.1  Titanium Corrosion in Alkaline Peroxide  D a t a w a s o b t a i n e d i n the absence o f p u l p o r c a l c i u m o n e l e c t r o p o l i s h e d c o u p o n s u s i n g the equipment and procedures described in Chapter 5.  6.1.1  Effects of H 0 Concentration, p H , and Temperature 2  2  E a c h test s o l u t i o n c o n t a i n e d a h y d r o g e n p e r o x i d e c o n c e n t r a t i o n o f 0 M , 0.05 M , 0.15 M , o r 0 . 2 5 M at a p H o f 1 0 o r 11 a n d a t e m p e r a t u r e o f e i t h e r 5 0 ° C o r 7 0 ° C .  44  Chapter 6  Results and Discussion  T i t a n i u m s h o w e d passive behavior i n alkaline peroxide solutions, i n d u c e d b y a current p l a t e a u , w i t h the p a s s i v e p l a t e a u current d e n s i t y i n d i c a t i v e o f the c o r r o s i o n rate.  Weight  loss c o r r o s i o n rates i n F i g u r e 6.18, b a s e d o n an a p p r o x i m a t e l y three h o u r e x p o s u r e illustrate the trend b e t w e e n l o w e r w e i g h t loss c o r r o s i o n rates a n d l o w e r p a s s i v e densities.  The potentiodynamic  time,  current  s c a n s w e r e o b t a i n e d at t h e e n d o f t h e t h r e e h o u r  test.  A d d i t i o n o f h y d r o g e n p e r o x i d e c l e a r l y i n c r e a s e d the c o r r o s i o n rate a n d i n i t i a l l y r a i s e d the corrosion potential i n the anodic  d i r e c t i o n , as i s i l l u s t r a t e d i n F i g u r e  attributed to the r e d u c t i o n o f h y d r o g e n p e r o x i d e , w h i c h p r o v i d e s  6.18.  This  is  an additional  cathode  and h y d r o g e n p e r o x i d e concentration increased the  titanium  reaction.  Increasing p H , temperature, c o r r o s i o n rate.  A s the s o l u t i o n b e c a m e m o r e c o r r o s i v e , the rate o f o x i d e  dissolution  i n c r e a s e d a n d , as a result, the c o r r o s i o n p o t e n t i a l . d e c r e a s e d .  pH 10, 50 C no peroxide 0.05 M H 0  2  0.15 M H 0  2  2  2  pH 11,50 C 0.15MH O pH 11.70C 2  2  • 0.25 M H 0 2  2  1.5  0-35 \ \ 2.3 mm/y,'' \ mm/y  \ 5 8  \  No Peroxide With Peroxide  -0.5 10^  IO  i  JJ-J  - 3  10"  i  2  'I  10  • • _ 1  10°  • •  Current Density, A / m F i g u r e 6.18  The  titanium  10  1  10  2  2  c o r r o s i o n rate increases w i t h i n c r e a s i n g h y d r o g e n  concentration, temperature and p H .  45  peroxide  W e i g h t l o s s c o r r o s i o n rates are s h o w n .  Chapter 6  Results and Discussion  W h e n m o n i t o r i n g the c o r r o s i o n rate w i t h t i m e , i t b e c a m e apparent that the c o r r o s i o n rate decreased o v e r the three h o u r t i m e p e r i o d under the m i l d c o n d i t i o n s o f p H  10, 0.05  H 0 ,  oxide  2  2  (see  Figure  6.19).  This  is probably  dueto a thickening  o f the  M  layer  [21,23,24,47].  10  •  • •  s  E o  o  pH 10, 0.05 M H 0 2  d 10  EIS data  A  LPR data 10"  2  0.03 mm/y  CD O  •  A  i  4  c  cc<  pH 11, 0.05 M H 0 _ 0.11 mm/y 2  „  2  10"  10  rz o w o i_ i_ o O  3  pH 10, 0.15 M H 0 0.35 mm/y 2  10  10°  2  73 I—IjD  3  2  20  40  60  80  Time, F i g u r e 6.19  o —\ o w o'  CD  or  o  100 120 140 160  180  min  U n d e r m i l d l y c o r r o s i v e c o n d i t i o n s , at 5 0 ° C , the c o r r o s i o n rate decreases w i t h time.  W e i g h t l o s s c o r r o s i o n rates are i n d i c a t e d i n b o l d font.  160 140 E o  CD O  c cc  'o CO CL CD  o  5 0 C , p H 10, 0 . 0 5 M H 0  2  •  50 C, p H 1 0 ,  0.15MH O  2  o  50 C,  pH11,0.05MH O  2  50 C , p H 11, 0.15 M H 0  2  70 C,  2  2  •  1  1  •  120 -  60  :A  2  i  A  •  1  A  • ••  •  i  1  _  A  • •  A  : •  pH11,0.15MH O  A  100 80  2  2  A  _  2  •  #  •  •  40  •  20 i  0  20  o"  a a i  40  .  o  •  i  60  i  80  .  • i  100  •  •  120  140  D  160  Time, min F i g u r e 6.20  T h e d o u b l e layer capacitance is greater i n a m o r e c o r r o s i v e  46  environment.  Chapter 6  Results and  Discussion  T h e charge transfer capacitance increased w i t h increasing p H , temperature, and h y d r o g e n p e r o x i d e concentration. T h i s c o u l d either be the result o f a thinner o x i d e o r an o x i d e w i t h a greater dielectric constant. T h e capacitance is described b y :  K - A  where K  dielectric constant  A  exposed surface area  d  oxide thickness  T h e o x i d e c o u l d g r o w i n thickness but b e c o m e increasingly m o r e hydrated and  porous,  thereby increasing K and C . c t  B a s e d o n literature i n f o r m a t i o n (see C h a p t e r 2), i t appears that a c r y s t a l l i n e anatase o x i d e layer m a y H S0 2  4  have superior corrosion resistance.  T w ocoupons were anodized i n 0.5 M  at 6 5 V f o r 1 a n d 3 0 m i n u t e s , r e s p e c t i v e l y .  B o t h coupons had a green appearance  w i t h the longer a n o d i z e d sample b e i n g m o r e intense green.  A third coupon was anodized  for 10 m i n u t e s at 9 5 V . A t this voltage, the o x i d e s h o u l d consist entirely o f crystalline anatase [17].  T h e sample had a d u l l gray appearance.  A l l three c o u p o n s w e r e tested i n 0.15 M H 0 2  presented i n Figure 6.21. corrosion resistance.  2  a t p H 11 a n d 5 0 ° C a n d t h e r e s u l t s a r e  N o n e o f the three c o u p o n s s h o w e d a s i g n i f i c a n t increase i n the  T h e green coupons q u i c k l y lost their green c o l o r u p o n  i n the a l k a l i n e p e r o x i d e .  submersion  A s the anatase s a m p l e w a s s u b m e r g e d i n the s o l u t i o n , the gray  anatase turned s l o w l y w h i t e o v e r a t w e n t y m i n u t e p e r i o d .  A t the e n d o f 4 0 m i n u t e s , the  w h i t e c o l o r d i s a p p e a r e d a n d the u s u a l dark g r a y c o l o r o f a c o r r o d e d , u n a n o d i z e d was observed.  47  coupon  Chapter 6  E o  Results and Discussion  10  4  d  CD O  •  fresh electropolished surface  A  anodized f o r 1 min @ 65 V , green  V  anodized f o r 30 min @ 65 V , green  •  anodized f o r 1 0 min @ 95 V , gray  10"  1  10 b 3  O o —i o CO  o'  c  10  "oo  CO CD  CC c q  • •  0)  V  I—I-  • •  CD  ^10  CO  o i_  o O  F i g u r e 6.21  10  1  u  10  o  15  20  25  30  1  3  35  Time, min  A n o d i z a t i o n does not i m p r o v e the t i t a n i u m c o r r o s i o n resistance i n 0.15 H 0 2  2  atpH  M  11,50°C.  C r y s t a l l i n e anatase d i d not increase the p o l a r i z a t i o n resistance. A s w a s apparent f r o m the disappearing green a n o d i z a t i o n c o l o r s , the p e r h y d r o x y l i o n c l e a r l y reacted w i t h the o x i d e . T h e o x i d e , as i n d i c a t e d b y the w h i t e c o l o r o f the anatase s a m p l e , d i d n o t d i s s o l v e a w a y immediately.  T h e s e results s e e m t o suggest that the o x i d e p a r t i c i p a t e d i n the c o r r o s i o n  process.  6.1.2  Corrosion  Mechanism  M e t a l - c a t a l y z e d d e c o m p o s i t i o n o f h y d r o g e n p e r o x i d e i s w e l l k n o w n i n the literature [48, 49, 50, 51].  T h e hydrogen peroxide decomposition sequence is given by:  48  Chapter 6  Results and  Discussion  E°,  V  S H E  [10]  Step 1  H 0  Step 2 a  O O H " + H 0 + 2e" <=> 3 O H "  0.878  (6.13)  Step 2 b  O O H " + O H " <=> 0  0.076  (6.14)  Overall  2H 0  2  + O H " <=> O O H " + H 0  2  (2.2)  2  2  2  + H 0 + 2e 2  _+  In  2  <=> 2 H 0 + 0  2  2  (6.15)  2  s o l u t i o n , t h e e l e c t r o n s o f step 2 a r e c a r r i e d b y s u r r o u n d i n g  ions  a n d the solvent  resulting i n intermediate species s u c h as the h y d r o x y l radical, H O \ S u c h species a r e s h o r t - l i v e d w i t h v e r y h i g h r e c o m b i n a t i o n r e a c t i o n rates. metal  ions,  such  as iron  d e c o m p o s i t i o n [51].  a n d copper,  are k n o w n  to catalyze  intermediate  Dissolved  hydrogen  heavy  peroxide  These metal ions participate through redox reactions a n d can b e  c o n s i d e r e d as m o r e efficient electron carriers i n the d e c o m p o s i t i o n process.  Hydrogen  peroxide d e c o m p o s i t i o n m a y f o l l o w a similar m e c h a n i s m o n metal surfaces, w i t h the metal o r metal oxides participating through redox reactions.  F o r this m e c h a n i s m to b e  t h e r m o d y n a m i c a l l y feasible, the m e t a l r e d o x potential m u s t b e b e l o w the r e d o x potential o f step 2 a a n d a b o v e the r e d o x p o t e n t i a l o f step 2 b [80]. a l k a l i n e p H v a l u e s , t i t a n i u m exists o n l y as T i  I v  .  W i t h i n these b o u n d a r i e s a n d at  A s a result, t i t a n i u m o r t i t a n i u m o x i d e s  c a n not react w i t h h y d r o g e n peroxide through redox reactions and w i l l r e m a i n i n a T i o x i d a t i o n state. 0 ~  and the T i  2  0  2  Instead, there i s e v i d e n c e that a p e r o x i d e c o m p l e x i s f o r m e d I V  o x i d a t i o n state [63].  h a s b e e n o b s e r v e d at p H 2 - 7  Consequently,  I V  between  The formation o f oxygen radical complexes with  [81].  i n t h e presence o f the p e r h y d r o x y l  ion, O O H ,  the breakdown  -  o f the  T i 0 H 0 f i l m m a y b e e n v i s a g e d as p r o c e e d i n g t h r o u g h s t e p s 1 a n d 2 : 2  2  Step 1  2 T i 0 H 0 + 2 0 0 H " + 4 H 0 <=> 2 T i 0 - 2 H 0 + 2 0 H "  (6.16)  Step 2  T i + 2 T i 0 - 2 H 0 + 2 0 H " <=> T i 0  (6.17)  Overall  T i+ T i 0  2  2  2  3  2  3  2  + 2 0 0 H " <=> 2 H T i 0 " 3  49  2  2  + 2HTi0 " + 4 H 0 3  2  (6.18)  Chapter 6  Results and  Discussion  I n r e a c t i o n 6.16, the o x i d e reacts w i t h the p e r h y d r o x y l i o n , w h i c h i s m o r e stable t h a n the peroxide  ion, 0  titanium  peroxide  2  2  " ,over  the range  complex.  o f p H values tested [ 4 0 ] , t o f o r m t h e metastable  T h e titanium  atom  i s i n the T i  l v  oxidation  c o m p l e x e s w i t h the a d s o r b e d p e r o x i d e i n the f o l l o w i n g suggested m a n n e r  state a n d  [83]:  T h e structure reflects adsorption a n d subsequent deprotonation o f the p e r h y d r o x y l The titanium atom is still positioned i n the oxide film.  Subsequent  ion.  electron transfer  o c c u r s w i t h t h e u n d e r l y i n g m e t a l i n c o r r o s i o n r e a c t i o n s a n d t h e o x i d e f i l m g r o w s at t h e m e t a l - o x i d e interface (reaction 6.17).  T h e t h r e s h o l d h y d r o g e n p e r o x i d e concentration i s d e f i n e d as t h e c o n c e n t r a t i o n  below  w h i c h the o x i d e t h i c k e n s and b e c o m e s m o r e protective:  T i + 2 T i 0 . 2 H 0 <=> 3 T i 0 3  2  2  (6.19)  + 2H 0 2  I n the p r e s e n c e o f a l o w p e r o x i d e c o n c e n t r a t i o n a b o v e the t h r e s h o l d c o n c e n t r a t i o n , t h e oxide grows but becomes more hydrated and hydroxylated:  Ti + 2TiOy2H 0 2  «• T i 0  2  + TiGy2H 0 + Ti(OH) 2  (6.20)  4  Solubilization ismore likely to give titanium perhydroxyl complexes favoring octahedral c o o r d i n a t i o n [82].  A p p r o a c h i n g s o l u b i l i z a t i o n a n d as a r e s u l t o f e x t e n s i v e h y d r o x y l a t i o n ,  w e m a y n o w understand t h e structure o f T i O y 2 H 0 2  structure o f H T i 0 HTiOj + 3H 0 2  peroxide  3  to consist o f T i ( O H ) O O H . 3  c a n b e u n d e r s t o o d to c o n s i s t o f h y d r o x y l a t e d a n d h y d r a t e d T , I V  is equivalent to T i ( O H ) ( H 0 ) \ 5  2  I n thepresence o f a high  concentration, the o x i d e layer thickness decreases as the titanium  50  The where  hydrogen peroxide  Chapter 6  Results and Discussion  c o m p l e x further reacts w i t h the p e r h y d r o x y l  i o n to form soluble complexes.  I n this  reaction, the p e r o x i d e d e c o m p o s e s t o o x y g e n :  T i G y 2 H 0 + O O H " + H 0 <=> T i ( O H ) ( H 0 ) " + 0 2  2  Overall, the increased breakdown  5  2  o f the T i 0  2  (6.21)  2  f i l m i n the presence o f O O H '  attributed to the f o r m a t i o n o f the a d s o r b e d p e r o x i d e o r p e r h y d r o x y l  m a y be  complex.  A s d i s c u s s e d i n t h e p r e v i o u s s e c t i o n , the effect o f o x i d e t h i c k n e s s w a s n o t o b s e r v e d at a l l i n a n a l k a l i n e p e r o x i d e s o l u t i o n c o n t a i n i n g 0.15 M H 0 2  2  at p H 1 1 , 5 0 °C.  Therefore,  e l e c t r o n transfer a n d i o n m i g r a t i o n t h r o u g h the o x i d e l a y e r are n o t rate l i m i t i n g a n d , s i n c e the c o r r o s i o n rate i s v e r y s e n s i t i v e t o p H a n d h y d r o g e n p e r o x i d e c o n c e n t r a t i o n , the rate c o n t r o l l i n g step i s either: (a)  m a s s t r a n s f e r o f the p e r h y d r o x y l i o n to the o x i d e / s o l u t i o n i n t e r f a c e , o r  (b)  reaction 6.16 i nw h i c h the p e r h y d r o x y l i o n reacts w i t h the o x i d e .  F o r m a t i o n o f the t i t a n i u m p e r o x i d e i n t e r m e d i a t e m a y further e n h a n c e the c o r r o s i o n rate b y i n c r e a s i n g the potential across the H e l m h o l t z layer and thereby the d r i v i n g force f o r the p o t e n t i a l c o n t r o l l e d m i g r a t i o n o f the p e r h y d r o x y l i o n to the o x i d e - s o l u t i o n interface.  6.1.3  SEM  Surface Morphology  photographs  environments. boundaries.  illustrate the progress  o f titanium  corrosion  i n alkaline  peroxide  A t the m i l d e r c o r r o s i o n c o n d i t i o n s , s m a l l pits f o r m m o s t l y a l o n g the grain  A f e w s m a l l p i t s , h o w e v e r , are a l s o present o n the g r a i n s u r f a c e (see F i g u r e  6.22).  51  Chapter 6  Figure 6.22  Results and  Discussion  Surface morphology o f a titanium coupon after exposure to an alkaline peroxide solution o f p H 10, 0.15 M H 0 at 50 °C for 6.4 hrs. 2  2  A s the solution corrosivity increases, the surface pits become more defined.  The pits  along the grain boundaries increase in number and join to form grooves (see Figure 6.23). In Figure 6.24, grain boundary corrosion has progressed to the extent that grain drop-out may occur.  Simultaneous general corrosion o f the grain surface is illustrated by the  numerous cusps i n Figure 6.25.  The surface roughness o f Figure 6.24 is shallow and  crevices do not extent into the metal beyond the top layer o f grains as illustrated by Figure 6.26.  Second phase particle segregation is not observed and grain boundary  corrosion is probably the result o f an imperfect higher energy structure at the boundary with a lower corrosion resistance. N o evidence was found to suggest that grain boundary corrosion was associated with beta spheroids (see Figure 5.11).  Subsequent studies on  high purity titanium single crystals showed similar corrosion rates to Grade 2 titanium suggesting that the spheroids played little or no role i n the corrosion behavior.  52  Chapter 6  Figure 6.24  Results and  Discussion  Surface morphology o f a titanium coupon p e r o x i d e s o l u t i o n o f p H 11, 0.25 M H 0 2  53  2  after e x p o s u r e  a t 7 0 °C f o r 2 . 6 h r s .  to a n alkaline  Chapter 6  F i g u r e 6.25  Results and Discussion  A higher magnification clearly shows m a n y cusps and general corrosion o f the g r a i n surfaces.  54  Chapter 6  6.1.4  Results and  Surface  Discussion  Roughness  T h e i n c r e a s e d r o u g h n e s s o f a 6 0 0 grit surface resulted i n a s l i g h t l y h i g h e r c o r r o s i o n rate i n alkaline p e r o x i d e solutions than electropolished surfaces.  T h i s i s l i k e l y the result o f a  greater true surface area p e r unit area o f s p e c i m e n o n the 6 0 0 grit surfaces.  In Figures  6.27 a n d 6.28, the w e i g h t loss c o r r o s i o n rate o f a 6 0 0 grit surface i n 0.15 M H 0 , p H 11, 2  50 °C is 3.3 m m / y  as compared w i t h 2.3 m m / y  2  f o r a n electropolished sample.  The  p o l a r i z a t i o n runs i n p e r o x i d e free a l k a l i n e solutions s h o w the same s m a l l difference i n the passive current density.  F e w s m a l l r a n d o m pits are o b s e r v e d o n the u n c o r r o d e d e l e c t r o p o l i s h e d s a m p l e i n F i g u r e 6.29.  T h e u n c o r r o d e d 6 0 0 grit surface o f F i g u r e 6.30 i s m u c h rougher.  A f t e r three hours  i n 0.15 M H 0 , at p H 1 1 , 5 0 ° C , the s u r f a c e a p p e a r s m u c h s m o o t h e r (see F i g u r e 6.31). 2  2  P i t t i n g as w e l l a s t h e o n s e t o f g r a i n b o u n d a r y c o r r o s i o n a r e o b s e r v e d .  Current Density, A / m F i g u r e 6.27  2  T h e p o l a r i z a t i o n curves o f electropolished and 6 0 0 grit s a m p l e s i n 0.15 H 0 , p H 1 1 , 5 0 °C. 2  2  55  M  Chapter 6  Results and  10  ;  E o  Discussion  1  1  '  1  I  •  I  1  i  i  1  i  1  •  A  EIS Data  •  •  •  LPR Data  m  i  1  -  d  o  electropolished sample  o  2.3  ro  A A  10'  •  A.A  •  a  A  A  m  o"  A  A A A ^ A A • a • _  A  •3 J}  A  •  "  :•  o-% o  10°  mm/y  (a  cr: c g  1  •j  .  i-H-  CD  \  '(/) O i_ s_  600 grit sample 3.3  o CJ  mm/y 1  0  20  40  60  80  ,  1  10 ,  100  1  ,  120  1  ,  140  1 160  1  180  Time, min Figure 6.28  T h e E I S data o f electropolished a n d 6 0 0 grit samples i n 0.15 M 11, 5 0 °C.  Figure 6.29  T h e uncorroded electropolished sample has a f e w cusps.  56  H 0 , 2  2  Chapter 6  Results and  Discussion  Chapter 6 6.1.5  Results and Discussion  Cold work  To investigate the effect o f cold work on the corrosion rate, an electropolished titanium sample was cold rolled to 4 5 % o f the original thickness from 1 . 8 4 m m to 0 . 8 m m thick. The  sample was subsequently corroded in 0 . 1 5 M H 0 , p H 1 1 , 5 0 °C. N o significant 2  2  difference was observed i n the corrosion rate o f the sample ( 2 . 7 mm/y) as compared to an annealed electropolished sample (2.3 mm/y). The corroded surface displayed no grain boundary corrosion, although some grooves were present i n the rolling direction (see Figure 6.32). Overall the surface appeared rougher which, i n part, is caused by the slight roughening o f the surface by the rolling process.  Figure 6.32  The cold worked surface after exposure to 0 . 1 5 M H 0 , p H 1 1 , 5 0 °C for 3 hrs. 2  58  2  Chapter 6  6.1.6  An  Results and  Discussion  Pickle Bath Conditions  alkaline peroxide pickle bath could eliminate environmental  associated w i t h hydrofluoric acid p i c k l i n g .  