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Design of high temperature and pressure electrochemical cell and corrosion chemistry of alloy 625 in… Ubah, Chinedu Gideon 2010

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Design of High Temperature and Pressure Electrochemical Cell and Corrosion Chemistry of Alloy 625 in High Temperature and High Pressure Aqueous Media Using a Two-Electrode Electrochemical Method      by   Chinedu Gideon Ubah  B. Eng., Saimaa University of Applied Science, Finland 2008      A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of    Master of Applied Science   in   The Faculty of Graduate Studies  (Materials Engineering)     The University of British Columbia  (Vancouver)    August 2010  © Chinedu Gideon Ubah 2010  ii ABSTRACT    As aqueous processing moves to higher temperatures and pressures to take advantage of increased kinetics, there is a need to develop and test appropriate reactor materials to ensure that corrosion is minimized.  Corrosion testing often requires an electrochemical approach for a comprehensive understanding of the range of behaviors exhibited from a corroding metal or alloy in different environments.  Prior art of designs for electrodes, associated pressure vessels and sealing technology is presented.  The development of an apparatus and methods for high temperature and high pressure electrochemical corrosion testing are discussed.  The final flow-through electrochemical cell design, the Flow-Through External Pressure-Balanced Reference Electrode (FTEPBRE) design, working/counter electrode and other components, which were developed for temperatures and pressures in excess of 500ºC and 5000 PSI is presented.  A two-electrode electrochemical testing method is presented, using Stainless Steel (SS 316) as both Quasi Reference Electrode (QRE) and Counter Electrode (CE), and Alloy 625 (Ni-062.8%, Cr-21.8%, Mo-7.35, Fe-3.97%, Nb-2.7%) as the Working Electrode (WE). The effects of pressure, and its combination with temperature on OCP and corrosion rate of alloy 625 (WE) in both naturally aerated and de-oxygenated environments in 0.1 M sodium sulphate (Na2SO4) solution with a flow rate of 7 mL/min were investigated and discussed. The effect of pressure represented as a change in activation volume and reaction volume for the homogenous and heterogeneous phases is also presented.  The corrosion rate was observed to increase with both temperature and pressure: higher for naturally aerated conditions than the corresponding de-aerated ones. Results also show that the instability of the QRE affected the result and direction of the OCP tests. A reduction in the corrosion current was observed above 207 bar (3000 PSI) in the polarization tests and was attributed to the increasing stability of the passive film formed on the surface of the alloys.  iii TABLE OF CONTENTS  ABSTRACT !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ""! TABLE OF CONTENTS !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! """! LIST OF TABLES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!#! LIST OF FIGURES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! #"! LIST OF SYMBOLS AND ABBREVIATIONS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$! ACKNOWLEDGEMENTS !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!$"""! DEDICATION !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! $"#! 1.! INTRODUCTION !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %! 2. ! LITERATURE REVIEW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! &! 2. 1       Electrochemical Apparatus !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! "! 2. 1. 1   The Cell Body """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""#! $"!%"!$!!!&'(!)*+,-./01*2.3(+!45(63+*7(!8)40149 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""":! 2. 1. 3   The Reference Electrode""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""";! 2. 2  ! Solution Chemistry !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! #$! 2. 3  ! Pressure and Aqueous Solutions !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! #$! 2. 3. 1   The Theoretical Effect of Pressure on Reaction Rate """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" %<! 2. 3. 2   Effect of Pressure on Conductivity of Aqueous Solutions """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" %;! 2. 3. 4   Volume Effect of Pressure on Electrode Potential"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" $$! 2. 4  ! Temperature and Pressure Effect on Aqueous Solution !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %"! 2. 4. 1   Supercritical Water """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" <=! 3      OBJECTIVES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'(! 4 ! ELECTROCHEMICAL CELL DESIGN!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!')! 4. 1 ! Design Assembly and Operational Principle!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! $&! 4. 2 ! The Cell Body !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! $'! 4. 2. 1   Structural Analysis""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" >=! 4. 3 ! Working/Counter Electrode !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! "'! 4. 4 ! Reference Electrode !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (#! 4. 5 ! Challenges of the Reference Electrode!