Hydrogen  a n d safety concerns  absorption i s also a concern i n  r e d u c i n g acids s i n c e e m b r i t t l i n g h y d r i d e f o r m a t i o n has dramatic effects o n the m a t e r i a l ' s mechanical properties.  I f an alkaline peroxide solution were to replace a hydrofluoric  a c i d p i c k l e bath, t h e c o r r o s i o n rates w o u l d h a v e t o b e o f c o m p a r a b l e m a g n i t u d e . addition, the surface m o r p h o l o g y i s important.  In  General corrosion i s preferred because it  produces a relative s m o o t h surface.  A  regular 6 0 0 grit w e i g h t loss c o r r o s i o n c o u p o n w a s submerged  for 10 minutes i n a  hydrofluoric acid pickle bath solution containing 2 . 2 % H F and 3 0 % nitric acid b y weight (5% HF, 35% H N 0  3  o f concentrated solutions).  A s the c h e m i c a l s w e r e m i x e d , the bath  temperature rose t o r o u g h l y 4 0 °C. T h e w e i g h t loss c o r r o s i o n rate w a s 1600 m m / y . T h e surface appeared shiny. U n d e r the s c a n n i n g electron m i c r o s c o p e , m a n y s h a l l o w pits w e r e observed together w i t h s o m e deeper ones.  A l t h o u g h some fissures appeared along grain  b o u n d a r i e s , w i d e s c a l e p i t t i n g g a v e a g e n e r a l c o r r o s i o n appearance (see F i g u r e 6.33).  Similar  corrosion coupons  were  submerged  for 10 minutes  i n alkaline p i c k l e baths  c o n t a i n i n g 2.5 M N a O H at - 9 5 ° C . O n e b a t h c o n t a i n e d a n a v e r a g e o f 0.2 M H 0 2  weight  loss c o r r o s i o n rate o f 7 7 0 m m / y w a s measured.  Whereas  grain  c o r r o s i o n w a s o b s e r v e d , i t w a s n o t as s e v e r e as at l o w e r c o r r o s i o n rates.  2  and a  boundary  Apparently,  these c o n d i t i o n s are s o s e v e r e , that the g r a i n s u r f a c e c o r r o d e s at a rate c o m p a r a b l e t o that o f the g r a i n b o u n d a r y .  F i g u r e 6.34 clearly s h o w s roughening o f the grain surface. T h e  c o u p o n a p p e a r e d d u l l w i t h faint b l u e a n d g o l d c o l o r e d areas. indicate the presence o f a t h i c k e n e d o x i d e f i l m .  59  T h einterference colors  Chapter 6  Figure 6.34  Results and  Roughening  Discussion  o f the g r a i n surface leads t o a d u l l appearance i n a n a l k a l i n e  peroxide bath.  60  Chapter 6  Results and Discussion  A second bath contained 2.5 M NaOH plus -0.5 M H 0 . The 2300 mm/y weight loss 2  2  corrosion rate measured in this pickle bath exceeded that obtained in the conventional hydrofluoric acid bath. The coupon displayed a mixture of gold and blue colors which were more intense than the colors obtained in the 0.2 M H 0 bath. Coloration of the 2  2  surface would indicate thickening of the surface oxide. This oxide, however, is not very protective and might be extensively hydrated and porous.  Figure 6.35 displays more  extensive grain boundary corrosion at 2300 mm/y in 0.5 M H 0 than was observed at 2  2  770 mm/y in 0.2 M H 0 . 2  Figure 6.35  Grain boundary corrosion in an alkaline peroxide solution of 2.5 M NaOH, 0.5 M H 0 , 95 °C. 2  6.1.6.1  2  A d d i t i o n of  2  Calcium  Even though the titanium surface pickled in alkaline peroxide appeared somewhat rougher and darker than titanium samples pickled in hydrofluoric acid, the corrosion mode was mostly one of general corrosion and the appearance is mostly a matter of  61  Chapter 6  esthetics.  Results and Discussion  W h e n further c o a t i n g o f the t i t a n i u m is desired, a s l i g h t l y rougher surface m a y  g i v e better adhesive properties.  The m a i n disadvantage o f alkaline peroxide  pickling  baths is the h i g h d e c o m p o s i t i o n rate o f the p e r o x i d e w h i c h leads to h i g h c h e m i c a l costs and difficulties w i t h composition control.  T h e t i t a n i u m c o r r o s i o n rate depends l i n e a r l y o n the concentration o f H 0 2  F i g u r e 6.36.  2  as i l l u s t r a t e d i n  A s the h y d r o g e n p e r o x i d e concentration increases, so does the  peroxide  d e c o m p o s i t i o n rate.  0.0  0.1  0.2  0.3  0.4  0.5  0.6  [HA], M F i g u r e 6.36  T h e t i t a n i u m c o r r o s i o n rate d e p e n d s l i n e a r l y o n the h y d r o g e n p e r o x i d e c o n c e n t r a t i o n . T h e h y d r o g e n p e r o x i d e d e c o m p o s i t i o n rate is s h o w n i n b o l d font.  S i n c e c a l c i u m w a s f o u n d to p r o m o t e p e r o x i d e s t a b i l i z i n g properties i n a l k a l i n e p e r o x i d e solutions, the effect o f adding relatively large concentrations o f c a l c i u m o n the peroxide decomposition Unfortunately,  rate  as  well  as  on  the  titanium  corrosion  rate  was  investigated.  a d d i t i o n o f c a l c i u m n o t o n l y s t a b i l i z e d the p e r o x i d e c o n c e n t r a t i o n , b u t it  also d e c r e a s e d the t i t a n i u m c o r r o s i o n rate.  W h e n 0.1 M  62  CaC0  3  w a s a d d e d t o 0.1  M  Chapter 6  Results and Discussion  H 0 , 2.5 M N a O H , at 95 °C, the corrosion rate decreased to 3.5 mm/y. U p o n addition o f 2  2  calcium carbonate, a gas evolved which is likely carbon dioxide gas as the hydrogen peroxide reacts with calcium to form calcium peroxide.  The peroxide was fairly stable  and decreased from 0.097 M to 0.096 M over a period o f 30 minutes.  The addition o f ~0.1 M C a C 0  3  to a pickle bath containing ~0.2 M H 0 2  2  resulted in a  titanium corrosion rate o f 440 mm/y, which compares well with a corrosion rate o f 415 mm/y i n 0.1 M H 0 . The peroxide consumption rate decreased to -0.82 ml/min. This 2  2  small benefit o f calcium with a slightly higher corrosion rate and lower peroxide decomposition rate is small and is likely not significant. The surface morphology o f the two samples was very similar (see Figures 6.37 and 6.38).  Figure 6.37  A titanium corrosion rate o f 415 mm/y was measured in 0.1 M H 0 , 2.5 M N a O H , at 95 °C. 2  63  2  Chapter 6  F i g u r e 6.38  Results and  Discussion  I n t h e p r e s e n c e o f 0.1 M C a C 0  3  and 0.2 M H 0 2  2  ,the c o u p o n m o r p h o l o g y  a n d c o r r o s i o n r a t e r e s e m b l e t h a t o b t a i n e d i n 0.1 M H 0 . 2  6.1.6.2  Thermal Oxidation  Ordinarily, a p i c k l e bath i snot used toremove layer  underneath  processing.  2  t h e scale  produced  freshly  b y thermal  polished metal, but an oxygen-rich oxidation during  T w o c o u p o n s w e r e h e a t e d i n a i r at 1 1 0 0 ° C f o r s i x h o u r s .  high  temperature  T h e presence o f  n i t r o g e n p r o d u c e d a -0.33 m m b r i t t l e g r a y o u t e r s c a l e , w h i c h w a s e a s i l y r e m o v e d b y hand. pm  The subsequently exposed o x y g e n stabilized alpha layer i s estimated to b e - 1 0 0  t h i c k [ 5 2 ] . T h e o x i d e r e m o v a l rate w a s m e a s u r e d i n a h y d r o f l u o r i c a c i d p i c k l e  s o l u t i o n a n d a n a l k a l i n e p e r o x i d e s o l u t i o n as b e f o r e .  Dissolution reactions occured w i t h  the t i t a n i u m o x i d e l a y e r w h i c h i s t h e r m o d y n a m i c a l l y m o r e stable than the t i t a n i u m metal. C o r r o s i o n rates are, therefore, e x p e c t e d t o b e l o w e r than those o f the titanium metal.  64  freshly  polished  Chapter 6  The  Results and  average  mm/y.  weight  Discussion  l o s s c o r r o s i o n rate i n t h e h y d r o f l u o r i c  acid pickle bath was 7 6 0  T h e s u r f a c e d i s p l a y e d w i d e s p r e a d c r a c k i n g w i t h s o m e p i t s n e a r the c r a c k s (see  Figure 6.39).  C r a c k i n g c o u l d be the result o f l o c a l i z e d c o r r o s i o n c o m b i n e d w i t h a h i g h l y  brittle surface oxide.  T h e alkaline peroxide concentration w a s slightly higher than before w i t h a n average o f 0.62 M H 0 2  2  at 9 5 C i n 2.5 M N a O H .  T h e o x i d e r e m o v a l r a t e w a s m e a s u r e d at - 1 8 0 0  m m / y w h i c h i s s u b s t a n t i a l l y h i g h e r than the c o r r o s i o n rate m e a s u r e d i n h y d r o f l u o r i c a n d o n l y s l i g h t l y l o w e r t h a n the d i s s o l u t i o n rate o f the corrosion l e d to a roughening  freshly  polished metal.  General  o f the surface w i t h n o c r a c k s o r pits (see F i g u r e  B o t h samples had a dull silvery color.  T h e surface f i n i s h i s n o t that i m p o r t a n t  stage since after ten m i n u t e s the o x y g e n - r i c h surface l a y e r has not b e e n entirely  acid  6.40). at this  removed  yet i n either p i c k l e bath.  F i g u r e 6.39  In a hydrofluoric  a c i d p i c k l e bath, surface alpha case leads to widespread  cracking.  65  Chapter 6  Figure 6.40  Results and Discussion  Extensive roughening  o f an alpha case titanium surface is observed i n an  alkaline peroxide p i c k l e bath.  T i t a n i u m Single Crystal Corrosion  6.1.7  T i t a n i u m ' s c o r r o s i o n b e h a v i o r i n a l k a l i n e p e r o x i d e e n v i r o n m e n t s m a y be affected b y the crystallographic  orientation  of  the  metal.  To  explore  the  possibilities o f  preferred  dissolution o f certain crystallographic planes, an electropolished single crystal ( ~ 2 m m ~2mm  square x ~ 7 m m )  M  2  H 0 , at p H 2  was suspended in an alkaline peroxide solution containing  11 a n d 5 0 ° C ,  for three hours.  No  x  0.15  differences w e r e o b s e r v e d i n the  c o r r o s i o n appearance o f the orthogonal sides, a l l o f w h i c h d i s p l a y e d r a n d o m pitting.  A n o t h e r s a m p l e w a s p r e p a r e d as d e s c r i b e d i n C h a p t e r 5 a n d i l l u s t r a t e d b y F i g u r e T h e three o r t h o g o n a l surfaces w e r e r a n d o m l y  l a b e l e d s u r f a c e 1, 2 a n d 3 .  By  5.13.  shielding  t w o s u r f a c e s w i t h s i l i c o n e s e a l a n t , t h e t h r e e s u r f a c e s w e r e e x p o s e d o n e at t h e t i m e t o 0 . 2 5 M  H 0 , p H 1 1 , 7 0 °C. 2  2  T h e c o r r o s i o n rate w a s m o n i t o r e d o v e r the d u r a t i o n o f the run.  66  Chapter 6  Results and Discussion  T h e plane orientations were determined b y illustrated in Figure 6.41.  the b a c k reflection L a u e method  and  are  T h e s i n g l e c r y s t a l p l a n e o r i e n t a t i o n is p a r a l l e l w i t h the c o u p o n  surface a n d is d r a w n i n F i g u r e 6.41  i n s u c h a w a y a s i f o n e w e r e l o o k i n g at a c r o s s -  section o f the c o u p o n .  surface 1 (1 5 4 4 ) F i g u r e 6.41  surface 2  1  (4  surface 3 (2  3 1)  4 2  13)  C r y s t a l orientations o f single c r y s t a l surfaces 1 , 2 , a n d 3.  T h e c o r r o s i o n rates r e m a i n e d r e l a t i v e l y c o n s t a n t t h r o u g h o u t t h e d u r a t i o n o f the test f o r a l l three surfaces.  T h e p o l a r i z a t i o n resistances, as m e a s u r e d b y E I S a n d L P R , d i d not v a r y  m u c h b e t w e e n surfaces 1 and 3, w h i c h c o m p a r e d v e r y w e l l w i t h the general G r a d e specimens.  2  S u r f a c e 2, w h i c h is o n l y 8.6 d e g r e e s f r o m the c l o s e s t p r i s m plane, h a d a  s l i g h t l y h i g h e r p o l a r i z a t i o n resistance as o b s e r v e d i n F i g u r e 6.42. the c r y s t a l l o g r a p h i c appearance o f surface 2.  67  F i g u r e 6.44 illustrates  Chapter 6  CM  Results and Discussion  10'  E o  •  ,  o  EIS data  •  ,  v  LPR data O  a  Surface 2  O o  c "GO GO  o  v  v  o  v  W  ^10  o  o  v  o-t o  1  CO  o  o" 10  1  • • A •  •  *  A  0 or  1—1-  58 mm/y  Surface 1  c o  73  Grade 2 Titanium  CD  3 3  10^  GO O s_ i_  O O  10°  0  20  40  60  80  100  120  Time, min F i g u r e 6.42  T h e p o l a r i z a t i o n r e s i s t a n c e o f s u r f a c e 2 i s s o m e w h a t h i g h e r t h a n that o f the other surfaces.  A l t h o u g h the p o l a r i z a t i o n resistances w e r e v i r t u a l l y the same f o r surfaces 1 a n d 3 , quite distinct crystallographic patterns w e r e observed o n S E M photographs 6.45,  a n d 6.46).  Surface  3 , w h i c h i s 2 6 degrees  from the basal plane,  extensive p i t t i n g w h i c h leads t o significant r o u g h e n i n g o f the surface. only  10.2degrees  from a pyramidal  plane,  suffers  (see F i g u r e s  from  suffers  6.43, from  S u r f a c e 1, w h i c h i s  the formation  o f directional  corrosive alkaline peroxide conditions, d i dpreferred  dissolution o f  grooves and r o u g h e n i n g o na smaller scale.  Only  under  highly  certain c r y s t a l l o g r a p h i c planes l e a d t o distinctive surface patterns. orientation  o n the polarization  resistance w a srelatively  conditions.  68  T h e effect o f crystal  small under  these  corrosive  Chapter  6  Results  and  Discussion  Chapter 6  Results and  Discussion  Chapter 6  Results and Discussion  Inhibition by Calcium  6.2  A t first sight, it appears that c a l c i u m i s a n e f f e c t i v e i n h i b i t o r .  Addition of  100  ppm  c a l c i u m d e c r e a s e d the c o r r o s i o n rate to b e l o w the d e t e c t i o n l i m i t i n F i g u r e 6.47a, b, c.  and  T h e p a s s i v e c u r r e n t d e n s i t i e s o f t h e s e p o t e n t i o d y n a m i c s c a n s , t a k e n at t h e e n d o f t h e  ~ 3 h o u r r u n , appear to b e i n d i c a t i v e o f the w e i g h t l o s s c o r r o s i o n rate.  H o w e v e r , as t h e  p e r o x i d e c o n c e n t r a t i o n and temperature increased, the c a l c i u m i n h i b i t e d p a s s i v e current d e n s i t y b e c a m e less r e p r e s e n t a t i v e o f the w e i g h t l o s s c o r r o s i o n rate, as i l l u s t r a t e d b y F i g u r e 6.47e.  T h i s i n d i c a t e s that the c o r r o s i o n rate i n the c a l c i u m i n h i b i t e d s o l u t i o n m u s t  have increased w i t h time.  In F i g u r e s 6.47a, b, a n d c , the a d d i t i o n o f c a l c i u m l o w e r e d the o p e n c i r c u i t p o t e n t i a l .  The  c a t h o d i c c u r r e n t d e n s i t y m a y h a v e d e c r e a s e d as t h e a d d e d c a l c i u m i n h i b i t s t h e r e d u c t i o n of hydrogen peroxide.  At  the m o r e  severe conditions o f Figures  6.47d and  e,  the  m e c h a n i s m a p p e a r e d t o b e d o m i n a t e d b y a n o d i c i n h i b i t i o n . A n i n c r e a s e from 5 0 ° C t o 7 0 °C  at p H  11 a n d 0 . 0 5  M  H 0 2  2  i n F i g u r e 6.47c, increased the a n o d i c current  density  w i t h o u t a n y effect o n the o p e n c i r c u i t potential.  P o t e n t i o d y n a m i c scans, h o w e v e r , do not s h o w h o w the c o r r o s i o n b e h a v i o r changes w i t h time.  S c a n s t h r o u g h a large potential range alter the surface.  O x i d e s a r e r e d u c e d at  c a t h o d i c p o t e n t i a l s a n d t h e o x i d e l a y e r t h i c k e n s at a n o d i c p o t e n t i a l s .  Linear polarization  resistance tests ( L P R ) a n d e l e c t r o c h e m i c a l i m p e d a n c e s p e c t r o s c o p y ( E I S ) are m u c h m o r e suitable techniques for c o r r o s i o n m o n i t o r i n g w i t h m i n i m a l effects o n the c o u p o n surface. Using L P R  and EIS  to m e a s u r e the i n h i b i t i v e effect o f c a l c i u m w i t h t i m e , it is  o b s e r v e d t h a t t h e c o r r o s i o n r a t e i n c r e a s e d d r a m a t i c a l l y o v e r t h e t h r e e h o u r r u n at p H 0.15 M  H 0 , 7 0 °C 2  2  (see F i g u r e 6.48e).  These effects w e r e less p r o n o u n c e d  conditions, but, nevertheless, still present.  71  now 11,  at m i l d e r  Chapter 6  Results and  Discussion  Current Density, A / m Figure 6.47a  Current Density, A / m  5 0 °C, p H 10, 0.05 M H 0 2  2  F i g u r e 6.47b  5 0 °C, p H 10, 0.15 M H 0 2  2  no Ca  1.5  -100 ppm Ca  1.0  0.5  °-  oo  £o  10"*  Current Density, A / m Figure 6.47c  •4  0.04 mm/y  10  3  10'  2  10 ' -  10°  Current Density, A / m  5 0 °C, p H 11, 0.05 M H 0 2  2  2-3 mm/y  •%  •5  Figure 6.47d  10  1  2  5 0 °C, p H 11, 0.15 M H 0 2  2  Current Density, A / m F i g u r e 6.47e  7 0 ° C , p H 11, 0.25  M H 0  Figure 6.47  Inhibition b y calcium.  2  2  W e i g h t loss c o r r o s i o n rates are s h o w n i n b o l d font.  72  Chapter 6  O  d  <u o c ro  10*  Results and Discussion  •  A  EIS data  •  A  L P R data  E  o  1 0 0 ppm C a  A<*  s  ^  A A A  co CD CC c g 'to S o O  • •in" 10'  o  2  0.03 mm/y  10' 3  10  io  100  e  •  A  •  A  •  1 0 0 ppm  A  A  A A A A &  co cu  .  C»o» 1 0 0 ppm C a  io"  70 C,  ° •  c %  c»o»  0.04 mm/y  O  j  10'  3  0  20  o  d  CD O  co cu  10  4  60  80  l> CO  73  CC C  3 3  o "co 2  180  •  A  EIS data  o  A  L P R data  A  AA  Ca  10'  0.43 mm/y  20  40  60  g  ro "co 'co  o  10"  80  0  20  40  60  6?  io  80  100  140  160  CC o  'co o o O  180  Time, min  180  2  A .  10"  120  | •<"  5 0 ° C , p H 10, 0.15 M H 0  5  •  A  •  A  A  A A  L P R data  o  q  io-  2  A  Ca  3  0.04 mm/y  CO  1 0 0 ppm  10"'  g  10°  CD  3  10  !  • o"  B O  ••• 4 •• Q• • ••  Q)  no C a  20  40  60  80  100  3  10'  2.3 mm/y  ,  2  E I S data  10  120  140  Time, min  160  180  5 0 ° C , p H 11, 0.15 M H 0 2  10  2  3  •  A  E I S data  •  A  L P R data  O 10°  1 0 0 ppm  10  2  10'  •  •  A  . A  0  A  A  A  10'  A  7J 03  A A  3 3  • • • • C* • no C a 58  10"  o'  mm/y  A  A  q  o  CO  Ca  6.6 A  CD  3 3 100 120 140 160  CD  Time, min  CD  CO  jj?  10-  10°  d  o , 3  1 4 mm/y  0  o  o  10  no C a  10'  no C a  Figure 6.48d  2  E  r  1 0 0 ppm  2  0.35 mm/y  o 2  n  Ca  3  o «>< 0  5 0 ° C , p H 11, 0.05 M H 0  A 4 /  cu  CD  D  100 120 140 160  o c ro  CU  tu  UA  0C o 'co o l_ 1— o O  40  1'  c  ro ^—'  .<2  *  o  Time, min  Figure 6.48c  E  —  d  o'  0*>  0.11 mm/y  10  o O  °  : „ c a "  O o —I CO  2  10'' .  10  o  —i  <0.01 m m / y  . A  io=  cc  E  Ca  O o -i , 3 10 w o 1 0  A A AA  E I S data  J  L P R data  <0.01 m m / y  10- ^  F i g u r e 6.48b  2  L P R data  0  10 A .  100 ppm  o 2  A *  A  E I S data  A  !  o  5 0 ° C , p H 10, 0.05 M H 0  A ^  A  A  •  s  150  Time, min  10  A  •  CO  3  50  * ^ A  .<2  CO CD  1  A  8  o  cn o'  10"  ° i  CD~  no C a  Figure 6.48a  E  s  A  CO  d  £  3  <0.01 mm/y  10  10-  io'  20  •  c  mm/y  40  60  80  Time, min  100  120  F i g u r e 6.48e  7 0 ° C , p H 11, 0.15 M H 0  F i g u r e 6.48  T h e i n h i b i t i v e e f f e c t o f c a l c i u m i s s h o r t - l i v e d at m o r e c o r r o s i v e c o n d i t i o n s .  2  Figure 6.48f  2  7 0 ° C , p H 11, 0.25 M H 0  W e i g h t l o s s c o r r o s i o n rates are s h o w n i n b o l d font.  73  2  2  Chapter 6  Results and Discussion  35  1 1'  1  1  -1  O  r  -  noCa  30 o  •  25  .-2  20  100 ppm Ca  O CO  Q-  15 10  0  40  80  120  160  200  240  280  0  20  40  60  Time, min  Figure 6.49a  50 °C, p H 10, 0.05 M H 0 2  70  E o  • • •  60  o  CO CL CO  110  —, 1  100  N  O  •  90  30  •  •  °  •  60 |-  c  50  CO O CO CL  40  O 20  40  60  80  •  180  100 120 140 160  1  111 .  100 ppm Ca  20 0  20  40  60  °  1  •  60  • 0  i  20  •  i  40  • 60  •  450 N  g  •  i  400  80  •  :  i  160  200  co a  • 140  250  £  g.  160  8  180  2  •  •  300  <0 O  100 ppm Ca 120  •  LL.  • •  . 100  140  i  1  •  •  -  •  •  •  150  100 .  •  •  •  0  50 0  Time, min  •  20  40  60  80  100  120  140  Time, min  Figure 6.49e 70 °C, p H 11, 0.15 M H Q 2  Figure 6.49  no Ca 100 ppm Ca  350  •  20  120  2  •—,  •  •  •  40  100  Figure 6.49d 50 °C, p H 11, 0.15 M H 0  2  -  0  100 80  80  B  •  120  0  1[ .  1  no Ca  140  o c ro o co  1  °  •  Time, min 2  1  2  10  180  50 °C, p H 11, 0.05 M H Q  • i •-|  160  •  D  30  Time, min  Figure 6.49c  160  noCa  CD O  .CO  20  O 0  140  , . ! . 1. — 1  70  40 o  120  2  80  CO  100  Figure 6.49b 50 °C, p H 10, 0.15 M H 0  2  no Ca, 50 C 100 ppm Ca, 50 C 100 ppm Ca, 70 C  50  CD  80  Time, min  Figure 6.49f  2  70 °C, p H 11, 0.25 M H 0  T h e effect o f c a l c i u m o n the charge transfer capacitance.  74  2  2  Chapter 6  Results and  Discussion  T h e E I S d a t a w e r e f i t t e d t o a o n e - t i m e - c o n s t a n t m o d e l (see F i g u r e 5.14). transfer capacitance, C  c t  , w h i c h i s associated w i t h theoxide, increased w i t h increasing  c o r r o s i o n rate, i.e. i t i n c r e a s e d as t h e p H , p e r o x i d e i n c r e a s e d (see F i g u r e 6.49).  concentration, a n d temperature  The addition o f calcium reduced C . c t  w a s r e l a t i v e l y c o n s t a n t at 1 5 u F 7 c m . 2  period.  T h e charge  A t 7 0 °C, p H 11, C  c t  A t 5 0 °C, p H 11, C  c t  A t 5 0 ° C , p H 10, C  c t  i n c r e a s e d s l i g h t l y o v e r the test  i n c r e a s e d s i g n i f i c a n t l y o v e r the three h o u r test r u n . A n y  effect o f c a l c i u m h a d d i s a p p e a r e d at 7 0 ° C , p H 1 1 , 0.25 M H 0 . 2  2  I n the a b s e n c e o f h i g h c o r r o s i o n rates, the o x i d e i s n o t e x p e c t e d t o g r o w r a p i d l y i n the presence o f c a l c i u m . A s m a l l charge transfer capacitance is, therefore, m o r e l i k e l y related to a l o w d i e l e c t r i c c o n s t a n t o f the s u r f a c e o x i d e .  A s the s o l u t i o n c o r r o s i v i t y increased,  the o x i d e g r e w , b e c a m e hydrated, a n d increased i n p o r o s i t y w h e r e pores m a y have f i l l e d up  with  conductive  ions.  Consequently,  the dielectric constant  increased  to give  capacitance values comparable tothose o f uninhibited solutions.  T h e e n d o f e a c h r u n w a s o r d i n a r i l y f o l l o w e d b y a p o l a r i z a t i o n r u n t o 1.5 V sample polarized i n 0.15 M removed,  washed  alkaline peroxide concentration. was  H 0 , p H 11, 7 0 °C containing 2  w i t h water solution  2  a n d ethanol  o f the same  a n d subsequently  p H , temperature  .  The  100p p m calcium, w a s resubmerged  and peroxide  in a  fresh  and calcium  P r o b a b l y as a r e s u l t o f the p o l a r i z a t i o n treatment, the c o r r o s i o n r e s i s t a n c e  i n i t i a l l y as h i g h  as the freshly  electropolished sample.  resistance h a d d r o p p e d ~ 1 0 0 fold, the sample w a s r e m o v e d w i t h water.  S H E  from  When  the polarization  the s o l u t i o n and w a s h e d  A w h i t e c a l c i u m deposit w a s present o n the surface w h i c h w a s  r e m o v e d b y r u b b i n g w i t h a g l o v e d finger.  readily  U p o n r e s u b m e r s i o n , the c o r r o s i o n resistance  w a s as l o w a s i t w o u l d h a v e b e e n i f t h e s a m p l e h a d n o t b e e n r e m o v e d at a l l .  T h i s p r o c e d u r e w a s r e p e a t e d t o test f o r r e p r o d u c i b i l i t y .  T h ecoupon was n o w  cleaned  w i t h w a t e r a n d ethanol, dried, a n d c o o l e d to r o o m temperature i n a n attempt to restore the original  condition  o f the  fresh  coupon.  However,  upon  resubmersion  the coupon  p r o c e e d e d t o c o r r o d e at the h i g h rate, a l m o s t a s i f there w a s n o c a l c i u m i n h i b i t o r present  75  Chapter 6  Results and Discussion  i n t h e s o l u t i o n at a l l (see F i g u r e 6.50).  T o c o n f i r m that this effect w a s t h e result o f  changes i n the c o u p o n surface oxide, a freshly electropolished sample w a s submerged a n d tested. runs,  T h e latter d i s p l a y e d a h i g h i n i t i a l c o r r o s i o n resistance as e x p e c t e d f r o m i l l u s t r a t i n g that t h e s o l u t i o n w a s n o t t h e r e a s o n  previous  for the drop i n the corrosion  resistance o f the o l d sample.  E o  d  ~I  1  1  '  T"  •  EIS data  •  o  L P R data  io Hi 4  O  oi" o c  to  •  100 ppm C a g  0.43 mm/y  GO  o'  1 0  CO CD  tr c  o o  10°  QJ. CD  icr °m  O  O  •q  14 mm/y  O  ° ° #  9  10  3 3  no C a  'GO GO  o  o 3  10"  1  #  o oO #  c o co  #  10  1  #  1  0  20  40  60  80  100 120 140 160 180  Time, min Figure 6.50a  L o s s o f p a s s i v a t i o n at 7 0 ° C , p H 1 1 , 0 . 1 5 M H 0 , 100 p p m C a . 2  E o  d  •  10 r_ 4  1  1  1  2  •  EIS data  •  L P R data  Fresh solution, Coupon has been anodized and cleaned  0  o c CD -2 io k •4—>  ^10"  1  Fresh coupon  o S  3  Coupon was removed,  GO  rubbed and cleaned  'oo  10°  CD  01  c  o  CD  10  1  washed with water, alcohol, dried and cooled to R.T.  10  o' «-t-  Coupon was removed,  2 i_  o  GO  73 Di  10'  tn  O  o  3 3 «<•  1  50  100  150  200  250  Time, min Figure 6.50b  R e s u b m e r s i o n at 7 0 ° C , p H 1 1 , 0 . 1 5 M H 0 , 100 p p m C a .  Figure 6.50  C a l c i u m affects the t i t a n i u m o x i d e rendering it less c o r r o s i o n resistant.  2  76  2  Chapter 6  Results and Discussion  A d d i t i o n a l tests w e r e p e r f o r m e d t o c o n f i r m t h e a b o v e results a n d t o further s t u d y t h e effects o f c a l c i u m o n the t i t a n i u m surface o x i d e a n d the r a m i f i c a t i o n s f o r the c o r r o s i o n behavior  i n subsequent  solutions o f different  concentrations.  Several coupons  were  submerged i n uninhibited a n d calcium inhibited alkaline peroxide solutions containing 0.15 M H 0 , p H 11, 7 0 °C. A t the e n d o f the c o r r o s i o n run, the c o u p o n s w e r e r e m o v e d , 2  2  w a s h e d and tested i na m i l d e r environment consisting o f 0.15 M H 0 , p H 10, 5 0°C w i t h 2  2  a n d w i t h o u t 1 0 0 p p m C a . T h e results are r e p o r t e d i n F i g u r e s 6.51 a n d 6.52.  A  A  Freshly polished sample  %  O  Previously corroded for -2.5 hrs at 14 mm/y in  •  •  Previously corroded f o r - 3 hrs at 1.7 mm/y in  10"  0.15 M H 0 , pH 11, 70 C 2  2  0.15 M H 0 , pH 11, 70 C , 100 ppm C a 2  2  ^10"  1  O o  3  AA*A «^  10-  CO  o"  A  10'  0  20  40  A  AA  o  60  o  80  o oo  100  120  •  73 10  U  i—iCD  3 3  140  Time, min F i g u r e 6.51  I n 0.15 M H 0 , p H 1 0 , 5 0 ° C , t h e c o r r o s i o n rate appeared independent o f 2  2  previous exposure to c a l c i u m (solid s y m b o l s - E I S data, open s y m b o l s L P R data).  77  Chapter 6  CN  Results and Discussion  • 0  A O  •  •  2  10 d  E o  Freshly polished sample Previously corroded for -2.5 hrs at 14 mm/y in 0.15MH O , pH 11.70C Previously corroded for-3 hrs at 1.7 mm/y in 0.15 M H 0 , pH 11, 70 C, 100 ppm Ca  b  2  d  2  2  o  0"  o c cc  1 0 -3  AA  -1—> CO CO CD  10  CC d  •  o  a  o —1 oco o'  O  — i »-  CO  o  CD  •  — t  O  O  0  20  40  60  • 80  •  100  10"  3 3  120  Time, min Figure 6.52  I n 0.15 M H 0 , p H 10, 5 0 °C, w i t h 100 p p m C a , p r e v i o u s e x p o s u r e to 2  2  c a l c i u m affected the t i t a n i u m o x i d e r e n d e r i n g it less c o r r o s i o n resistant ( s o l i d s y m b o l s - E I S data, o p e n s y m b o l s - L P R data).  When  a  coupon,  containing 0.15  M  which  had  H 0 , pH 2  2  corroded  for  3  hours  in  11, 70 C , w a s submerged  a  calcium inhibited  in a  fresh  calcium  solution inhibited  a l k a l i n e p e r o x i d e s o l u t i o n , the c o r r o s i o n rate w a s r o u g h l y t e n t i m e s h i g h e r t h a n that o f a freshly  polished coupon.  A c o u p o n w h i c h h a d c o r r o d e d at a h i g h r a t e f o r 2 . 5 h o u r s i n a  c a l c i u m - f r e e s o l u t i o n , w a s c o r r o d i n g at t h e s a m e l o w r a t e a s t h e (see F i g u r e 6 . 5 2 ) .  freshly  polished coupon  T h e s e results raise a serious c o n c e r n for the l o n g t e r m effectiveness o f  c a l c i u m as a n i n h i b i t o r i n a p l a n t e n v i r o n m e n t w i t h p l a n t upsets l e a d i n g o c c a s i o n a l l y to high p H and peroxide concentrations.  78  Chapter 6  Results and Discussion  0.22 LU X  Freshly polished sample - O - Previously corroded for -2.5 hrs at 14 mm/y  0.20  2  2  Previously corroded for -3 hrs at 1.7 mm/y in  0.18 ro  in  0.15MH O , pH11,70C 0.15 M H 0 , pH 11, 70 C, 100 ppm Ca 2  2  0.22 0.20 0.18  0.16  0.16  0.14  0.14  CL  0.12  0.12  rz g 'oo  0.10  0.10  0.08  0.08  o O  0.06  c CD  o  2 i_  0.04  Time, min Figure 6.53  C o r r o s i o n p o t e n t i a l i n 0.15 M H 0 , p H 10, 5 0 °C. 2  2  A -—  Freshly polished sample  -O—  Previously corroded for -2.5 hrs at 14 mm/y  in  0.15MH O , pH 11.70C 2  2  Previously corroded for -3hrs at 1.7 mm/y  0.4  0.15 M H 0 , pH 11, 70 C, 100 ppm Ca 2  LU X C/3  •S  in  0.4  2  0.3  0.3  0.2  0.2  0.1  0.1  0.0  0.0  CD »  O Q_ c o GO o o O  0.1 U 0  • 1_ 20  40  60  80  -0.1 100 120 140 160 180  Time, min Figure 6.54  C o r r o s i o n potential i n 0.15 M H 0 , p H 10, 5 0 °C, 100 p p m C a . 2  79  2  Chapter 6  Results and Discussion  In a c a l c i u m - f r e e s o l u t i o n , t h e c o r r o s i o n potential i s independent o f the sample's history (see F i g u r e 6.53).  I n a calcium-inhibited solution, the presence o f c a l c i u m lowers the  o p e n c e l l potential o f the  freshly  p o l i s h e d s a m p l e (see F i g u r e 6.54).  T h e freshly polished  s a m p l e e x p e r i e n c e d a c o r r o s i o n rate i n 0.15 M H 0 , p H 1 0 , 5 0 ° C , 100 p p m C a , w h i c h 2  2  w a s e q u i v a l e n t t o the c o r r o s i o n rate o f the s a m p l e w h i c h h a d p r e v i o u s l y b e e n c o r r o d e d i n a c a l c i u m - f r e e s o l u t i o n . T h e latter, h o w e v e r , w a s c o n s i s t e n t l y at a h i g h e r p o t e n t i a l .  Even  the s a m p l e w h i c h h a d p r e v i o u s l y b e e n c o r r o d e d i n a c a l c i u m i n h i b i t e d s o l u t i o n and w h i c h s h o w e d a h i g h e r c o r r o s i o n rate t h a n the  freshly  p o l i s h e d sample i n 0.15 M H 0 , p H 10, 2  50 °C, 100 p p m C a , h a d a more anodic corrosion potential than the freshly  2  polished  sample.  120  A  Freshly polished sample  O  Previously corroded for -2.5 hrs at 14 mm/y in  •  Previously corroded for -3 hrs at 1.7 mm/y in  0.15 M H 0 , pH 11,70 C 2  100  LL zL CD  O JZ  0.15 M H 0 , pH 11,70 C, 100 ppm Ca 2  2  80  • • •  60  CD -t— *  'o  40  o  20  CO Q_ CO  2  O  0  °  • O  o  •o  A AAA A A 20  40  60  A  80  100  0  0  120 140  Time, min  F i g u r e 6.55  C h a r g e transfer capacitances i n 0.15 M H 0 , p H 10, 5 0 °C, 100 p p m C a . 2  2  Consequently, a shift towards a m o r e anodic corrosion potential does not i m p l y a passive c o n d i t i o n w i t h l o w e r c o r r o s i o n rates.  T h e o b s e r v e d o p e n c e l l p o t e n t i a l s are a f u n c t i o n o f  the nature o f the d o u b l e l a y e r a n d t h e u n d e r l y i n g o x i d e , itsp o r o s i t y a n d i t s affinity f o r ions.  T h e c h a r g e transfer c a p a c i t a n c e s o f the a b o v e s a m p l e s are p l o t t e d i n F i g u r e  6.55.  The previously corroded samples h a d comparable capacitances w h i c h were significantly  80  Chapter 6  Results and  Discussion  h i g h e r than the capacitance o f the freshly p o l i s h e d sample.  A more porous, thicker oxide,  w i t h a h i g h e r dielectric constant i s expected t o b e the reason f o r the h i g h e r capacitance values o f previously corroded specimens.  6.2.1  Effect o f Carbonate  T h e added 100 p p m C a i n the f o r m o f C a ( O H )  2  never dissolved entirely.  The  solution  had a m i l k y w h i t e c o l o r regardless o f p H , temperature o r peroxide concentration. T h e s u s p e n d e d s o l i d s w e r e f i l t e r e d o f f at t h e e n d o f t h e r u n a n d a n a l y z e d b y X - r a y d i f f r a c t i o n . A t a l o w e r p H o f 10, the s o l i d s w e r e entirely c o m p r i s e d o f c a l c i u m carbonate. a m i x t u r e o f c a l c i u m carbonate and c a l c i u m p e r o x i d e w a s present. w a s the C 0  2  f r o m the s u r r o u n d i n g air.  A t p H 11,  T h e carbonate source  W i t h the apparatus u s e d f o r the experiments, a  nitrogen p u r g e d i d not s h i e l d the s o l u t i o n effectively e n o u g h t o prevent the absorption o f carbon dioxide.  T o investigate the effect o f carbonate o n the t i t a n i u m c o r r o s i o n resistance, several runs were performed  w i t h the addition o f c a l c i u m carbonate instead o f c a l c i u m  F i g u r e 6.56 s h o w s h o w the a d d i t i o n o f C a C 0 more q u i c k l y than addition o f C a ( O H ) . 2  3  hydroxide.  seems to l o w e r the c o r r o s i o n resistance  N o n e t h e l e s s , X R D analysis o f the s o l i d s i n b o t h  cases i n d i c a t e d the presence o f c a l c i u m carbonate. N o c a l c i u m p e r o x i d e w a s detected.  T h e faster d e c a y i n c o r r o s i o n resistance w i t h C a C 0  3  addition i spuzzling because C a C 0  w a s i n t h e f i n a l s u s p e n s i o n w h e t h e r c a l c i u m w a s a d d e d as C a C 0 change  i n the solution  phenomenon.  chemistry  i s therefore  3  orCa(OH) . 2  A gradual  not likely to b e the reason  It i s m o r e p l a u s i b l e that t h e c h a n g e  3  i n oxide occurred more  f o r this rapidly,  p o s s i b l y because o f a n i m p u r i t y w h i c h w a s present i nthe c a l c i u m carbonate but is l a c k i n g f r o m o r present i n s m a l l e r concentrations i nthe c a l c i u m h y d r o x i d e .  81  Chapter 6  Results and Discussion  E o  ~\  i  1  i  1  r  1  100 ppm Ca from Ca(OH) , 2  d  purged with N , <0.01 mm/y  ^JtQ^  2  CD O  c  10  CO "GO CO CD  c g o  \  10  <J  n •  s  *  100 ppm Ca from CaC0 0.03 mm/y  I  o —1 o  10 -2  3  4  72  2  pH 11, 70 C, 100 ppm Ca(OH)  3  ^A A*A A /f  A 20  40  A  Ca-free solution, 0.35 mm/y _l i l i I i l  60  CD  10"  2  0  o  Q) (—1-  -1.7 mm/y in 0.15 M H 0 ,  O O  (J) ZJ  previously corroded at 2  10  o  10"  80  100 120  140  160  10  u  180  Time, min F i g u r e 6.56  Addition o f C a C 0  3  decreases t h e c o r r o s i o n r e s i s t a n c e at a faster rate t h a n  the addition o f C a ( O H )  2  i n 0.15 M H 0 , p H 10, 5 0 ° C ( s o l i d s y m b o l s - E I S 2  data, o p e n s y m b o l s - L P R  2  data).  E v e n though the corrosion resistance i n calcium-containing solutions decreased, was  still some  solution.  protection obtained  f r o m the c a l c i u m as compared  there  to a calcium-free  H o w e v e r , as the c o r r o s i v i t y o f the s o l u t i o n i n c r e a s e s , a n y p r o t e c t i o n o f f e r e d b y  c a l c i u m disappears  quickly.  In 0.15 M H 0 , p H 11,7 0 °C, theaddition o f C a C 0 2  2  3  accelerates this process as i s illustrated i n F i g u r e 6.57.  A g a i n , i t i s u n l i k e l y that this i s  brought  calcium peroxide  carbonate  about  b y solution  concentrations  equilibration  appear  as the final  to b e a function  concentration and n o t the initial starting material. consist o f mixtures o f C a C 0  3  and C a 0  p H a n d peroxide  T h e analyzed solids were found to  w h e r e the height o f the m a i n C a C 0  2  about 3 0 % o f the height o f the m a i n C a 0  o f the solution  and calcium  2  peak.  82  3  peak  was  Chapter 6  Results and Discussion  <N  E o G  CO co CD  c g CO ot_ t_ o O  A O  •  •  Ca-free solution, 14 m m / y 100 ppm Ca from Ca(OH) , 0.43 m m / y 100 ppm Ca from CaC0 , 2.1 m m / y 2  3  10 «Q 4  <D o  3  A %  •  O •  M  • •  10  3  i 10"  o  • o • Pi •  3  o, 10  %o  10' A  A  10  A A  A  A A  A  4  A A  10  1  0  20  40  60  80  100  70  33 *< CD  A A  c  cn O <—»-  Plr A  O o—i  120  140  160  Time, min F i g u r e 6.57.  In 0.15 M  H 0 , pH 2  2  1 1 , 7 0 °C,  the addition o f C a C 0  all protection offered b y c a l c i u m i n ~ 2 hours,  3  virtually eliminates  (solid s y m b o l s - E I S data,  . o p e n s y m b o l s - L P R data).  P r o g r a m C A P E R (see A p p e n d i x A ) w a s m o d i f i e d to a c c o u n t for the a b s o r p t i o n o f f r o m the s u r r o u n d i n g air.  C0  2  E q u i l i b r i u m c a l c u l a t i o n s i n d i c a t e d t h a t as t h e p H i n c r e a s e d i n  an alkaline peroxide solution w i t h  100 p p m  o f c a l c i u m , at 5 0 ° C ,  the nature o f  the  u n d i s s o l v e d s o l i d s c h a n g e d f r o m c a l c i u m c a r b o n a t e at a p H < 1 0 . 3 t o c a l c i u m p e r o x i d e at a p H > 10.3 (see F i g u r e 6.58).  The actual soluble ionic c a l c i u m concentration remained  very s m a l l o v e r the entire p H range.  A s m a l l c h a n g e i n s l o p e i s o b s e r v e d at a p H o f 1.3.  F i g u r e 6 . 5 9 i l l u s t r a t e s t h e e f f e c t o f t e m p e r a t u r e at a p H o f 1 0 , 0 . 1 5 M of added calcium. CaC0  3  A t temperatures b e l o w 68 °C,  w h e r e a s at h i g h e r t e m p e r a t u r e s C a 0  2  83  2  2  and 100 p p m  the u n d i s s o l v e d s o l i d s consisted o f  w a s present.  solids w a s present.  H 0  A t 6 8 °C, a m i x t u r e o f the t w o  Chapter 6  Results and Discussion  o "co £c cu o c o o  I  I  1  1  io  1  I  '  I 7  o  's '\_  'N  +  HCO" _ ^  O  3  I  O  '  10"  10  -2  o o  ' S.  OOH^V  X  o  I  1  'x_  'v 's  CD  N  : * co 2  3  v  O O  . 0.0025 M CaCOj 0.0025 M C a 0  io  Ca v —*"'N. 2+  v  2  K. o  9  3  X  O  o  Figure 6.58  .  1  io-  10.0  1  .  1  10.6  2  ""•  1  .  10.8  1 11.0  3  1  2  below a p H o f - 1 0 . 3 and C a 0  1  2  above a p H o f - 1 0 . 3 .  r  1  C a,2+'  '-4—»  c  .  A t 5 0 C , a solution w i t h 0.15 M H 0 , 100 p p m C a i n equilibrium w i t h air,  c o CO  1 10.4  PH  contained C a C 0  £  1 10.2  IO"  1  CD O  10 HCO,  c o o  -7  OOH"  O O  CD  10-  <—K  0)  O O  I—»•  0.0025 M C a C 0 i 0.0025 M C a 0 3  X  o o  ro +  o  I  X  O O  o  10  2  1(T  o" 3  -3  20  30  40  50  60  70  80  90  Temperature, °C Figure 6.59  A t a p H o f 10, a solution w i t h 0.15 M w i t h air, contained C a C 0  3  H 0 , 2  2  100 p p m C a i n equilibrium  at t e m p e r a t u r e s b e l o w 6 8 ° C a n d C a 0  68 °C.  84  2  above  Chapter 6  Results and  Discussion  T h e f o r m a t i o n o f c a l c i u m carbonate reduces the i n h i b i t i v e effect o f c a l c i u m arising the f o r m a t i o n o f c a l c i u m p e r o x i d e .  Inhibition through calcium peroxide formation  from does  a p p e a r t o b e t h e l e a d i n g m e c h a n i s m at h i g h e r p H v a l u e s a n d m o r e a g g r e s s i v e c o r r o s i o n c o n d i t i o n s s u c h as p i c k l e b a t h t e m p e r a t u r e s a n d c o n c e n t r a t i o n s .  H o w e v e r , at a p H o f 10,  n o c a l c i u m p e r o x i d e w a s d e t e c t e d ; y e t , t h e i n h i b i t i o n a p p e a r s t o b e g r e a t e r at a l o w e r p H .  A p h y s i c a l l y adsorbed c a l c i u m surface f i l m w a s r e m o v e d b y gently w i p i n g w i t h a gloved finger and lightly rinsing w i t h distilled water.  Subsequent surface analytical techniques  ( A E S , X P S , S I M S w i t h depth profiling) showed n o evidence o f c a l c i u m integrated into t h e s u r f a c e o x i d e ( s e e A p p e n d i x D ) . I n f a c t , t h e s e t e c h n i q u e s s h o w e d n o d i f f e r e n c e s at a l l between a coupon corroded i n a calcium-free solution and a coupon inhibited b y calcium. C o n s e q u e n t l y , the i n h i b i t i v e effect o f c a l c i u m i snot caused b y the f o r m a t i o n o f a c a l c i u m titanate.  T h r o u g h o u t the runs, a c a l c i u m deposit appeared to c l i n g t o every a v a i l a b l e surface.  Acid  c l e a n i n g w a s f o u n d t o b e n e c e s s a r y t o ensure that a l l c a l c i u m h a d b e e n r e m o v e d f o r the subsequent  run.  It, therefore,  s e e m s p o s s i b l e that a c a l c i u m c a r b o n a t e o r a c a l c i u m  p e r o x i d e f i l m o n the c o u p o n surface affected the l o c a l c h e m i s t r y s u f f i c i e n t l y t o protect the u n d e r l y i n g m e t a l .  S u c h a f i l m w o u l d greatly i m p e d e the transport o f the p e r h y d r o x y l  i o n t o t h e t i t a n i u m o x i d e surface r e d u c i n g t h e c o r r o s i o n rate. temperatures  a n d higher  peroxide  A t higher p H ,  concentrations, the f i l m becomes  more  higher  and more  unstable and less and less protective.  T h e m o r e p e r m a n e n t effect o f c a l c i u m suggests a change i n the p h y s i c a l and/or c h e m i c a l stability o f the surface o x i d e .  W h e n protection is n o longer  obtained  i n a calcium  i n h i b i t e d s o l u t i o n , the o x i d e has b e e n a f f e c t e d t o s u c h a n extent that, e v e n i n a  freshly  prepared, less corrosive, c a l c i u m inhibited solution, a c a l c i u m f i l m has d i f f i c u l t y f o r m i n g and i s less protective than a f i l m f o r m e d o n a surface w h i c h has n o t p r e v i o u s l y  85  been  Chapter 6  Results and  Discussion  e x p o s e d t o c a l c i u m . W h e n this happens, the charge transfer resistance decreases l e a d i n g to i n c r e a s e d c o r r o s i o n rates.  Heavily corroded  samples displayed h i g h capacitances p r o b a b l y as a result o f m u c h  greater o x i d e d i e l e c t r i c constants.  T h e presence o f ions i n a porous oxide c o u l d lead to  higher corrosion potentials as observed i n Figure  6.54.  When combined with a l o w  c h a r g e t r a n s f e r r e s i s t a n c e , as i s the c a s e w h e n the s a m p l e h a s b e e n a c t i v e l y c o r r o d i n g i n a c a l c i u m inhibited solution, the result i s a higher  anode passive current density, a n d ,  subsequently, a lower corrosion potential.  6.2.2  Surface  Roughness  F i g u r e 6.60 s h o w s h o w the 6 0 0 grit s a m p l e e x p e r i e n c e d a c o r r o s i o n rate w h i c h i s r o u g h l y t w i c e that o f the e l e c t r o p o l i s h e d s a m p l e .  T h i s i s p r o b a b l y the result o f the m u c h greater  surface area o f the 6 0 0 grit s a m p l e , w h i c h i s affected b y c a l c i u m the s a m e w a y as the e l e c t r o p o l i s h e d s u r f a c e as t h e c o r r o s i o n p r o f i l e i s t h e s a m e o v e r t h e t e s t p e r i o d .  A t a h i g h e r temperature, as i l l u s t r a t e d i n F i g u r e 6.61, n o p r o t e c t i o n w a s o b s e r v e d o n the 6 0 0 grit sample.  It is p o s s i b l e that, either a n u n s t a b l e f i l m j u s t c a n n o t b e s u s t a i n e d o n the  r o u g h e r surface, o r the e l e c t r o p o l i s h e d surface o x i d e i s different than the 6 0 0 grit surface oxide a n dmore easily coated b y the calcium.  O v e r t i m e , t h e c o r r o s i o n rate o f the  e l e c t r o p o l i s h e d surface a p p r o a c h e d that o f the 6 0 0 grit surface.  86  Chapter 6  ^  Results and Discussion  10  0.15 M H 0 , pH 11, 50 C, 100 ppm Ca  s  2  2  O  d  10"'  CD O  electropolished  c  0.04  CD  10"'  mm/y  o'  GO  ZJ  "co  CD  c o  10  •  CD  O  •  4  O  CD  600 grit 0.09 j  0  o  73  O  CO  o o  o —i o CO  •+—•  Cd  o  20  mm/y i  i  40  10"  1  j  i  60  80  J  i  100  120  i  L  140  3 3  10" 160  Time, min  Figure 6.60  I n c r e a s e d s u r f a c e r o u g h n e s s l e a d s to a h i g h e r c o r r o s i o n rate.  Second  ' C o r r o s i o n R a t e ' scale has been corrected for anodic mass transfer control ( s o l i d s y m b o l s - E I S data, o p e n s y m b o l s - L P R data).  A n activation control corrosion model, w i t h a relatively h i g h anode Tafel slope o f - 0 . 2 5 V , a n d a c a t h o d e T a f e l s l o p e o f - 0 . 1 2 0 V ( S e c t i o n 5.2, C h a p t e r 5) a p p e a r e d to w o r k  very  w e l l for m o s t o f the data. Interestingly, i n F i g u r e 6.60, the w e i g h t loss c o r r o s i o n rate w a s h i g h e r t h a n e x p e c t e d f r o m the c a l c u l a t e d right h a n d c o r r o s i o n rate scale. I f c a l c i u m f o r m s a c o r r o s i o n protective f i l m w h i c h i m p e d e s the transport o f the p e r h y d r o x y l  i o n to the  t i t a n i u m o x i d e surface, the c o r r o s i o n process w o u l d be under a n o d i c d i f f u s i o n control. U n d e r a n o d i c d i f f u s i o n c o n t r o l , the anode T a f e l coefficient, b , approaches i n f i n i t y a  and  the c o r r o s i o n current i s d e s c r i b e d b y (see also e q u a t i o n 5.9):  : 0.052/  (6.22)  2.303 - R ,  87  Chapter 6  Results and Discussion  T h e corrected c o r r o s i o n rate scale i s also s h o w n i n F i g u r e 6.60, a n d c o m p a r e s better to the m e a s u r e d w e i g h t l o s s c o r r o s i o n rates.  I n Figure 6.61, the c a l c i u m f i l m might b e  unstable p l a c i n g the c o r r o s i o n c o u p o n a g a i n i n a n a c t i v a t i o n c o n t r o l l e d environment.  0.15 M H,0,, pH 11,70 C, 100 ppm Ca  o G  o  •  CD O  c  CD  -*-> CO  "GO CD  •  a  10H  3  electropolished D  _  ,_,  CO  0.43 mm/y  o'  Ql  c  600 grit  CO  6.4mm/y  o  2 o  10  io°  ^ •  2  o*°  #  #  0  CD  ofe  #  3 3  '•d  o  0  J  20  i  I  40  i  I  60  i  I  80  i  I  100  i  I  120  i  I  140  i  91  10 L_ 160 180  1  Time, min F i g u r e 6.61  N o p r o t e c t i o n i s o b s e r v e d at a l l o n 6 0 0 grit s a m p l e s w h e r e i n h i b i t i o n i s marginal  o n electropolished samples  (solid  symbols  -  E I S data,  open  s y m b o l s - L P R data).  6.2.3  Surface  Morphology  S E M p h o t o g r a p h s s h o w t h e f o r m a t i o n o f s m a l l p i t s at i n 0 . 1 5 M H 0 , p H 10, 5 0 ° C , a n d 2  100 p p m C a f r o m C a ( O H ) . 2  2  T h e s m a l l pits appear r a n d o m l y w i t h little preference f o r  g r a i n b o u n d a r i e s (see F i g u r e 6.62).  U n d e r the same c o n d i t i o n s , but w i t h added c a l c i u m  c a r b o n a t e , p i t s j o i n e d r a n d o m l y as i l l u s t r a t e d i n F i g u r e 6.63.  88  Chapter 6  F i g u r e 6.62  Results and  S m a l l p i t s are o b s e r v e d o n a c o u p o n c o r r o d e d i n 0.15 M H 0 , p H 10, 5 0 2  °C,  F i g u r e 6.63  Discussion  2  1 0 0 p p m C a f r o m C a ( O H ) , at a r a t e o f 0 . 0 1 m m / y f o r 7 h o u r s . 2  C o u p o n c o r r o d e d i n 0.15 M H 0 , p H 10, 5 0 ° C , 100 p p m C a f r o m C a C 0 , 2  2  at a r a t e o f 0 . 0 3 m m / y f o r 4 h o u r s .  89  3  Chapter 6  Results and  This  pitting  random  Discussion  seems to confirm  the presence o f a diffusion  limited calcium  c a r b o n a t e l a y e r o n the c o u p o n surface. T h e c o r r o s i o n rate appears t o b e c o n t r o l l e d b y the diffusion o f the p e r h y d r o x y l i o n through this thin f i l m .  L o c a l i z e d corrosion m a y then b e  c o n t r o l l e d b y the properties o f the f i l m and c o i n c i d e w i t h a s l i g h t l y thinner o r m o r e easily penetrable p o r t i o n o f the f i l m .  T h e c a l c i u m carbonate  film  m a y just have a few more  w e a k s p o t s w h e n c a l c i u m c a r b o n a t e i s a d d e d as o p p o s e d t o c a l c i u m h y d r o x i d e .  U n d e r m o r e c o r r o s i v e c o n d i t i o n s , the t i t a n i u m c o u p o n s u r f a c e a p p e a r a n c e r e s e m b l e d that o f corroded coupons i n calcium-free alkaline peroxide solutions w i t h preferred  grain  boundary dissolution.  under  T h i s i s t o b e e x p e c t e d as the effect o f c a l c i u m i s m i n i m a l  these c o n d i t i o n s a n d the c o r r o s i o n m e c h a n i s m s are e x p e c t e d t o b e the s a m e .  Figure 6.64  C o u p o n corroded i n 0.25 M H 0 , p H 11, 7 0 °C, 100 p p m C a 2  2  at a r a t e o f 6 . 6 5 m m / y f o r 2Vi h o u r s .  90  from  Ca(OH)  2  Chapter 6  Results and  Discussion  6.3  Inhibition b y Pulp  The much  l o w e r c o r r o s i o n rates obtained i n a m i l l e n v i r o n m e n t  as opposed to those  obtained i n the lab, h a v e a l w a y s b e e n ascribed t o the presence o f additives. pulp itself has not been investigated.  T h e effect o f  I n peroxide bleaching studies, pulp has been f o u n d  to p l a y a n a c t i v e r o l e [53], p o s s i b l y t h r o u g h a d s o r p t i o n o f h y d r o g e n p e r o x i d e o n the fiber pore surfaces [54].  I n l i g h t o f this theory, the t i t a n i u m c o r r o s i o n rates s h o u l d b e l o w e r i n  the presence o f p u l p .  T o study the effect o f p u l p , a f u l l y b l e a c h e d kraft p u l p w a s u s e d w h i c h served as inert fiber,  consuming  corrosion.  very  Inhibition  little  peroxide.  A s anticipated, thepulp  increased w i t h consistency.  inhibited  titanium  T h e inhibitive effect surpassed a l l  e x p e c t a t i o n s w i t h a n a c c e p t a b l e c o r r o s i o n rate i n the p r e s e n c e o f m e r e l y 1 % p u l p at p H 11, 0.15 M H 0 , 5 0 °C (see F i g u r e 6.65). 2  2  pH11,0.15MH O , 50 C 2  •  E o  d  cu o c CO CO  'oo Q)  10  I  •  EIS Data  •  LPR Data  i |  i  |  10"'  i  1 % pulp, <0.02 mm/y  -IQ  io  O o — 1  1  D  fa  ••• •  q« • •  ZJ  •  0.2% pulp, 0.2 mm/yr  4 10°  Q CO 10  _ •• • "  2  •  i  0  20  I o  •eft"  3  c  o O  i  •  4  01  t  2  .  i  40  .  i  60  i 80  73 CO «-+• CD  3 3  no pulp, 2.3 mm/yr i  i  i  i  100 120 140 160  180  Time, min F i g u r e 6.65  A n a c c e p t a b l e c o r r o s i o n rate is o b t a i n e d i n the presence o f 1 % p u l p . W e i g h t l o s s c o r r o s i o n rates s h o w n i n b o l d font.  91  Chapter  6  Results  10 CN  and  Discussion  pH 11, 0.15 M H 0 , 50 C 2  b  E o  2  •  ,  A  100 ppm calcium  •  ,  o  1% pulp  o o—i  d  oi" o c  10" 0.04  CO -t—>  3  CO  mm/y  o'  u2  ZJ  oo  0  73  10 h 4  o  g  < 0.02  00  0J  •  I—».  CD  3 3 ^<  mm/y  2  o O  0  20  40  60  80  100  120  140  160  10"  Time, min F i g u r e 6.66  1 % p u l p i s a s e f f e c t i v e as 1 0 0 p p m o f c a l c i u m at p H 1 1 , 0 . 1 5 M 5 0 °C.  W e i g h t l o s s c o r r o s i o n rates are s h o w n i n b o l d font.  H 0 , 2  2  and  Solid symbols -  E I S data, o p e n s y m b o l s - L P R data.  P u l p b e h a v e d as a m o r e s t a b l e i n h i b i t o r t h a n c a l c i u m . A t 5 0 ° C , p H 1 1 , 0 . 1 5 M  H 0 ,  1%  p u l p i s as e f f e c t i v e , i f n o t m o r e e f f e c t i v e , t h a n 1 0 0 p p m o f c a l c i u m ( s e e F i g u r e 6 . 6 6 ) .  At  7 0 °C, This  2  2  the c o r r o s i o n rate i n a 1 % s l u r r y i n c r e a s e d s l i g h t l y w i t h t i m e (see F i g u r e increase w a s  shown  to b e  the result o f  changes  i n the  suspension  6.67).  chemistry,  p r o b a b l y l i n k e d to c h a n g e s i n the p u l p t h r o u g h r e a c t i o n w i t h p e r o x i d e a n d a l k a l i . l o w i n i t i a l c o r r o s i o n rate w a s m e a s u r e d a g a i n i n a fresh s u s p e n s i o n o f p u l p .  The  In a m i l l  environment, a pulp mixture is continually refreshed.  W h e n 1 0 0 p p m C a w a s a d d e d to a 1 % p u l p s l u r r y , the c o r r o s i o n b e h a v i o r r e s e m b l e d that o f t i t a n i u m i n a p u l p - f r e e , c a l c i u m i n h i b i t e d s o l u t i o n ( s e e F i g u r e 6 . 6 7 ) . It a p p e a r s t h a t t h e calcium inhibition mechanism  dominates  in a pulp  containing solution.  The  h i g h e r c o r r o s i o n rate m i g h t b e due to a s c o u r i n g a c t i o n o f the p u l p , t h i n n i n g o r the i n h i b i t i n g c a l c i u m f i l m o n the m e t a l surface.  92  slightly removing  Chapter 6  Results and Discussion  pH 11, 0.15 M H 0 , 70 C 2  E o  10<  d © o c  CO -1—' GO CO CD  or  c o  •S^h 10  o  •  LPR data  1 % pulp, 0.15  1%pulp+  10'  10"  mm/y  •  1.7  a  10°  mm/y  0  o  20  a  o  B  40  =3  CD  110  no pulp or Ca, 14 i i i i i  60  o" 7J  1  • j  o-* o  CD  100 ppm Ca HiCLg •  O  CO  100 ppm Ca  D  2.7 mm/y  1  2  • rji  10  EIS data  3  CO  o o  1 3 0  •  1  3  3  mm/y i  1  80 100 120 140 160 180  Time, min F i g u r e 6.67  T h e corrosion resistance decreases rapidly w h e n both c a l c i u m and pulp present.  are  W e i g h t loss c o r r o s i o n rates are s h o w n i n b o l d .  T i t a n i u m , w h i c h h a d b e e n e x p o s e d to a c a l c i u m i n h i b i t e d p e r o x i d e  environment  and  w h i c h h a d s h o w n h i g h c o r r o s i o n r a t e s at t h e e n d o f i t s t h r e e h o u r c o r r o s i o n r u n , s t a b i l i z e d at t h e s a m e l o w c o r r o s i o n r a t e as a f r e s h l y p r e p a r e d s a m p l e i n a c a l c i u m - f r e e , 1 %  pulp  i n h i b i t e d s u s p e n s i o n (see F i g u r e 6.68).  I n F i g u r e 6 . 6 8 , t h e 6 0 0 g r i t s a m p l e c o r r o d e d r o u g h l y t w i c e a s f a s t as t h e e l e c t r o p o l i s h e d sample.  T h i s is l i k e l y due to the larger surface area o f the rougher 6 0 0 grit surface.  a c o r r o s i o n p e r s p e c t i v e , the d i f f e r e n c e i n the c o r r o s i o n rates i s r e l a t i v e l y s m a l l .  93  From  Chapter 6  Results and Discussion  pH 11, 0.15 M H 0 , 50 C, 1% pulp 2  2  •  ,  A  ,  8  ,  v  EIS data  •  ,  A  ,  e  ,  v  LPR data  previously corroded in  o  I®  d  calcium inhibited solution  1 0 %  CD"  o cz  1(T  electropolished surface,  x'"'  < 0.02 mm/y  I % £ ^ J  L  A  A  A ^ A  A  A  ^  A  \  A  A  O A a  o - i -I o  4A.  CD  •4—>  00  CD  t  r-t~  CD  in the absence of pulp  10*  o  T  V I  0  20  V  ^  ^10°  ^ T V T V T V T V  T  i  l  i  40  i l  I  60  5  73  electropolished surface  2.3 mm/y V • V •  1  ZJ  0.04 mm/y  irr  c • g  "oo o  10"  600 grit surface,  CO  i  80  l  l  i  100  i  120  l  3 3 *<*  i _  140  160  Time, min Figure 6.68  T h e titanium  c o r r o s i o n rate i n a 1 % p u l p  slurry  i s independent  o f the  s a m p l e ' s c o r r o s i o n history o r surface treatment.  T h e above results are supported b y whatever little m i l l experience has been m a d e public. R e i c h e r t [55] m e a s u r e d a c o r r o s i o n rate less than 0.003 m m / y i n 1 0 % p u l p , p H 11.2, 0.08 M  H 0 , 8 0 °C at t h e m i x e r outlet. 2  H o w e v e r , h i g h e r c o r r o s i o n rates, r a n g i n g f r o m 0.8 -  2  1.8 m m / y , w e r e m e a s u r e d a t h i g h e r p H v a l u e s i n t h e p r e s e n c e o f 2 0 - 3 6 p p m C a .  Clarke  a n d S i n g b e i l [5] o b t a i n e d a c o r r o s i o n rate o f 0.1 - 0 . 2 m m / y i n a p u l p m i x t u r e c o n t a i n i n g 0 . 0 9 M H 0 , 2 0 p p m M g , at p H 1 1 . 2 , 8 2 °C, j u s t d o w n s t r e a m o f t h e m i x e r . 2  2  Howe  Sound  P u l p a n d P a p e r L t d . [ 5 6 ] h a s b e e n r u n n i n g t i t a n i u m m i x e r s f o r t h e p a s t t h r e e y e a r s w i t h at least 15 c a m p a i g n s o f T C F p u l p w i t h n o p i t t i n g o r other v i s i b l e c o r r o s i o n . a r e r u n a t p H 11 w i t h - 0 . 0 8 M H 0 2  it h a s g e n e r a l l y  been  assumed  2  that  and added M g S 0 . 4  the added  inhibition.  94  T h e i r P stages  In a l l o f the above experiences,  calcium or magnesium  provided the  Chapter 6  Results and Discussion  6.3.1  A  Velocity Effects  v a r i a b l e speed m i x e r w i t h a stainless steel shaft a n d paddle b l a d e w a s p l a c e d i n the  p u l p s u s p e n s i o n w i t h t h e e d g e o f t h e p a d d l e b l a d e 1.3 c m f r o m t h e c o r r o s i o n c o u p o n . T h e c o u p o n w a s m o u n t e d i n a T e f l o n plate s u c h that the c o u p o n f a c e w a s f l u s h w i t h the T e f l o n surface. TM0270-72  T h e tangential velocity w a scomputed according to N A C E  Standard  from:  Velocity (m/s)  =  rpm  x  (m)  (6.23)  60  w h e r e R i s the distance b e t w e e n the center o f the shaft and the c o u p o n face. W h e r e a s the true v e l o c i t y i s s o m e w h a t less t h a n t h e c o m p u t e d v e l o c i t y , p a r t i c u l a r l y as t h e v i s c o s i t y increases w i t h increasing pulp consistency, it i s sufficient for comparative purposes. T h e c o r r o s i o n rate w a s m o n i t o r e d b y E I S .  pH 11, 0.17-0.18 M H 0 . 0 C 2  7  2  10'  1 CO  E io E  O  1  0  9  1% pulp, 4 % S i C  A  A  I  u  O O  73  A  T  10  o' 13  1% pulp, 8% SiC  a) CO  or c o "GO o  N Oi—tJ  no pulp or S i C  CD CO  co' r-t-  0)  o (D  1% pulp J  0  5  i  I  6  i  I  7  i  L  8  10'  6  3  Tangential Velocity, m/s Figure 6.69  I n c r e a s i n g v e l o c i t i e s d o not appear to affect the t i t a n i u m c o r r o s i o n rate.  95  Chapter 6  Results and  Discussion  F i g u r e 6.69 i l l u s t r a t e s h o w the c o r r o s i o n rate d o e s not appear t o b e a f f e c t e d b y v e l o c i t i e s up to 8 m/s.  T h e p r e s e n c e o f 1 % p u l p s e r v e d as a c o r r o s i o n i n h i b i t o r .  T h e a d d i t i o n o f S i C p a r t i c l e s ( 4 2 5 p m - 5 0 0 u m ) i n c r e a s e d the c o r r o s i o n rate, b u t again n o variation was observed w i t h increasing velocity.  T h eS i C particle surface appeared to  catalyze the decomposition o f hydrogen peroxide i n h i b i t i v e effect o f the p u l p .  and m a y have  interfered w i t h the  N o effect o f S i C l o a d i n g w a s observed i n pulp-free alkaline  peroxide solutions.  6.3.2  Figure 6.70  Surface  S m a l l g r a i n b o u n d a r y p i t s are p r e s e n t at 5 0 ° C , p H 1 1 , 0.15 M H 0 , a n d 2  0.2% pulp. In  a pulp  Morphology  environment,  2  T h e w e i g h t l o s s c o r r o s i o n r a t e w a s 0 . 2 m m / y o v e r 3V2 h r s . t h e t i t a n i u m s u r f a c e r e s e m b l e s that o f c o u p o n s  c o r r o d e d i n a p u l p free a l k a l i n e p e r o x i d e solution.  96  which  have  F i g u r e 6.70 illustrates h o w s m a l l pits  Chapter 6  Results and Discussion  f o r m a l o n g t h e g r a i n b o u n d a r y a n d a f e w s h a l l o w p i t s a r e p r e s e n t o n t h e g r a i n s u r f a c e at 5 0 °C, p H 11, 0.15 M H 0 , a n d 0 . 2 % p u l p . 2  2  T h e i n h i b i t i o n i s q u i t e a p p a r e n t e v e n at s u c h  a l o w consistency o f p u l p w h e n c o m p a r e d w i t h F i g u r e 6.23. p u l p , the surface appeared v i r t u a l l y untouched.  97  In the presence o f  more  Chapter  7  Conclusions  The corrosion o f Grade 2 titanium in alkaline peroxide environments weight  loss  corrosion  tests,  electrochemical impedance  was studied  spectroscopy  (EIS),  polarization resistance ( L P R ) measurements and potentiodynamic polarography. titanium  oxide  and  corrosion  information  was  obtained  by  studying  c o r r o s i o n b e h a v i o r as a f u n c t i o n o f t i m e t h r o u g h the u s e o f E I S a n d L P R . c o r r o s i o n rate w a s  f o u n d to increase w i t h i n c r e a s i n g p H , temperature,  peroxide concentration.  by  linear Unique  the  titanium  The  titanium  and  hydrogen  A t the s a m e t i m e , the c h a r g e t r a n s f e r c a p a c i t a n c e i n c r e a s e d as  the surface o x i d e b e c a m e less p r o t e c t i v e a n d d i s p l a y e d a greater d i e l e c t r i c constant.  The  o p e n c i r c u i t potential shifted into the a n o d i c d i r e c t i o n w i t h a n increase i n the cathodic reduction current density.  W i t h more prominent  o x i d e d i s s o l u t i o n at m o r e  corrosive  c o n d i t i o n s , the c o r r o s i o n potential shifted into the cathodic direction.  A  corrosion mechanism  decomposition  rate  hydroxylation,  as  concentration.  and  was  constructed w h i c h incorporated  which  well  as  accounts  oxide  for  thinning  oxide  the h y d r o g e n  thickening,  depending  on  oxide  the  peroxide  hydration  hydrogen  and  peroxide  S i n c e v e l o c i t y e x p e r i m e n t s s h o w e d n o i n c r e a s e i n t h e c o r r o s i o n r a t e as a  f u n c t i o n o f t a n g e n t i o n a l v e l o c i t y , the c o r r o s i o n c o n t r o l l i n g m e c h a n i s m is thought to b e the r e a c t i o n o f the o x i d e w i t h the p e r h y d r o x y l i o n . with a preference  for grain boundaries, probably  S h a l l o w pits f o r m e d o n the surface because o f a local imperfect  higher  energy structure w i t h a slightly l o w e r corrosion resistance.  Preferred  dissolution o f certain crystallographic planes was  corrosion o f a titanium single crystal. at 5 0 ° C ,  pH  11, 0.15  M  H 0 . 2  2  investigated through  the  R a n d o m pitting w a s observed o n all c o u p o n sides  A t 7 0 °C,  pH  11, 0.25  M  H 0 , the S E M 2  2  showed  crystallographic related surface patterns w h i c h v a r i e d b e t w e e n three orthogonal surfaces. T h e surface o r i e n t a t i o n d i d not appear to affect the c o r r o s i o n b e h a v i o r and,  98  therefore,  Chapter 7  Conclusions  texturing i s not relevant t o i n c r e a s i n g the m a t e r i a l ' s c o r r o s i o n resistance.  S i n c e the h i g h  p u r i t y s i n g l e c r y s t a l c o r r o s i o n b e h a v i o r r e s e m b l e s that o f G r a d e 2 t i t a n i u m , i t c a n also b e c o n c l u d e d that t h e c o r r o s i o n b e h a v i o r o f G r a d e 2 i s e s s e n t i a l l y c o n t r o l l e d b y the a l p h a phase and not the s m a l l portion o f dispersed beta phase.  Replacement o f hydrofluoric investigated.  acid with alkaline peroxide for pickling o f titanium  T i t a n i u m c o r r o s i o n rates i n a l k a l i n e p e r o x i d e e x c e e d e d those o b t a i n e d i n  the conventional h y d r o f l u o r i c  acid bath.  A c o r r o s i o n rate o f u p t o 2 3 0 0 m m / y  o b t a i n e d at 9 5 ° C , 2 . 5 M N a O H , a n d - 0 . 5 M extensive roughening  H2O2.  was  General corrosion was observed with  o f the surface g i v i n g a d u l l gray appearance.  pitting i n hydrofluoric surface.  was  acid solutions resulted i n a shiny  Widescale shallow  appearance  o f the titanium  A d d i t i o n o f calcium stabilized the hydrogen peroxide through formation o f  c a l c i u m p e r o x i d e , b u t a l s o d e c r e a s e d the e f f e c t i v e h y d r o g e n p e r o x i d e c o n c e n t r a t i o n as the c a l c i u m p e r o x i d e d i d not participate i n the d i s s o l u t i o n process.  An  alkaline peroxide pickle bath could eliminate environmental  associated w i t h hydrofluoric  a n d safety  concerns  acid p i c k l i n g as w e l l as h y d r o g e n p i c k u p concerns w h i c h  can lead to embrittling hydride formation.  W h e r e a s the r e q u i r e d m e t a l r e m o v a l rate c a n  be obtained i n alkaline peroxide solutions, the surface finish m a y n o t b e esthetically desirable f o r m a n y applications. rougher  surface m a yb e ideal.  which would  have  W h e n f o l l o w e d b y subsequent coating processes, the Hydrogen  peroxide stability is still a n economic issue  to b e resolved, possibly through i n situ production  o f hydrogen  peroxide.  Calcium  w a s investigated  stability diagrams  as a corrosion inhibitor.  Aqueous  thermodynamic  phase  c o n s t r u c t e d f o r t h e Ti-Ca-H20 s y s t e m s h o w e d that c a l c i u m titanate  f i l m f o r m a t i o n i s p o s s i b l e i n the water stability region. ions need t o b e present.  O n l y trace quantities o f c a l c i u m  H o w e v e r , surface analytical techniques ( A E S , X P S , and  SIMS)  h a v e s h o w n n o e v i d e n c e o f c a l c i u m i n the s u r f a c e o x i d e f i l m s o f test c o u p o n s e x p o s e d t o calcium-inhibited alkaline peroxide solutions.  99  Chapter 7  The  Conclusions  effectiveness o f c a l c i u m o n corrosion inhibition over extended time periods  found to b e questionable.  Inhibition  b y c a l c i u m i s attributed to a n adsorbed  c a l c i u m carbonate o r c a l c i u m peroxide w h i c h forms a physical barrier to i o n and affects t h e solution chemistry lo c a lly , rendering metal surface.  was  film o f transport  t h e s o l u t i o n less c o r r o s i v e at t h e  It has b e e n s h o w n that the i n h i b i t i n g effect o f c a l c i u m i s t e m p o r a r y .  t i m e , c a l c i u m affects the c h e m i c a l and/or p h y s i c a l stability o f the surface o x i d e ,  With  thereby,  i n c r e a s i n g c o r r o s i o n rates. T h i s effect o n the surface o x i d e i s o f a m o r e p e r m a n e n t nature w h i c h i s not affected b y surface cleaning o r resubmersion into freshly prepared inhibited  solutions.  It m a y h a v e  become  more  difficult  for the oxide  calcium  to sustain a  protective c a l c i u m film.  P u l p w a s i n v e s t i g a t e d as a c o r r o s i o n i n h i b i t o r i n p e r o x i d e s o l u t i o n s a n d w a s f o u n d to b e a more  effective  and more  stable corrosion inhibitor  c o n c e n t r a t i o n d e c r e a s e d the c o r r o s i o n rate.  than  calcium.  Raising  the pulp  C o r r o s i o n rates r e m a i n e d r e a s o n a b l e constant  t h r o u g h o u t the test, s h o w i n g that the p u l p b e h a v e d as a stable c o r r o s i o n i n h i b i t o r . T h e i n h i b i t i n g effect o f p u l p m a y b e related to the a d s o r p t i o n a n d i n t e r a c t i o n o f the p u l p with  H2O2,  thereby decreasing the peroxide concentration i n the solution and  the s o l u t i o n less c o r r o s i v e .  The  addition  fibers  rendering  T h e surface o x i d e d i d not appear to b e affected.  o f calcium to a pulp  slurry negatively  affected the inhibition b y pulp,  p o s s i b l y b y interacting w i t h the adsorption o f h y d r o g e n p e r o x i d e onto the p u l p fiber. T h e presence o f p u l p i n a c a l c i u m i n h i b i t e d s o l u t i o n appeared t o h a v e a negative effect o n the calcium  film  stability o n the t i t a n i u m surface, p o s s i b l y t h r o u g h a s c o u r i n g a c t i o n o f the  pulp.  Surface stresses i n t r o d u c e d into the m a t e r i a l t h r o u g h c o l d r o l l i n g , h a d n o effect o n the titanium corrosion behavior.  Electropolished samples showed slightly higher  resistances than 600 grit samples.  polarization  W h e r e a s this effect i s contributed m a i n l y t o the larger  true surface area, t h e e l e c t r o p o l i s h e d surface o x i d e c o u l d b e s l i g h t l y m o r e resistant than the o x i d e o n the 6 0 0 grit surface.  100  corrosion  Chapter 7  Conclusions  T h e results o f this w o r k suggest that e x i s t i n g t i t a n i u m b l e a c h p l a n t e q u i p m e n t m a y b e operated  i n well-mixed  concentrations < 0.15  M  alkaline peroxide  pulp  suspensions  at p H <  11, peroxide  ( ~ 5 % H2O2 o n 1 0 % p u l p ) , t e m p e r a t u r e s u p t o 7 0 ° C , a n d  c o n s i s t e n c i e s > 1 % . W h e r e a s t h e c o r r o s i o n rate w a s o b s e r v e d t o b e i n d e p e n d e n t o f v e l o c i t y , i t i s r e c o m m e n d e d that these r e s u l t s b e s u p p l e m e n t e d w i t h a c t u a l p l a n t test data.  101  Chapter 8  Recommendations for Further Research  T h e suitable operating boundaries for the use o f t i t a n i u m i n a l k a l i n e p e r o x i d e b l e a c h i n g e n v i r o n m e n t s c o u l d b e e x p l o r e d further b y s t u d y i n g the effects o f a d d i t i v e s s u c h as: •  chelants, e.g.  DTPA  •  peroxide stabilizers  •  metal ions  T h e t i t a n i u m c o r r o s i o n b e h a v i o r i n the presence o f h i g h e r p u l p consistencies c o u l d b e investigated.  T h i s w o r k s h o u l d b e s u p p l e m e n t e d b y a c t u a l p l a n t test d a t a u n d e r a v a r i e t y  of flow conditions.  Future w o r k c o u l d b e conducted over a broader range o ftemperatures to determine  the  Arrhenius activation energy i n support o f an activation controlled reaction mechanism.  T h e corrosion behavior o f other titanium alloys i nalkaline peroxide environments be studied. Al.  could  S i g a l o v k a y a s t u d i e d the e f f e c t o f a l l o y i n g a d d i t i o n s s u c h as P d , M o , T a , a n d  H i g h d i s s o l u t i o n rates w e r e m e a s u r e d for a l l o f t h e m .  A nalloy containing 6 % A l  s h o w e d s l i g h t l y l o w e r c o r r o s i o n rates, w h i c h is s o m e w h a t surprising g i v e n the instability o f a l u m i n u m at h i g h p H . of aluminum  F u r t h e r r e s e a r c h c o u l d b e a i m e d at i n v e s t i g a t i n g t h e s u i t a b i l i t y  containing titanium alloys as construction materials o f bleaching  equipment w h i c h is tobe used i nalkaline peroxide and chlorine bleaching  plant  environments.  Orientation image m i c r o s c o p y c o u l d be used to correlate crystallographic orientations o f alpha  a n d beta  phases  to the material's  corrosion  behavior  i n alkaline  peroxide  environments.  Further  research  could  focus  replacement for hydrofluoric  o n high  temperature  acid pickling.  102  alkaline peroxide  p i c k l i n g as a  Emphasis should beplaced o n economics  either through stabilizing the h y d r o g e n peroxide u s i n g intermediates and/or the h y d r o g e n p e r o x i d e l o c a l l y , i n situ, p o s s i b l y t h r o u g h the use o f catalysts.  103  generating  Chapter 9  Nomenclature  A  e x p o s e d surface area, o r otherwise defined constant  B  i s a constant, V  b , b a  anode a n d the cathode Tafel coefficients, respectively, V  c  C C  i o n concentration,  charge transfer capacitance, u F / c m  c t  AC  M  I  p  d  2  a p p r o x i m a t e d a s t h e a v e r a g e h e a t c a p a c i t y at 2 9 8 K a n d T , J / m o l - K 2  oxide thickness, or distance between atoms n o r m a l to the incident beam, n m  F  Faraday's constant, 96,489  AG  free energy o f reaction, J / m o l  AG°  2  i i  2  2  exchange current density, A / m  TP!  i i  G i b b s f r e e e n e r g y at t e m p e r a t u r e T , k J / m o l current density, A / m  0  C/mol  c o r  corrosion current density, A / m  2  Ios  i o n i c strength,  K  dielectric constant, o r otherwise defined constant  Kca  equilibrium constant for the dissolution o f c a l c i u m hydroxide,  KCACARB  (y )-[Ca 2  2 +  M  ].[C0  2 3  1,M  M  2  Kcaoh  e q u i l i b r i u m constant o f c a l c i u m hydroxide i o n formation, M "  Kcap  e q u i l i b r i u m constant o f c a l c i u m peroxide dissolution,  Ko  solubility constant o f carbon dioxide, M-atm"  Kperox  e q u i l i b r i u m constant for the acidic dissociation o f h y d r o g e n peroxide,  K w  e q u i l i b r i u m constant o f water, M  Ki  (l/ )-[H C03]/[H ]-[HC0 -], M" Y  2  +  3  104  1  2  1  M  1  M  Chapter 9  K  1  Nomenclature  (l/ )-[HC0 -]/[H ]-[C0  2  Y  +  3  3  2  -], M"  1  PCO2  partial carbon dioxide pressure,  n  order o f reflection, o r otherwise defined constant  R  G a s L a w constant, 8.3143 J / K - m o l , 1.9872 c a l . / g m o l e - K , o r c o r r o s i o n rate,  s o  mm/y  charge transfer resistance, o h m s - m  Ret R  atm  i  2  solution resistance, o h m s - m  Tc  temperature, degrees C e l s i u s  r  the crystal radius,  S  salinity, %  ASjog  standard entropy, J / m o l - K , c a l / m o l - K  T orT k  temperature,  T2  either 333 o r 373 degrees K e l v i n  W  equivalent weight,  x  constant, 2 for positive ions, 1 for negative ions  Z  complex impedance,  z  ion charge  z+, z .  i o n i c v a l e n c e o f the p o s i t i v e and negative i o n , r e s p e c t i v e l y  AES  A u g e r electron spectroscopy  B H N  B r i n e l l hardness n u m b e r  CPE  constant phase  EIS  electrochemical impedance  LPR  linear polarization resistance  OCP  open circuit potential  P stage  p e r o x i d e b l e a c h i n g stage  SIMS  secondary ion mass spectroscopy  SHE  standard h y d r o g e n electrode  S E M  scanning electrode microscope  TCF  total chlorine  XPS  X - r a y photoelectrbn  A  K  g/mol  ohms.cm  2  element spectroscopy  free spectroscopy  105  Chapter 9  Nomenclature  a  anodic charge transfer coefficient  a  cathodic charge transfer coefficient  y  m e a n activity coefficient  r|  overpotential, V  X  scattered wavelength, n m  6  angle o f incidence w h i c h equals the angle o f reflection  p  density, g / c m  ©  function o f temperature,  co  frequency, H z  {T -298-T dn(T /298)} 2  2  106  2  Bibliography  1. 2.  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HOGFELDT,  E . , Stability constants o fmetal-ion complexes - Part A:Inorganic  Ligands. T U P A C C h e m i c a l D a t a Series, N o 2 1 , Pergamon Press, O x f o r d (1982). 67.  H A M E R , W . J., "Theoretical M e a n Activity Coefficients o f Strong Electrolytes i n A q u e o u s S o l u t i o n s from 0 t o 1 0 0 ° C " , N a t i o n a l S t a n d a r d R e f e r e n c e D a t a S e r i e s . N a t i o n a l B u r e a u o f Standards 2 4 , (1968).  Ill  Bibliography  68.  W H I T F I E L D , M . , T U R N E R , D.R.,  "The Carbon D i o x i d e System i nEstuaries - A n  Inorganic Perspective", T h e S c i e n c e o fthe T o t a l E n v i r o n m e n t , 49: 2 3 5 - 2 5 5 69.  B L A C K B U R N , M.J., W I L L I A M S ,  (1986).  J . C , "The preparation o f thin foils o f titanium  a l l o y s , " T r a n s a c t i o n s o f the M e t a l l u r g i c a l S o c i e t y o f A I M E , 2 3 9 , p p . 2 8 7 - 2 8 8 70.  (1967).  M e t a l s H a n d b o o k . N i n t h E d i t i o n , V o l . 16, A S M , M e t a l s P a r k , O H , p 2 8 , p p 4 3 0 - 4 3 1 (1980).  71.  D I M I T R O V , O., F R O I S , C , "Properties o f Superpure Metals," Physical Metallurgy, edited b y C a h n , R.W., N o r t h - H o l l a n d Publishing C o m p a n y , A m s t e r d a m ,  pi053  (1965). 72.  B A R R E T T , C.S., M A S S A L S K I , Structure o f Metals. Crystallographic Methods. Principles, and Data. 3  73.  r d  edition, M c G r a w - H i l l : 2 1 1 - 2 1 7  T A Y L O R , A . , L E B E R , S., "Crystallographic A n g l e s for H e x a g o n a l M e t a l s , " Journal o f Metals: 190-192 (February,  74.  1954).  M C H A R G U E , C . J . , "Crystallographic A n g l e s for Titanium and Z i r c o n i u m , " Journal o f M e t a l s : 6 6 0 (June  75.  (1966).  1952).  D A V I S , L.E., M A C D O N A L D ,  N . C , P A L M B E R G , P.W., R I A C H , G.E.,  WEBER,  R . E . , H a n d b o o k o fA u g e r E l e c t r o n Spectroscopy. P h y s i c a l E l e c t r o n i c Industries, E d e n Prairie (1976). 76.  B R I G G S , D., S E A H , M . P . , editors, Practical Surface A n a l y s i s . V o l . 1 A u g e r and X ray photoelectron spectroscopy, John W i l e y & Sons (1990).  77.  B E N N T N G H O V E N , A., R U D E N A U E R , F.G., W E R N E R ,  H.W.,  Secondary  Ion  M a s s Spectroscopy - B a s i c Concepts. Instrumental Aspects. Applications and Trends. J o h n W i l e y & Sons, N e w Y o r k (1987). 78.  B A R R E T T . C S . . Structure o fMetals. Crystallographic Methods. Principles, and Data. 2  79.  n d  edition, M c G r a w - H i l l : 6 4 8 (1952).  L E V I N E , T.E., N A S T A S I , M . , A L F O R D , T.L., S U C H I C I T A L , C , R U S S E L L , S., L U P T A K , K., PIZZICONI,  V . , M A Y E R , J.W., "Ion B e a m M i x i n g o f Titanium  Overlayers W i t h Hydroxyapatite Substrates," M a t . Res. Soc. S y m p . P r o c , 356, pp.791-796 80.  (1995).  L I D E N , J . , O H M A N , " O n the P r e v e n t i o n o f F e - and M n - C a t a l y z e d H 0 2  2  D e c o m p o s i t i o n U n d e r B l e a c h i n g Conditions," J P u l p Paper Sci., 24, 9, pp. (1998).  112  269-276  Bibliography  81.  T E N G V A L L , P., W A L I V A A R A ,  B., W E S T E R L I N G ,  J., L U N D S T R O M ,  "Stable  Titanium Superoxide Radicals i nAqueous T i - P e r o x y Gels and Ti-Peroxide S o l u t i o n s , " L e t t e r to the E d i t o r , J . C o l l o i d Interface S c i . , 143, 2 , p p 5 8 9 - 5 9 1 (1991). 82.  O R G E L , L . E . , A n I n t r o d u c t i o n to T r a n s i t i o n M e t a l C h e m i s t r y L i g a n d - F i e l d T h e o r y . M e t h u e n and C o . L t d . , L o n d o n , U K , p. 4 6 (1960).  83.  G M E L F N , L . , Deutsche C h e m i s c h e Gesellschaft, G m e l i n ' s H a n d b u c h der anorganischen Chemie, Verlag Chemie G . M . B . H . , Berlin, 8 269 (1924-198-).  113  t h  edition, 41, pp. 268-  Appendix A  Thermodynamic Data  A.1  Determination of Thermodynamic Data  Table 3.2 summarizes  theGibbs  energy  o f formation  compounds considered i n the E h - p H diagrams.  at 2 5 , 6 0 a n d 1 0 0 ° C f o r a l l  T h e G i b b s free e n e r g i e s o f f o r m a t i o n at  r o o m t e m p e r a t u r e as w e l l as d a t a o f p u r e s u b s t a n c e s at h i g h e r t e m p e r a t u r e s w e r e o b t a i n e d f r o m the literature [57,58].  E n t r o p i e s and heat capacities o f aqueous i o n i c substances a n d  s o m e h y d r a t e d c o m p o u n d s h a d t o b e e s t i m a t e d b y u s i n g L a t i m e r ' s m e t h o d [ 5 9 ] at r o o m temperature a n d the Criss a n d C o b b l e theory f o r extrapolation to higher  temperatures  [60,61].  A.1.1  Ti, TiH , TiO, CaTi0 ,3CaO 2Ti0 , 4CaO 3TiO , Ca, Ca , CaH , +2  2  3  2  z  2  Ca(OH) , Ca0 , OH 2  The  2  G i b b s energies at t h e h i g h e r temperatures w e r e determined  from  equation A . 2 4  [60,61]:  AG°  = AG°  2  9 8  + AC;]";  g  -0 - ( T  2  (A.24)  - 2 9 8 ) A S 298 °  where: T  either 3 3 3 o r 3 7 3 degrees K e l v i n  2  AG°  t h e G i b b s f r e e e n e r g y at t e m p e r a t u r e T , k J / m o l 2  2  1298  a p p r o x i m a t e d as t h e a v e r a g e heat c a p a c i t y at 2 9 8 K a n d T , J / m o l K 2  0  function o f temperature  {T -298-T -ln(T /298)}  AS°  standard entropy, J / m o l - K  2  2  114  2  Appendix A  Thermodynamic Data  F o r e x a m p l e , f o r T i at 3 3 3 K , 0 i s - 1 . 9 7 8 3 5 a n d AG  333 =  0  A.1.2  + ( 2 5 . 3 ) ( - 1 . 9 7 8 3 5 ) - (35)(30.76) = -1.1 k J / m o l  Tf and Ti 2  +3  Entropy values were determined using the method outlined b y Latimer [59], w h o gives a general equation f o r the entropies o f m o n a t o m i c ions i n water solutions:  3 S°  29S  270•7  -,- + 3 7  =|R.ln(at.wt)-^  (A.25)  (r + x j  where: R  - the universal gasl a w constant, 1.9872 c a l . / g m o l e - K  Z  - n u m e r i c a l v a l u e o f the charge  r  - t h e c r y s t a l r a d i u s , A, [ 2 9 , p a g e  x  - 2 forpositive ions  F164]  - 1 for negative ions AS °  9 8  - standard entropy, c a l / m o l - K  For example, for T i AS°  9 8  + 2  , Z = 2 , r = 0 . 9 4 A, x - 2 , a t . w t . = 4 7 . 9 g / m o l  = 3/2 (1.9872)-ln(47.9) - (270)(2)/(0.94 + 2 ) + 37 = -13.94 c a l / m o l - K 2  = -58.4 J / m o l - K  In  order  t o establish a linear relationship  [60,61], the above  value  needs  AS°  9 8  entropies  at various  t o b e adjusted to a n e w "absolute"  S°(FT>Z, w h e r e S ° ( H ) = - 2 0 . 9 2 J / m o l - K . +  between  Then,  = -58.4 J / m o l - K - (2)(-20.92) = -100.2 J / m o l - K  115  temperatures  scale b y adding  Appendix A  Thermodynamic  Data  T h e i o n i c heat c a p a c i t y w a s predicted u s i n g the C r i s s and C o b b l e theory [60,61]:  AC^  a ( T ) + P(T )-AS°  29g  2  2  (A.26)  98  w h e r e a ( T ) a n d P ( T ) a r e c o n s t a n t s at t e m p e r a t u r e T . 2  2  2  F o r e x a m p l e , at T = 3 3 3 , a = 1 4 8 a n d p = - 0 . 4 1 . F o r T i 2  A C  PC  =  1  4  8  (-°-  +  4 1  )(-  1 0 0  - )= 2  1  8  + 2  :  J/mol-K  9  E q u a t i o n A . 2 4 w a s t h e n u s e d t o d e t e r m i n e t h e G i b b s f r e e e n e r g y at T , 2  A.1.3  TiO , Ti0 + 2  + 2 2  , and H T i 0  AG!;  3  It i s m o r e d i f f i c u l t t o e s t i m a t e e n t r o p y v a l u e s o f c o m p l e x i o n s s u c h a s T i O H T i 0 " , s i n c e n o t m a n y d a t a o r c o r r e l a t i o n s are a v a i l a b l e . 3  are aqueous  complexes,  it c a nb e assumed  c o n t r i b u t i o n i s t h e s a m e as i n s o l i d c o m p o u n d s attributed  to the oxide  or hydroxide  ligand(s).  2  + 2  , and  L a t i m e r lists t h e entropy  c o n t r i b u t i o n o f t h e e l e m e n t s i n s o l i d c o m p o u n d s at 2 9 8 K [59]. ions  , Ti0  Generally, entropy increases  w i t h size a n d mass a n d decreases w i t h increasing charge.  complex  + 2  E v e n t h o u g h the above  that t h e e l e m e n t  a n dthe remainder  entropy  o f the entropy i s  T h e latter c o n t r i b u t i o n  should be  a p p r o x i m a t e l y the s a m e a m o n g different aqueous c o m p l e x e s w i t h the same ligands. T h e entropy  o f the above  complexes  contribution o f the h y d r o x i d e  c a nthen b e evaluated b y determining  the entropy  or oxide ligand, using similar complexes with  entropy values, a n d a d d i n g this t o the entropy c o n t r i b u t i o n o f the element.  116  known  Appendix A  Thermodynamic  Data  For HTi0 ", 3  AS  Ion C o m p l e x  9  298, el.  g  cal/K  cal/K 11.6  18.8  HS0 -  26  8.5  17.5  HCO3HT1O3-  22.7 ?  + 2  17.5  9.8  17.9 (avg)  , A S> ! 8  AS° 298,  i J l J  29  Ion C o m p l e x VO  +  TiO  2  + 2  cal/K  el.  cal/K  A 2S9°8 ,  i J L J  9  lig  cal/K  -32  10.1  -42.1  ?  9.8  -42.1  = 9.8 - 42.1 = -32.3 c a l / K = -135.2 J / K  For T i 0  2  ,  + 2  AS Ion C o m p l e x U 0 Ti0 AS°  5.2  = 9.8 +17.9 = 27.7 c a l / K = 116.0 J / K  For TiO  9 8  cal/K  30.4  3  AS°  "^298,%  HSe0 3  A S L  2  +  2  2  cal/K  2  9 8 i e l  .  cal/K  -23.3  2  +  AS°  ?  = 9.8 - 3 9 . 3 = -29.5 c a l / K - -123.5 J / K  117  AS2  98ilig  cal/K  16.0  -39.3  9.8  -39.3  Appendix A  A.1.4  Thermodynamic Data  Ti0  H 0 a n d Ti(OH) 2  2  3  L a t i m e r ' s a p p r o a c h b a s e d o n s u m m a t i o n o f entropy contributions o f the v a r i o u s parts o f the c o m p o u n d w a s also u s e d f o r T i 0 H 0 2  2  and Ti(OH) . 3  T h i s approach w a s applied to  several similar c o m p o u n d s w i t h k n o w n entropies f o r comparison:  AC  Ion C o m p l e x  AS  0  A l 3  cal/K  AC  0  0  AC  ^ ° 2 9 8 , lig  298,el.  cal/K  cal/K  0  ^ ° 2 9 8 , calc.  % error  cal/K  Sn(OH)  2  37  13.1  4.5  22.1  40  Zn(OH)  2  19.5  10.9  4.5  19.9  2  21  15.4  5  20.4  3  Tl(OH) Ni(OH)  2  21  10.5  4.5  19.5  7  Fe(OH)  3  25.5  10.4  3.0  19.4  24  Ti(OH)  3  ?  9.8  3.0  18.8  -  H e a t c a p a c i t y v a l u e s w e r e estimated b y addition o f the heat c a p a c i t y o f t i t a n i u m o x i d e and H 0 . F o r example, T i ( O H ) 2  of T i 0 2  3  3  c a n also b e considered as T i 0 - 3 H 0 . 2  3  2  T h e heat c a p a c i t y  i s ~ 1 0 0 J / m o l - K a n d the heat c a p a c i t y o f w a t e r i s 7 5 J / m o l - K .  capacity o f T i ( O H )  3  T h e n , the heat  is roughly equal to (100+ 3 x 75)/2 = 163 J / m o l - K .  E q u a t i o n A . 2 4 c o u l d t h e n b e u s e d t o d e t e r m i n e t h e f r e e e n e r g i e s at v a r i o u s t e m p e r a t u r e s .  A.1.5 The  H 0 2  2  a n d Ca(OH)  free energies o f H 0 2  2  +  and Ca(OH)  +  were determined from  e q u i l i b r i u m constants  reported i n the literature w h i c h are d i s c u s s e d m o r e f u l l y i n the n e x t s e c t i o n , s e c t i o n A . 2 .  118  Appendix A  A.2  Thermodynamic Data  Quickbasic Computer Program CAPER  A.2.1  Equilibrium Constant of H 0 2  T h e water e q u i l i b r i u m constant i s available as a f u n c t i o n o f temperature [10] and i o n i c strength  [62,63,64].  temperature  I n curve  w a s independent  fitting t h e data, o f t h e effect  it w a s assumed  o f ionic  strength.  that A  t h e effect o f  reasonably  good  a p p r o x i m a t i o n o f the e q u i l i b r i u m constant w a s o b t a i n e d b y (see a l s o F i g u r e A . 7 1 ) :  -(14.95-4.34E-2-Tc + 2.26E-4-Tc - 5.92E-7Tc - 0.474 + 0.474 e 2  Kw=10  3  (_123Ios)  + 0.126-Ios) (A.27)  where K w  e q u i l i b r i u m constant o fwater, M  2  K w = [OH]-[H ]  (A.28)  +  Tc  temperature, degrees C e l s i u s  Ios  Ionic Strength, M  2  Ios where  (A.29)  z  ion charge  C  ion concentration, M  •  14.2 14.0  Literature data Equation A.27  13.8 13.6 £ 13.4 TO O  1 3 2  _J 13.0 12.8 12.6 12.4  0.0  0.5  1.0  1.5  2.0 2.5  3.0  Ionic Strength, M Figure A.71  E q u a t i o n A . 2 7 g i v e s a r e a s o n a b l e f i t o f t h e w a t e r e q u i l i b r i u m c o n s t a n t as a f u n c t i o n o f i o n i c strength and temperature.  119  Appendix A A.2.2  Data  Thermodynamic Data  Equilibrium Constant of Hydrogen Peroxide  o n the equilibrium between  function o f temperature  hydrogen  peroxide  a n dtheperhydroxyl  a n d i o n i c strength i s available i n t h e literature  i o n as a  [65]. T h e  f o l l o w i n g r e l a t i o n s h i p w a s f o u n d t o h o l d o v e r a temperature r a n g e o f 2 0 - 8 0 °C a n d a concentration range o f s o d i u m ions o f 0.04 to 3 M :  Kperox=  K  w  /  j r  10  where  -(1330/Tk - 2.13 + 0.15  Kperox  —^  (A.30)  i  Na | ) +  thee q u i l i b r i u m constant f o rthe acidic dissociation o f h y d r o g e n peroxide, M H 0 2  Tk  <=> H  2  +  + OOH"  (A.31)  temperature i n degrees K e l v i n  In program  CAPER,  thesodium  i o nconcentration has been replaced w i t h the ionic  strength. I n T e d e r ' s experiments, the s o d i u m i o n w a s the o n l y p o s i t i v e i o n i n the s o l u t i o n and the relevant i o n i c strength w a s , therefore, a s s u m e d to equal t h e i o n i c strength o f the sodium i o n [65].  A.2.3  The Dissolution of Calcium Hydroxide  Ca(OH)  2  dissolves toform C a  2 +  , Ca(OH)  +  andOH".  describes the e q u i l i b r i u m o f the s o l i d C a ( O H ) OH".  w i t h the free i o n s i n s o l u t i o n i.e. C a  2 +  and  T h i s e q u i l i b r i u m constant i s presented i n the literature as a f u n c t i o n o f temperature  [63,64,66].  T h i s data w a s plotted and curve fitted w i t h a second order p o l y n o m i a l :  K c a = 10<- 4  where  2  T h e conventional solubility product  K c a  9 6  "  L  8  5  E  "  3  T  c  - 803E-5-TC ) 2  (  A  3  2  the e q u i l i b r i u m constant f o r the d i s s o l u t i o n o f c a l c i u m h y d r o x i d e , M Ca(OH)  2  o  Ca  2 +  + 20H"  120  )  3  (A.33)  Appendix A  Thermodynamic  Data  N o data w a s f o u n d o n the effect o f i o n i c strength. the d e v i a t i o n f r o m i d e a l i t y i n real solutions.  A c t i v i t y c o e f f i c i e n t s are a m e a s u r e o f  A s the concentration o f i o n s i n a s o l u t i o n  increases, i n t e r i o n i c interaction, i nparticular, interionic attraction affects the m o b i l i t y o f the i o n s a n d , h e n c e , the i o n a c t i v i t y . D e b y e a n d H u c k e l , b y a s s u m i n g that i o n s are p o i n t c h a r g e s , d e r i v e d a n e q u a t i o n f o r the a c t i v i t y c o e f f i c i e n t as a f u n c t i o n o f t e m p e r a t u r e  and  i o n i c strength [67]:  log y = -z+z_  • A • VIos  where  y  mean activity coeffcient  z , z.  i o n i c v a l e n c e o f the p o s i t i v e and negative i o n , r e s p e c t i v e l y  A  f u n c t i o n o f temperature a n d the dielectric constant o f the solvent  +  (A.34)  E q u a t i o n A . 3 4 is v a l i d f o r dilute solutions up to a n i o n i c strength o f ~ 0 . 1 .  Guggenheim,  a n d later D a v i e s , i m p r o v e d the D e b y e - H u c k e l equation b y a d d i n g a term  linear i n the i o n i c strength [67]:  log  y  = -z+z.-A-Vlos  +  0  2  .  A  .  .  I  o  s  (  A  3  5  )  l + Vios  T h e D a v i e s m o d e l i s intended t o g i v e a m o r e accurate a c t i v i t y coefficient o v e r the same range o f i o n i c strength.  O v e r the range o f concentrations o f interest for this w o r k ,  i o n i c strength is generally less than 0.1.  However, w h e n absorption o fcarbon dioxide is  taken into consideration, t h ei o n i c strength m a y alkaline p H .  The  Davies  the  i n c r e a s e t o h i g h e r v a l u e s at a m o r e  equation i s used u p to a n i o n i c strength o f 1 to obtain a  correction factor f o r the equilibrium  constant.  Tabulated  values  of A  f o r aqueous  s o l u t i o n s f r o m 0 ° C t o 1 0 0 °C [67] w e r e p l o t t e d a n d least squares c u r v e f i t t e d t o a s e c o n d order p o l y n o m i a l :  121  Appendix A  A  =0.49  Thermodynamic Data  + 6.60E-4-Tc  +  5.08E-6-Tc  (A.36)  2  The m e a n activity coefficient was then determined from equation A . 3 5 . that t h e p o s i t i v e i o n i c v a l e n c e w a s e q u a l t o one.  It w a s a s s u m e d  T h enegative i o n i c valence w a s also  equal to one unless the absorption o f carbon d i o x i d e w a s taken into consideration.  Then  the negative i o n i c v a l e n c e w a s w e i g h t e d b y the relative concentrations o f univalent a n d divalent negative ions.  T h e m e a n activity coefficient w a s used t o correct the solubility  constant accordingly:  Kca  ( C 0 I T e c t e d )  = y -Kca  (A.37)  3  A l t h o u g h often neglected, C a ( O H ) (see F i g u r e 3.4).  +  i s the d o m i n a n t species present i n a l k a l i n e solutions  D a t a o n the effect o f temperature o n the e q u i l i b r i u m constant d e s c r i b i n g  the e q u i l i b r i u m o f f o r m a t i o n o f C a ( O H )  +  i s available i n the literature. T h e effect o f i o n i c  s t r e n g t h w a s e s t i m a t e d u s i n g the D a v i e s r e l a t i o n s h i p as d e s c r i b e d a b o v e , g i v i n g :  Kcaoh = i - 1 0 ( 1  1  1  +  6-27E-3.TC + 2.51E-4-TC ) 2  (  A  3  g  )  Y  where  Kcaoh  the e q u i l i b r i u m constant d e s c r i b i n g the e q u i l i b r i u m o f f o r m a t i o n o f the c a l c i u m h y d r o x i d e i o n , M " Ca  A.2.4  2 +  + OH" o  Ca(OH)  1  +  The Formation ofCalcium Peroxide  A s illustrated b y F i g u r e 3.4, c a l c i u m p e r o x i d e i s n o t stable i n the w a t e r s t a b i l i t y region. However,  i n the presence  thermodynamically favoured.  o f hydrogen  peroxide,  formation  o f calcium peroxide is  C a l c i u m p e r o x i d e i sm o r e stable than c a l c i u m h y d r o x i d e o r  hydrogen peroxide under alkaline conditions.  122  Appendix A  Thermodynamic  Data  T h e effect o f temperature o n the G i b b ' s free energy o f f o r m a t i o n o f C a 0  2  has  been  d e t e r m i n e d as o u t l i n e d i n s e c t i o n A . 1.1. T h e e q u i l i b r i u m c o n s t a n t w a s d e t e r m i n e d b y :  lnKcap = - ^ ^ R T k  where  Kcap  V  e q u i l i b r i u m constant describing the e q u i l i b r i u m o f c a l c i u m peroxide dissolution, M Ca0  AG  (A.39) '  2  + H 0 o 2  3  Ca  2 +  +H00" + OH"  (A.40)  free energy o f reaction A . 4 0 (free energy o f f o r m a t i o n o f products free energy o f f o r m a t i o n o f reactants), J / m o l  R  T h e u n i v e r s a l gas l a w constant, 8.314 J / m o l - K  E n o u g h data points w e r e obtained t o a l l o w a reliable fit o f a s e c o n d order p o l y n o m i a l t o the e q u i l i b r i u m data as a f u n c t i o n o f temperature.  T h e effect o f i o n i c strength  was  incorporated t h r o u g h the D a v i e s relationship g i v i n g :  Kcap=y -(-6.89E-ll 3  + 5.16E-13Tk-8.55E-16-Tk ) 2  123  (A.41)  Appendix A  A.2.5  '  Thermodynamic  Data  L i s t i n g o fQ u i c k b a s i c C o m p u t e r  Program  C A P E R  by Jenny Been  'Determination of aqueous equilibrium concentrations in alkaline 'Ca-H202 solutions. This program computes the changesfromthe 'initial concentrations as the temperature increases from 25 to 90 C 'in 5 degree intervals. 'Input variables COLOR 15, 1: CLS : LOCATE 5, 12 PRINT "Enter the initial pH": LOCATE 6, 18: INPUT PHSET# LOCATE 8, 12: PRINT "Enter the concentration of H202, mol/1" LOCATE 9, 18: INPUT H202I# LOCATE 11,12: PRINT "Enter the concentration of Ca(OH)2, mol/1" LOCATE 12, 18: INPUT CAOH2I# 'Determination of equilibrium constants at specified temperatures IOS#=.00001 NAOH# = .00001 FOR TC# = 25 TO 90 STEP 5 TK# = TC# + 273.15 'temperature in kelvin 'kw is equilibrium constant of water (=[H+]*[OH-]) 5 KW# = 10 -(14.94511# - .043441967# * TC# + .0002256955# * TC# 2 - .00000059206034# * TC# A  A  3 . .474 + .474 * EXP(-12.338 * IOS#) + .126 * IOS#) 'Value of the Debye-Huckel constant A A# = .49158708# + .00065983042# * TC# + .0000050834452# * TC# 2 A  GAMMA# = 10 -(A# * (IOS# .5)) A  A  'kca is the dissociation constant of Ca(OH)2 (=[Ca+2]*[OH-] 2) A  KCA# = 10 (-4.95855 - .001849 * TC# - 8.0289E-05 * TC# 2) A  A  'Correct kca for the ionic strength (based on Debye-Huckel equation) KCA# = GAMMA# 3 * KCA# A  'kperox is the dissociation constant of H202 (=[OOH-]*[OH-]/[H202]) KPEROX# = KW# / (10 -(1330 / TK# - 2.13 + .15 * IOS# .5)) A  A  124  Appendix A  Thermodynamic Data  'kcap is the dissociation constant of Ca02 (=[Ca+2]*[OOH-]*[OH-]) KCAP# = -6.8945E-11 + 5.1592888D-13 * TK# - 8.5545965D-16 * TK# 2 A  'Correct kcap for the ionic strength KCAP# = GAMMA# 3 * KCAP# A  'kcaoh refers to the formation of Ca(OH)+ (=[CaOH+]/([Ca+2]*[OH-]» KCAOH# = (1 / GAMMA#) * 10 (1.1143268# + .006269765# * TC# + 2.505413E-04 * TC# 2) A  A  'ksol is the equilibrium constant of Ca(OH)2 --> Ca02 (=[H202], in excess Ca) KSOL# = 10 (-14.25703 + .02094 * TK#) A  'Set initial conditions NAOHMIN# = 0: NAOHMAX# = 3: OH# = .0000001# OOH# = H202I# * KPEROX# * OH# / KW# 10 OHNEG# = 1E-16: OHPOS# = 2: FX# = 100 'Determine the added NaOH concentration to obtain setpoint pH. DO UNTIL ABS(FX#) < 5E-13 FFX#= 100 IF CAOH2I# > 0 THEN CA# = KCA# / (OH# 2) A  CAOH# = KCAOH# * CA# * OH# IF CA# > CAOH2I# THEN CA# = CAOH2I# END IF IF CAOH2I# > H202I# THEN FX# = NAOH# - KCAP# * OH# / KCA# + CAOH# + 2 * CA# - OH# END IF IF CAOH2I# = 0 THEN OOH# = H202I# * KPEROX# / (KPEROX# + KW# / OH#) FX# = NAOH# - OOH# - OH# END IF IF CAOH2I# < H202I# AND CAOH2I# > 0 THEN Kl# = KPEROX# * OH# * (H202I# - CAOH2I#) / KW# K2# = (KPEROX# * KCAP# / KW#) * (1 + KCAOH# * OH#) K3# = 1 + KPEROX# * OH# / KW# OOH# = (Kl# + (Kl# 2 + 4 * K2#) .5) / (2 * K3#) A  A  CA# = KCAP# / (OOH# * OH#) CAOH# = KCAP# * KCAOH# / OOH#  125  Appendix A  Thermodynamic  Data  FX# = NAOH# - 00H# + CAOH# + 2 * CA# - 0H# END IF IF FX# < 0 THEN OHPOS# = 0H# IF FX# > 0 THEN OHNEG# = 0H# OH# = (OHPOS# + OHNEG#) / 2 LOOP PH# = -LOG(KW# / OH#) / 2.302585 IF ABS(PH# - PHSET#) > .00001 AND TC# = 25 THEN IF ABS(NAOHMAX# - NAOHMIN#) < 1E-15 THEN CLS : LOCATE 16, 5: PRINT "PH CAN NOT BE OBTAINED WITHOUT ADDITION OF ACID": SLEEP: STOP END IF IF PH# > PHSET# THEN NAOHMAX# = NAOH# END IF IF PH# < PHSET# THEN NAOHMIN# = NAOH# END IF NAOH# = (NAOHMAX# + NAOHMIN#) / 2 GOTO 10 END IF 'Determination of final equilibrium concentrations IF CAOH2I# > H202I# THEN OOH# = (KCAP# / KCA#) * OH# H202# = KW# * OOH# / (KPEROX# * OH#) CA02# = H202I# - H202# - OOH# CAOH2# = CAOH2I# - CA# - CA02# - CAOH# END IF IF CAOH2I# < H202I# AND CAOH2I# > 0 THEN H202# = H202I# - CAOH2I# + CAOH# + CA# - OOH# CA02# = CAOH2I# - CA# - CAOH# CAOH2# = 0 END IF IF CAOH2I# = 0 THEN  126  Appendix A  Thermodynamic  Data  H202# = H202I# - 00H# END IF 'Determine the ionic strength IOSOLD# = IOS# IOS# = .5 * (OH# + OOH# + NAOH# + 4 * CA#) IF ABS(IOSOLD# - IOS#) > .000001 THEN GOTO 5 'Print results to screen COLOR 15, 1: CLS LOCATE 5, 5: PRINT USING "TEST TEMPERATURE ###.# CELSIUS"; TC# LOCATE 7, 5: PRINT USING "KW = ##.#### "; KW# AAAA  LOCATE 9, 5: PRINT USING "KCA = ##.#### "; KCA# AAAA  LOCATE 11, 5: PRINT USING "KPEROX = ##.#### "; KPEROX# AAAA  LOCATE 13, 5: PRINT USING "KCAP = ##.#### "; KCAP# AAAA  LOCATE 14, 5: PRINT USING "KSOL = ##.#### "; KSOL# AAAA  LOCATE 16, 5: PRINT USING "SOLUTION pH IS ##.####"; PH# LOCATE 16, 35: PRINT USING "[OH-] ##.#### M"; OH# AAAA  LOCATE 18, 5: PRINT USING "[Ca+2]  ##.####  AAAA  M"; CA#  LOCATE 20, 5: PRINT USING "[Ca(OH)2] ##.####  AAAA  LOCATE 22, 5: PRINT USING "[Ca02]  ##.####  AAAA  LOCATE 18, 35: PRINT USING "[OOH-] ##.####  AAAA  LOCATE 20, 35: PRINT USING "[H202] ##.####  AAAA  LOCATE 24, 5: PRINT USING "[Ca(OH)+] ##.####  M"; CAOH2#  M"; CA02# M"; OOH# M"; H202#  AAAA  M"; CAOH#  LOCATE 22, 35: PRINT USING "IONIC STRENGTH IS ##.##### M"; IOS# SLEEP NEXT STOP  127  Appendix A  A.2.6  Thermodynamic  Data  The Solubility of Calcium  X R D  analyses  carbonate.  o f filtered  Whereas  solids  Carbonate  after  t h e tests,  revealed  the presence  o f calcium  n o carbonate salt w a s added t o the s o l u t i o n , c a r b o n d i o x i d e  absorbed f r o m the air, m a k i n g carbonate available t o f o r m c a l c i u m carbonate.  was  T h e test  apparatus w a s not designed t ob e air proof, and even a nitrogen purge w a s not sufficient to e x c l u d e C 0  2  f r o m the solution.  The surrounding  air p r o v i d e d a n u n l i m i t e d source o f  C 0 , a n d the quantity w h i c h is absorbed depends o n the s o l u t i o n chemistry. 2  T h e carbon dioxide solubility data obtained b y W e i s s w a s used, w h i c h is summarized b y the equation  [68]:  In K o = - 5 8 . 0 9 3 1 + 9 0 . 5 0 6 9 ( 1 0 0 / T k ) + 2 2 . 2 9 4 0 l n ( T k / 1 0 0 ) + [ 0 . 0 2 7 7 6 6 0.025888(Tk/100) + 0.0050578  where K o  solubility constant o f carbon dioxide, Ko =  pC0 S  (Tk/100)2]-S  2  [H C0 ]/pC0 2  3  M-atm"  (A.42)  1  2  partial carbon dioxide pressure salinity, %  T h e i o n i z a t i o n constants o f c a r b o n i c a c i d w e r e m o d e l e d to data f r o m S i l l e n [63,64]. T h e data w a s least squares c u r v e fitted w i t h the results d i s p l a y e d i n F i g u r e A . 7 2 .  The  first  i o n i z a t i o n constant c a n b e w r i t t e n as:  l o g K , = 6.355 - 0.0017 T c + 2.7 E - 5 T c where  (A.43)  2  K ,= (l/y)-[H C0 ]/[H ]-[HC0 '] 2  3  +  3  (A.44)  T h e s e c o n d i o n i z a t i o n constant c a n b e w r i t t e n as:  log K  1 2  where  = 10.461 - 0.0074 T c + 4.3E-5 T c K  1 2  (A.45)  2  = (l/y)-[HC0 ]/[H ]-[C0 3  +  128  3  2  -]  (A.46)  Appendix A  Thermodynamic Data  Data from Sillen  m  Least Squares Model  0  50  100  150  200  250  T e m p e r a t u r e , °C  Figure A.72  Ionization constants o f carbonic acid.  T h e s o l u b i l i t y o f c a l c i u m carbonate as a f u n c t i o n o f temperature w a s also m o d e l e d to data obtained f r o m S i l l e n [63,64].  T h e results are d i s p l a y e d i n F i g u r e A . 7 3 .  T h e data are  fitted b y a straight line:  Kc ACARB = - 7 . 8 3 7 - 0 . 0 1 8 T c where  (A.47) (A.48)  KCACARB = ( Y ) - [ C a ] - [ C 0 - ] 2  2 +  3  2  Data from Sillen  Least Squares Model  -7 -8 m UL  A o  ra o  _i  -9  -10 -11 -12 -13 50  100  150  Temperature,  Figure A.73  200 Of  250  C  S o l u b i l i t y o f c a l c i u m carbonate as a f u n c t i o n o f temperature.  129  Appendix A  Thermodynamic  Data  A.2.7  L i s t i n g o fQ u i c k b a s i c C o m p u t e r  '  by Jenny Been  Program  CAPER.v2  'Determination of aqueous equilibrium concentrations in alkaline 'Ca-H202 solutions, exposed to air, over a temperature range of 25 to 100 C. 'This program assumes a constant pH. 'Input variables COLOR 15,1: CLS : LOCATE 5,12 PRINT "Enter the pH": LOCATE 6, 18: INPUT PHSET# LOCATE 8, 12: PRINT "Enter the concentration of H202, mol/1" LOCATE 9,18: INPUT H202I# LOCATE 11,12: PRINT "Enter the concentration of Ca(OH)2, mol/1" LOCATE 12, 18: INPUT CAOH2I# 'Determination of equilibrium constants at specified temperatures S# = 0: OH# = 1: OOH# = 1: HC03# = 1: C032# = 1 IOS# = .00001 FOR TC# = 25 TO 90 STEP 5 TK# = TC# + 273.15 'temperature in kelvin 'kw is equilibrium constant of water (=[H+]*[OH-]) 5 KW# = 10 -(14.94511# - .043441967# * TC# + .0002256955# * TC# 2 - .00000059206034# * ' A  A  3 . .474 + .474 * EXP(-12.338 * IOS#) + .126 * IOS#) 'Value of the Debye-Huckel constant A A# = .49158708# + .00065983042# * TC# + .0000050834452# * TC# 2 A  ZMINUS# = (OH# + OOH# + HC03# + 2 * C032#) / (OH# + OOH# + HC03# + C032#) GAMMA# = 10 -((ZMINUS# * A# * (IOS# .5)) / (1 + IOS# .5) + .2 * A# * ZMINUS# * IOS#) A  A  A  'kca is the dissociation constant of Ca(OH)2 (=[Ca+2]*[OH-] 2) A  KCA# = 10 (-4.95855 - .001849 * TC# - 8.0289E-05 * TC# 2) A  A  'Correct kca for the ionic strength (based on Davies equation) KCA# = GAMMA# 3 * KCA# A  'kperox is the dissociation constant of H202 (=[OOH-]*[H+]/[H202])  130  Appendix A  Thermodynamic  Data  KPEROX# = KW# / (10 -(1330 / TK# - 2.13 + .15 * IOS# .5)) A  A  'kcap is the dissociation constant of Ca02 (=[Ca+2]*[OOH-]*[OH-]) KCAP# = -6.8945E-11 + 5.1592888D-13 * TK# - 8.5545965D-16 * TK# 2 A  'Correct kcap for the ionic strength KCAP# = GAMMA# 3 * KCAP# A  'kcaoh refers to the formation of Ca(OH)+ (=[CaOH+]/([Ca+2]*[OH-])) KCAOH# = (1 / GAMMA#) * 10 (1.1143268# + .006269765# * TC# + 2.505413E-04 * TC# 2) A  A  'kcacarb refers to the solubility of calcium carbonate =[Ca+2]*[C03-2] KCACARB# = GAMMA# 2 * 10 (-7.83701 - .01825 * TC#) A  A  'ko = H2C03/PC02, moles/l.atm KO# = EXP(-58.0931 + 90.5069 * (100 / TK#) + 22.294 * LOG(TK# /100) + S# * (.027766 - .025888 * (TK# / 100) + ,0050578# * (TK# /100) 2)) A  'kl2 = [HC03-]/[H+]*[C03-2] K12# = (1 / GAMMA#) * 10 (10.461 - .0074 * TC# + .000043 * TC# 2) A  A  'kl = [H2C03]/[H+]*[HC03-] Kl# = (1 / GAMMA#) * 10 (6.355 - .0017 * TC# + .000027 * TC# 2) A  A  'ksol is the equilibrium constant of Ca(OH)2 --> Ca02 (=[H202], in excess Ca) KSOL# = 10 (-14.25703 + .02094 * TK#) A  'Set initial conditions PC02# = .000333 'atm OH# = KW# /10 (-PHSET#) A  CAC03# = 0: CAOH2# = 0 H2C03# = PC02# * KO# C032# = (H2C03# * OH# 2) / (KW# 2 * Kl# * K12#) A  A  'Determine Ca from Ca(OH)2 and CaC03 CAl# = KCA#/OH# 2 A  CA2# = KCACARB# / C032# IF CA1# > CA2# THEN CA# = CA2# IF CA2# > CA1# THEN CA# = CA1# 'Determine Ca02 and CaOH+ concentrations CA02# = H202I# - (KCAP# * KW# + KCAP# * KPEROX# * OH#) / (CA# * (OH#) 2 * KPEROX#) A  IF CA02# < 0 THEN CA02# = 0 CAOH# = KCAOH# * CA# * OH#  131  Appendix A  Thermodynamic  Data  CASUM# = CA# + CAOH# + CA02# IF CASUM# < CA0H2I# AND CA 1 # > CA2# THEN CAC03# = CA0H2I# - CASUM# END IF IF CASUM# < CAOH2I# AND CA2# > CA1# THEN CA0H2# = CA0H2I - CASUM# END IF IF CASUM# > CAOH2I# THEN CAMAX# = CA# CAMIN# = 0 DO UNTIL ABS(CASUM# - CAOH2I#) < .00000001# CA# = (CAMIN# + CAMAX#) / 2 CA02# = H202I# - (KCAP# * KW# + KCAP# * KPEROX# * OH#) / (CA# * (OH#) 2 * KPEROX#) A  CAOH# = KCAOH# * CA# * OH# CASUM# = CA# + CAOH# + CA02# IF CASUM# < CAOH2I# THEN CAMIN# = CA# IF CASUM# > CAOH2I# THEN CAMAX# = CA# LOOP END IF OOH# = (H202I# - CA02#) / (1 + KW# / (OH# * KPEROX#)) H202# = OOH# * KW# / (KPEROX# * OH#) HC03# = H2C03# * OH# / (KW# * Kl#) 'Correct carbonate concentrations for salinity SOLD# = S# S# = (C032# * 60 + HC03# * 61) /10 IF ABS(SOLD# - S#) > .001 THEN GOTO 5 'Print results to screen and file COLOR 15, 1: CLS LOCATE 5, 5: PRINT USING "TEST TEMPERATURE ###.# CELSIUS"; TC# LOCATE 7, 5: PRINT USING "KW = ##.#### "; KW# AAAA  LOCATE 9, 5: PRINT USING "KCA = ##.#### "; KCA# AAAA  LOCATE 11,5: PRINT USING "KPEROX = ##.#### "; KPEROX# AAAA  LOCATE 13, 5: PRINT USING "KCAP = ##.#### "; KCAP# AAAA  132  Appendix A  Thermodynamic  Data  LOCATE 14, 5: PRINT USING "KSOL = ##.#### "; KSOL# AAAA  LOCATE 16, 5: PRINT USING "SOLUTION pH IS ##.####"; PHSET# LOCATE 16, 35: PRINT USING "[OH-] ##.#### M"; OH# AAAA  LOCATE 18, 5: PRINT USING "[Ca+2]  ##.####  AAAA  M"; CA#  LOCATE 20, 5: PRINT USING "[Ca(OH)2] ##.####  AAAA  LOCATE 22, 5: PRINT USING "[Ca02]  ##.####  AAAA  M"; CAOH2#  M"; CA02#  LOCATE 18, 35: PRINT USING "[OOH-] ##.####  M"; OOH#  LOCATE 20, 35: PRINT USING "[H202] ##.####  M"; H202#  AAAA  AAAA  LOCATE 24, 5: PRINT USING "[Ca(OH)+] ##.####  M"; CAOH#  LOCATE 22, 35: PRINT USING "[CaC03] ##.####  M"; CAC03#  LOCATE 23, 35: PRINT USING "[C032-] ##.####  M"; C032#  AAAA  AAAA  AAAA  LOCATE 24, 35: PRINT USING "[HC03-] ##.####  AAAA  M"; HC03#  NA# = OH# + OOH# + 2 * C032# + HC03# - 2 * CA# - CAOH# IOSOLD# = IOS# IOS# = .5 * (NA# + OH# + OOH# + 4 * CA# + 4 * C032# + HC03# + CAOH#) IF ABS(IOSOLD# - IOS#) > .0001 THEN GOTO 5 SLEEP NEXT STOP  133  Appendix  B  Electropolishing Titanium  T h e e l e c t r o p o l i s h i n g s o l u t i o n c o n s i s t e d o f 3 0 0 m l o f m e t h y l a l c o h o l , 1 7 5 m l o f «-butyl alcohol, and 30 m l o f 6 0 % perchloric acid. above  order  to  ensure  T h e c h e m i c a l s w e r e added a n d m i x e d i n the  that the p e r c h l o r i c a c i d w a s  immediately  diluted.  At  these  concentrations, the m i x t u r e is n o n - e x p l o s i v e and c a n be stored for several w e e k s .  This  solution is suitable for a w i d e range o f alloys.  E l e c t r o p o l i s h i n g w a s c o n d u c t e d b e l o w - 2 5 °C, f i l m i s p r o d u c e d o n the c o u p o n [69]. worked well. vessel hydride  as a b o v e t h i s t e m p e r a t u r e a b r o w n  anodic  K e e p i n g the temperature b e t w e e n - 4 0 and - 3 0  °C  T h e temperature w a s k e p t l o w t h r o u g h the a d d i t i o n o f l i q u i d n i t r o g e n to the  containing  the  electropolishing assembly.  Electropolishing  did  not  result  in  formation.  S t i r r i n g s h o u l d p r o d u c e a g o o d f l o w a l o n g the surface o f the sample.  If the f l o w w a s too  l o w , the v i s c o u s l a y e r f l o w e d d o w n i n v i s c o u s streams and g r o o v e s w e r e f o r m e d o n the surface.  It w a s v e r y i m p o r t a n t t o m a i n t a i n a n e v e n c u r r e n t d i s t r i b u t i o n .  I f a l l sides o f the sample  n e e d e d to b e p o l i s h e d , the s a m p l e w a s p l a c e d i n the center o f a stainless steel beaker, w h e r e the b e a k e r s e r v e d as the c a t h o d e .  I f o n l y one side w a s to b e p o l i s h e d , a stainless  steel cathode plate w a s p o s i t i o n e d p a r a l l e l to the sample.  This produced a more  even  current d i s t r i b u t i o n a n d better results than the beaker.  The optimum  current density was - 0 . 1  A/cm . 2  - 2 0 - 3 0 V o l t s for a cell gap o f a f e w centimeters.  T h i s corresponds to a c e l l potential  T h e current density w a s controlled, not  the p o t e n t i a l as the p o t e n t i a l v a r i e d w i t h f l u c t u a t i n g t e m p e r a t u r e .  134  of  Appendix B  A  Electropolishing  Titanium  g e n e r a l r u l e o f t h u m b states that e a c h p o l i s h i n g step s h o u l d r e m o v e  m e a n p a r t i c l e s i z e o f the p r e v i o u s p o l i s h i n g step. to 6 0 0 grit before e l e c t r o p o l i s h i n g . [70].  T h e Metals  handbook  three times t h e  The sample was mechanically polished  T h e m e a n particle size f o r grit size 6 0 0i s 8 m i c r o n  [70] suggests  that  residual  stresses a r e o b s e r v e d  m a x i m u m d e p t h o f 13 m i c r o n s u n d e r g e n t l e , l o w s t r e s s g r i n d i n g c o n d i t i o n s . 25 m i c r o n s should, therefore, b e sufficient to remove  a l l r e s i d u a l stresses.  ~ 2 5 m i c r o n , the electropolishing w a s continued for ~ 1 5  minutes.  135  at a  Removal o f T o remove  Appendix C  Determination of Titanium Single Crystal Orientation  T h e t i t a n i u m single crystal u s e d i n the c o r r o s i o n studies o f this report w a s prepared b y electron  beam  melting  from  commercially  pure  titanium  i n Prof.  l a b o r a t o r y at t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a ( s e e C h a p t e r 5 . 1 . 2 ) .  Alec  Mitchell's  The orientation o f  the single c r y s t a l w a s d e t e r m i n e d b y the b a c k r e f l e c t i o n L a u e m e t h o d .  C.l  The Back-Reflection Laue Method  T h e orientation o f a single crystal c a n b e d e t e r m i n e d accurately b y the b a c k - r e f l e c t i o n L a u e m e t h o d [72].  T h i s m e t h o d is b a s e d o n the B r a g g l a w o f d i f f r a c t i o n :  nX = 2dsinQ where  (C.49)  n  the order o f r e f l e c t i o n  X  scattered wavelength,  d  distance b e t w e e n atoms n o r m a l to the i n c i d e n t b e a m ,  e  angle o f incidence w h i c h equals the angle o f reflection  T h e m e t h o d i t s e l f is straight forward. o p e r a t i n g at 2 0 m A a n d 3 5 k V . and impinged  nm  I n this w o r k a c o p p e r target X - r a y tube w a s u s e d ,  The X - r a y s passed through a pinhole i n an X - r a y  upon thesingle crystal specimen.  The  e x a c t l y 3 c m from t h e f i l m , d i f f r a c t e d t h e i n c o m i n g r a y s . caught b y the X - r a y  f i l m and recorded as L a u e  reflected, diffracted X - r a y s  from  nm  a specific plane.  spots.  crystal, w h i c h was  film  positioned  S o m e o f the reflected rays w e r e E a c h spot w a s the result o f  L a u e spots lie o n h y p e r b o l a s w i t h the  r e p r e s e n t i n g p l a n e s b e l o n g i n g t o the s a m e z o n e , i.e., these p l a n e s are a l l p a r a l l e l t o the s a m e z o n e a x i s (see F i g u r e C . 7 4 ) .  The o p t i m u m exposure time was 25 minutes.  The X -  r a y f i l m w a s m a r k e d to ensure that the o r i e n t a t i o n o f the f i l m w o u l d b e k n o w n after f i l m d e v e l o p m e n t a n d d u r i n g the a n a l y s i s p r o c e s s . T h e f i l m w a s a n a l y z e d as i f it w e r e v i e w e d from  the p o s i t i o n o f the X - r a y tube.  136  Appendix C  Determination of Titanium Single Crystal Orientation  \  \ X —ray  Ti  crystal  X-ray  Figure C.74  beam  film  B a c k - r e f l e c t i o n L a u e pattern o f planes o f a zone i n a crystal.  D e t e r m i n i n g the c r y s t a l orientation i s not a l w a y s easy a n d m a y i n v o l v e a fair a m o u n t o f trial a n d error.  Step 1.  T h e f o l l o w i n g steps w e r e p e r f o r m e d :  Despite  the use o f a nickel  filter, considerable  fogging  from  radiation resulted i n L a u e spots w h i c h w e r e difficult to distinguish. results w e r e obtained b y h o l d i n g the f i l m t o the light.  fluorescent T h e best  I n order to perform  m e a s u r e m e n t s , the X - r a y f i l m w a s c o v e r e d w i t h a sheet o f t r a c i n g paper, a n d the L a u e spots w e r e traced o n a light table.  Step 2.  G r e n i n g e r ' s c h a r t , a c o l l e c t i o n o f h y p e r b o l a s d r a w n a t 2° i n t e r v a l s ( s e e F i g u r e C . 7 5 ) , w a s u s e d t o obtain the angle o f i n c l i n a t i o n , a.  and the angle o f rotation,  I f the angle o f i n c l i n a t i o n were zero, the h y p e r b o l a w o u l d reduce t o a  straight line t h r o u g h t h e center o f the f i l m . Greninger's  chart  A transparency was made o f a  for a 3 c m specimen to film  distance.  T h e center o f  G r e n i n g e r ' s chart transparency w a s p o s i t i o n e d o n the center o f the X - r a y as d r a w n o n t h e t r a c i n g p a p e r .  film  Greninger's chart w a s then rotated until one o f  its h y p e r b o l a s c o i n c i d e d w i t h a r o w o f spots.  T h e angles associated w i t h the  z o n e a x i s o f the h y p e r b o l a p l a n e s c a n t h e n e a s i l y b e r e a d o f f the chart (see Figure C.76).  137  Appendix C  Figure C.75  Determination of TitaniumSingle Crystal Orientation  Greninger's  c h a r t g r a d u a t e d i n 2° i n t e r v a l s i n t h e s i z e f o r 3 - c m  distance  f r o m s p e c i m e n to f i l m . S t e p 3.  T h e angles d e t e r m i n e d i n step 2 are the coordinates o f a z o n e a x i s w h i c h c a n b e transferred to a stereographic projection.  A  stereographic projection is a 2-  d i m e n s i o n a l representation o f a reference sphere w i t h a unit c e l l o f the single c r y s t a l e n v i s i o n e d at i t s c e n t e r .  Normals  surface o f the sphere, creating poles. one o r m o r e o f the crystal planes. tracing paper  o f the c r y s t a l p l a n e s intersect the  T h e p l o t t e d z o n e a x i s p o l e is n o r m a l to  A l l p o l e s o f step 2 w e r e p l o t t e d b y p l a c i n g  o n top o f a transparency o f a stereographic projection.  The  t r a c i n g p a p e r w a s m a r k e d to ensure that the a n g l e o f r o t a t i o n c o i n c i d e d w i t h that o n the f i l m .  The  angle o f inclination was measured  o u t s i d e e d g e o f the c i r c l e (see F i g u r e C . 7 7 ) .  138  inward from  the  Determination of Titanium Single Crystal Orientation  Appendix C  \ Y  L-45 \  aA  90  135  Figure C.76  Step 4 .  M e a s u r e m e n t o f the z o n e coordinates o f a zone axis.  T h e poles o n the stereographic projection were randomly  numbered a n d the  angles  t h e stereographic  between  the poles  were  measured  b y rotating  t r a n s p a r e n c y s u c h that b o t h p o l e s w e r e p o s i t i o n e d o n t h e s a m e l o n g i t u d e o r latitude line.  A t t e n t i o n w a s g i v e n to patterns, s y m m e t r y , a n d to ' n i c e '  angles  s u c h a s 30°, 60° a n d 90°, r e l e v a n t t o h e p c r y s t a l s .  T h e angle between  plane  n o r m a l s i s i d e n t i c a l to the angle b e t w e e n the planes.  A clay model, w i t h tooth  p i c k s as p l a n e n o r m a l s , w a s u s e d as a n a i d i n i d e n t i f y i n g the c o o r d i n a t e s o f the planes.  M e a s u r e d angles were compared to calculated angles  planes.  W h e n the p o l e s o f several planes h a d tentatively b e e n identified, they  1  [73,74] between  w e r e c h e c k e d against other p o l e s u n t i l c o n f i d e n c e as t o t h e orientation o f the crystal w a s obtained.  Electron Diffraction, Crystal Calculations - Engineering Materials Software Series by P.J. Goodhew, issued by the Materials Science & Engineering Department, University of Surrey, 1987. 1  139  Determination of Titanium Single Crystal Orientation  Appendix C  Figure C.77  C.2  T r a n s f e r r i n g the z o n e coordinates to a stereographic p r o j e c t i o n .  Orientation of T i t a n i u m Single Crystal  A t i t a n i u m s i n g l e c r y s t a l s a m p l e w a s p r e p a r e d as d e s c r i b e d i n C h a p t e r 5.1.2.  The  three  o r t h o g o n a l s u r f a c e s w e r e r a n d o m l y l a b e l e d as i n F i g u r e 5 . 1 3 .  F i g u r e s C . 7 8 , C . 8 0 , a n d C . 8 2 s h o w t h e L a u e p a t t e r n s o f s u r f a c e s 1, 2 , a n d 3 , r e s p e c t i v e l y , w i t h h y p e r b o l a s d r a w n t h r o u g h series o f spots w h i c h represent planes b e l o n g i n g to the same  zone.  previous Roman  The  coordinates o f the zone  section using Greninger's numbers and subsequently  C.79, C.81, and C.83). Tables C.5, C.6, and C.7.  chart.  axes were The  determined  as d e s c r i b e d i n  poles were numbered randomly  p l o t t e d o n a s t e r e o g r a p h i c p r o j e c t i o n (see  the with  Figures  A n g l e s b e t w e e n the p o l e s w e r e m e a s u r e d a n d are tabulated i n T h e crystal orientation o f surfaces 1 and 3 c o u l d positively be  i d e n t i f i e d a n d the coordinates o f the planes n o r m a l to the z o n e axes w i t h actual c a l c u l a t e d a n g l e s are a l s o l i s t e d i n T a b l e s C . 5 a n d C . 7 .  Surface 2 c o u l d not be identified through  this m e t h o d and its orientation w a s determined  from  surfaces 1 and 3. E a c h single crystal  s u r f a c e o r i e n t a t i o n c a n b e g e n e r a t e d i n t o p l a n e s o f the s a m e f o r m o r f a m i l y s u c h that a l l  140  Appendix C  Determination of Titanium Single Crystal Orientation  three s i n g l e c r y s t a l surfaces c a n b e represented i n the s a m e stereographic triangle Figure C.84).  B y m a i n t a i n i n g t h e c - c o o r d i n a t e c o n s t a n t , c r y s t a l s u r f a c e (1  rotated to the r e l a t e d s u r f a c e (4 1 5 4). surface ( 2  4 2  Figure C.78  13) is f a m i l y o f (2 2 4  S u r f a c e (4  13).  L a u e p a t t e r n o f s u r f a c e 1.  141  1 3  (see  5 4 4) c a n be  1) i s r e l a t e d t o (3 1 4  1)  and  Appendix C  Determination  of Titanium Single Crystal  Orientation  Appendix C  Figure C.80  Determination of Titanium Single Crystal Orientation  L a u e pattern o f surface 2.  143  Appendix C  Figure C.81  Determination  of Titanium Single Crystal  Stereographic projection o fsurface 2 .  144  Orientation  Appendix C  Determination of Titanium Single Crystal Orientation  VII  Figure C.82  L a u e p a t t e r n o f s u r f a c e 3.  145  Appendix C  Figure C.83  Determination  of Titanium Single Crystal  Stereographic projection o fsurface 3.  146  Orientation  Appendix C  Table C.5  Determination of Titanium Single Crystal Orientation  A n g l e s b e t w e e n i d e n t i f i e d p o l e s o f s u r f a c e 1. Indices o f  Pole  plane  Angles between poles  n o r m a l to  Measured (Calculated)  pole I  III  V  VI  IX  I  1010  -  90 (90)  58.5 (58.6)  30 (30)  75 (74.9)  II  8190  5 (5.8)  85 (85.8)  57 (58.8)  35 (35.8)  71 (72.2)  III  1213  90 (90)  -  53 (54.1)  68 (68.7)  24 (23.9)  IV  3210  39.5 (40.9)  60 (61.6)  67 (66.8)  10(10.9)  85 (84.3)  V  1013  58.5 (58.6)  53 (54.1)  -  65 (63.2)  30 (30.2)  VI  2110  30 (30)  68 (68.7)  65 (63.2)  -  89 (90)  VII  1123  50(51)  43 (42.6)  23 (23.9)  69 (68.7)  24 (23.9)  VIII  1218  90 (90)  26 (25)  37 (37.5)  78 (79.4)  17.5 (16.3)  IX  0113  75 (74.9)  24 (23.9)  30 (30.2)  89 (90)  -  Center  1544  74.2 (72.8)  72.3 (71)  76.5 (77.7)  80 (80.2)  86 (84.5)  147  Appendix C  Table C.6  Determination of Titanium Single Crystal Orientation  M e a s u r e d angles between poles o f surface 2. M e a s u r e d angles between poles  Pole I  II  III  IV  X  X'  I  -  77  87  65  54  61  II  77  -  47.5  73  57  41  III  87  47.5  -  28.5  36.5  32.5  IV  65  73  28.5  -  30  58  V  45  61.5  46  38.5  10  76  VI  88  51  6  30  41.5  29.5  VII  44.5  39  80  72  83.5  54  VIII  13  64  85  70  64.5  53  IX  24  51.5  77  77  75  -45  X  54  57  36.5  30  -  67  XI  64  70  26.5  5  25.5  58  XII  60  67  28.5  10.5  20  60  XIII  14  82.8  73.5  51  43.5  74  XIV  98  46  7  27.5  30  37.5  X V  4.5  78  88  67.5  54.5  60  X'  61  41  32.5  58  67  -  148  Appendix C  T a b l e C.l  Determination  of Titanium Single Crystal  Orientation  A n g l e s between identified poles o f surface 3. Indices o f  Pole  plane  Angles between poles Measured  normal to  (Calculated)  pole I  II  III  V  VII  I  0110  -  60 (60)  20(19.1)  80 (79.1)  61 (60)  II  1100  60 (60)  -  79 (79.1)  21 (19.1)  60 (60)  III  1230  20(19.1)  79 (79.1)  -  80 (81.8)  40.5 (40.9)  IV  1120  32 (30)  91 (90)  12(10.9)  68 (70.9)  29.5 (30)  V  3210  80 (79.1)  21 (19.1)  80 (81.8)  -  39 (40.9)  VI  2110  90 (90)  31 (30)  70 (70.9)  10.5 (10.9)  2 9 (30)  VII  1010  61(60)  60 (60)  40.5 (40.9)  39 (40.9)  -  VIII  4133  75 (75.4)  51 (50.8)  57 (58.3)  35 (35.8)  26.5 (27.9)  IX  4154  54 (53.8)  74 (72.8)  39 (38.6)  56 (56)  29 (27.6)  X  3143  51 (50.8)  75 (75.4)  35 (35.8)  57 (58.3)  27 (27.9)  XI  4132  76 (74.6)  49 (48.4)  57.5 (56,5)  31.5 (31.5)  24.5 (21.8)  XII  3122  80 (80)  44 (45.7)  61 (62.5)  27 (30.9)  25.5 (29.1)  XIII  2130  43 (40.9)  78 (79.1)  23 (21.8)  5 7 (60).  18(19.1)  XIV  3120  79 (79.1)  41.5 (40.9)  59 (60)  21 (21.8)  18(19.1)  X V  1212  44 (42.9)  44.5 (42.9)  57 (56.4)  58.5 (56.4)  9 0 (90)  XVI  1213  50 (51)  50 (51)  61 (61.6)  62 (61.6)  90 (90)  XVII  4843  33.5 (32.5)  32.5 (32.5)  52 (50.4)  51 (50.4)  91 (90)  XVIII  2421  33 (31.2)  31.5 (31.2)  51.5 (49.7)  51 (49.7)  91 (90)  XIX  1211  35 (34.3)  34 (34.3)  53 (51.4)  52 (51.4)  91 (90)  X X  4845  37 (36.3)  36 (36.3)  54 (52.5)  53 (52.5)  91(90)  Center  2 4 2 13  67 (67.7)  67.5 (67.7)  73 (73.3)  73.5 (73.3)  89.5 (90)  149  Appendix C  Figure C.84  Determination  of Titanium Single Crystal  Orientation  S i n g l e c r y s t a l s u r f a c e o r i e n t a t i o n i n a s t e r e o g r a p h i c t r i a n g l e . P o l e #1 c o r r e s p o n d s t o p l a n e ( 4 1 5 4 ) , p o l e # 2 r e p r e s e n t s p l a n e ( 3 1 4 1), a n d p o l e #3 i s p e r p e n d i c u l a r t o ( 2 2 4 1 3 ) .  150  Appendix D  Surface analysis  T h e i n i t i a l h y p o t h e s i s w a s that the p r e s e n c e o f c a l c i u m c o u l d l e a d t o the f o r m a t i o n o f c a l c i u m titanates w h i c h w o u l d subsequently oxide.  offer p r o t e c t i o n b y s t a b i l i z i n g the  surface  S e v e r a l surface a n a l y t i c a l techniques w e r e e m p l o y e d to f i n d out i f c a l c i u m titanate  w a s present.  A f t e r r e m o v a l o f t h e s a m p l e f r o m the test s o l u t i o n , a n y s u r f a c e c a l c i u m f i l m w a s b y r u b b i n g the s a m p l e lightly w i t h a g l o v e d finger.  removed  A l l samples were rinsed with distilled  water, dried i n air and stored i n a dessicator. T h e A E S analysis w a s carried out b y Fraser  University,  theX P Sa n d S I M S  analyses  were  performed  Simon  b y the Advanced  M a t e r i a l s & P r o c e s s E n g i n e e r i n g L a b o r a t o r y at the U n i v e r s i t y o f B r i t i s h C o l u m b i a . A l l three techniques  s h o w e d , w i t h i n the l i m i t s o f detectability, that n o c a l c i u m h a d  been  incorporated into the o x i d e surface f i l m .  D.l  Auger Electron Spectroscopy  A u g e r electrons are p r o d u c e d b y b o m b a r d i n g the surface w i t h l o w e n e r g y electrons k e V ) [75,76].  I n the A u g e r L M M  e m i s s i o n process o f titanium, an electron ejected f r o m  a L level m a y b ereplaced b y a M level electron. energy  t o eject a n A u g e r  emitted generally  T h i s drop i n level releases e n o u g h  electron f r o m a lower energy M level.  f r o m the outer  (1-10  Auger  electrons are  1 0 A o f the surface at characteristic e n e r g y  m a k i n g this technique a nexcellent tool for surface analysis.  levels,  The analyzer measures  the  n u m b e r o f ejected electrons as a f u n c t i o n o f the electron energies.  For titanium  energies are - 4 0 0 e V , for o x y g e n - 5 1 0 e V , a n d for c a l c i u m - 2 9 0 e V .  The detection limit  g e n e r a l l y r a n g e s f r o m 0.1 t o 1 a t o m i c p e r c e n t .  151  these  Appendix D  Surface Analysis  Figure D . 8 5 shows theAuger coupon. used.  spectrum o f a c a l c i u m irihibited titanium weight  loss  A n electron b e a m w i t h a kinetic energy o f 3 k e V a n d a current o f 85 n A w a s  C a l c i u m w a s n o t detected i n the A u g e r spectrum.  T h e p e a k at 2 7 0 e V i s d u e t o  carbon w h i c h apparently i s v e r y c o m m o n o n m o s t surfaces. T h epeak i s characteristic o f free  carbon, n o t a carbide such as titanium carbide [75].  uA/cm  2  A 3 0 s e c o n d sputter at 18  r e m o v e s r o u g h l y 5 A a n de l i m i n a t e d the c a r b o n peak.  T h e c a r b o n peak w a s also  present o n the original sample a n d t h e sample w h i c h h a d corroded alkaline peroxide.  i n calcium  free  Further sputtering decreased the intensity o f the o x y g e n peak a n d  increased the intensity o f the t i t a n i u m p e a k as the o x i d e layer w a s r e m o v e d .  *  '•• 210  240  300  270 —'  330  ........ ....... _. r  I  360  i.  420  t--H-  Figure D . 8 5 A n A u g e r spectrum o f a titanium weight loss c o u p o n corroded i n 0.15 M H 0 , p H 10, 5 0 °C, 100 p p m C a (horizontal axis - electron volts, vertical 2  2  axis - d [ E - N ( E ) ] / d E , w h e r e N ( E ) i s the secondary electron distribution).  152  Appendix D  D.l  Surface Analysis  X-ray Photoelectron Spectroscopy (XPS)  In X - r a y photoelectron spectroscopy, a source o f k n o w n , l o w - e n e r g y X - r a y s i sused t o bombard a sample under ultra-high vacuum conditions.  A s the X - r a y energy i s absorbed  b y l o w energy c o r e electrons, these p h o t o e l e c t r o n s are liberated w i t h a k i n e t i c  energy  w h i c h i s d i r e c t l y related t o the b i n d i n g energy o f the q u a n t u m energy l e v e l p r e v i o u s l y occupied.  T h i s binding energy  i s characteristic o f the a t o m i n v o l v e d a n d c a n thus b e  used for qualitative elemental analysis. C h e m i c a l shifts m a y be present depending o n the chemical environment.  T h e escaped electrons c o m e only  from  theouter  10 A o f the  surface.  Sntens (cpa]  lln  1000  Figure D.86  800  X P S analysis  shows  600  very  similar  400  binding  200 bind, energy  energy  profiles  [eV] l l n  f o r samples  c o r r o d e d i n c a l c i u m free o r c a l c i u m i n h i b i t e d s o l u t i o n s .  (blue, bottom - corroded in 0.15 M H 0 , 2  corroded  in 0.15 M H 0 , 2  corroded in 0.15 MH 0 , 2  2  2  2  pH 11, 50 °C, no Ca; black, top -  pH 11, 50 °C, 100 ppm Ca; red, middle -  pH 11, 70 °C, 100 ppm Ca)  153  Appendix D  Surface Analysis  A s a higher energy  electron falls into t h eavailable hole, the released energy m a y b e  passed o n t o a third electron w h i c h m a y n o w b e released as a n A u g e r electron.  These  electrons c a n b e u s e d to s u p p l y a d d i t i o n a l i n f o r m a t i o n .  The  advantage  specimen.  o f this technique  i s that m i n i m a l  surface damage  is imparted  to the  I n a d d i t i o n t o b i n d i n g energies, linetypes have been recorded i n the literature  and c a n be c o n s u l t e d for analysis, rendering X P S one o f the m o s t accurate o f a l l surface analytical techniques.  A s illustrated b y Figure D.86,  the surface b i n d i n g energy p r o f i l e i s v e r y s i m i l a r for a l l  three s a m p l e s o f w i d e l y v a r y i n g histories. U s i n g a M g X - r a y source, a s m a l l 2 p p e a k w a s d e t e c t e d at 3 4 7 e V a n d a s m a l l L M M  C a p e a k at 961 e V [76].  3  calcium  The C a peaks  h a d a 1 - 2 % relative intensity and w e r e present i n a l l three samples, i n c l u d i n g the s a m p l e w h i c h had not been exposed to calcium.  W i t h a detection limit f o rany element o f~1  a t o m i c percent, i t i s q u e s t i o n a b l e that these s m a l l c a l c i u m p e a k s a r e r e a l s i g n i f i c a n t results.  O x y g e n w a s present f r o m 5 1 - 5 8 % , titanium f r o m 1 2 - 1 5 % , and carbon f r o m 26 to  35%.  D.3  Secondary ion mass spectroscopy  In s e c o n d a r y i o n m a s s s p e c t r o s c o p y ( S I M S ) [77], a s o l i d surface i s b o m b a r d e d b y i o n particles  leading  bombarding Energy  to the emission  o f charged  atomic  and molecular  species.  The  p a r t i c l e s are g e n e r a l l y o f r e l a t i v e l y l o w e n e r g y o f the o r d e r o f 0 . 5 - 1 5 k e V .  andmomentum  are transferred  to surface material  leading  to the emission  ( s p u t t e r i n g ) o f c h a r g e d o r u n c h a r g e d s u r f a c e a t o m s a n d m o l e c u l e s as w e l l a s e l e c t r o n a n d p h o t o n e m i s s i o n . It i s , therefore, a destructive m e t h o d .  Sputtered secondary ions m a y be separated based o n charge and m a s s b y mass analyzers. In S I M S , even H  +  ions c a n be detected.  Quantification can be difficult.  154  Semi-empirical  Appendix D  Surface Analysis  i o n i z a t i o n m o d e l s are a v a i l a b l e a n d the use o f other techniques, s u c h as A u g e r electron spectroscopy or Rutherford backscattering spectroscopy, together w i t h S I M S is useful. v e r y h i g h s e n s i t i v i t y c a n b e obtained d o w n t o the parts p e r b i l l i o n b y w e i g h t  A  range.  W h e n u s e d f o r d e p t h p r o f i l i n g , the r e s o l u t i o n approaches that o f other surface techniques.  Corrosion coupons were analyzed forcalcium using a gallium primary ion gun with a p r i m a r y b e a m i m p a c t e n e r g y o f 3.3 k e V a n d a p r i m a r y i o n current d e n s i t y o f 8 n A . illustrated i n Figures D.87,  D.88,  and D.89,  A s  a c a l c i u m profile appears t o b e present i n  corrosion coupons corroded i n both calcium-free and calcium-inhibited environments. Since neither A E S n o r X P S detected any significant c a l c i u m , this must b e a baseline. Levine  et al.[79]  also observed  c a l c i u m signals i n regions  w h i c h were necessarily  deficient i n c a l c i u m a n d attributed this p h e n o m e n o n to the i s o t o p i c s i m i l a r i t y o f t i t a n i u m and calcium.  S p u t t e r i n g T i m e (min) Figure D.87  A c a l c i u m p r o f i l e is present i n the s a m p l e w h i c h w a s c o r r o d e d i n c a l c i u m f r e e 0 . 1 5 M H O , p H 1 1 , 7 0 °C. 2  2  155  Appendix D  Surface Analysis  156  Appendix D  Surface Analysis  T h e t w o samples c o r r o d e d i n 0.15 M H 0 , p H 11, 7 0 °C, s h o w e d s i m i l a r titanium 2  2  and  o x i d e (not s h o w n ) profiles, i n d i c a t i n g a s i m i l a r o x i d e c o m p o s i t i o n a n d thickness. T h e surface o f the s a m p l e c o r r o d e d i n 0.15 M H 0 , p H 10, 5 0 ° C , d i s p l a y e d a steeper o x i d e 2  2  and titanium profile indicating a m u c h thinner oxide. c o r r o s i o n p r o c e s s increases the o x i d e layer thickness.  157  T h i s s e e m s t o i n d i c a t e that the  

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