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (%! 4. 5. 1   Isothermal Liquid Junction Potential EILJP. """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" #$! 4. 5. 2   Thermal Liquid Junction Potential ETLJP """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" ##!  iv 4. 6 ! Heat Exchanger!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ()! 4. 6. 1   Structural Analysis""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" #?! 4.7        Other Component of the Design !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *+! 5 ! EXPERIMENTAL !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*%! 5. 1 ! Test Materials and Solution !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *#! 5. 2 ! OCP Measurements and Conditions !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *"! 5. 2. 1   Apparatus for OCP Tests under Pressure """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" @>! 5. 2. 2   Apparatus for OCP Tests under Pressure and Temperature """""""""""""""""""""""""""""""""""""""""""""""""""""""""""" @#! 6. ! Results and Discussion !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*(! 6. 1 ! Results !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! *)! 6. 1. 1    High Pressure Experiments"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" @:! 6. 1. 2    Temperature and Pressure Experiments """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" :$! 6. 2. 1    Results of Tests at Pressure below 207 Bar """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" :#! 6. 2. 2    Results of Test at Pressures above 207 Bar"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" ?$! 6. 2. 3    Quasi Reference Electrode""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" ??! 7.      CONCLUSION AND RECOMMENDATIONS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!+,! 7. 1 ! Conclusions !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! '+! 7. 2 ! Recommendations!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! '#! 7. 3 ! Future Work !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! '#! REFERENCES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!+-! APPENDIX A: Parts List !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %,'! APPENDIX B: Material’s Stress and Heat Exchanger Analyses !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %,&! APPENDIX C:  Plot of Standard Deviation of OCP of Alloy 625C Versus SS 316 QRE (Naturally Aerated) !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %%,! APPENDIX D:  Plot of Standard Deviation of OCP of Alloy 625C Versus SS 316 QRE (Deaerated) !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %%&! APPENDIX E: Drawings !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! %%)!   v LIST OF TABLES   Table 2.1 Comparison of properties of water at normal and supercritical conditions [86] ....33 Table 2.2 Materials and solution contribution to corrosion in aqueous media [116] . ............35 Table 5.1 Alloy 625 chemical compositions in weight percent...............................................61 Table 5.2 SS 316 alloy chemical compositions in weight percent. .........................................61 Table 6.1 OCP versus QRE and Ag/AgCl at 298K for a  alloy 625 electrode in 0.1M Na2SO4 solution at various pressures after 3 hours...............................................................................71 Table A.1 Parts list.................................................................................................................103 Table B.1 Material's properties and values ............................................................................104 Table B.2 Pressure induced stress distribution of the vessel .................................................104 Table B.3 Thermal stress distribution of the vessel...............................................................105 Table B.4 Superposition of the pressure and thermal stress of the vessel .............................106 Table B.5 Overall stress distribution - major and minor axis (thermal effects excluded) .....107 Table B.6 Overall stress distribution - major and minor axis (thermal effects included)......108 Table B.7 Calculated values for the heat exchanger..............................................................109   vi LIST OF FIGURES   Figure 1.1   Electrochemical and chemical processes in corrosion. ............................................... 2 Figure 2.1   Flow-through cell modified from Macdonald et al. [101]. ......................................... 6 Figure 2.2   Flow-through cell modified from Macdonald et al.  [11]. .......................................... 6 Figure 2.3   Cell assembly modified from Macdonald et al. [12].   ............................................... 7 Figure 2.4   Working electrode and holder assembly modified from Brown et al. [17]. ............... 9 Figure 2.5    Reference electrode assembly modified from Macdonald et al. [12, 28-29]. ......... 11 Figure 2.6   Variation of dielectric constant and ion product of water with pressure at 25˚C. Constructed with data from Franck et al.  [40-42].. ..................................................................... 14 Figure 2.7  Variation of density and viscosity of water with pressure at 25˚C. Constructed with data from Haar et al. [39] and Franck [40]. .. . ............................................................................. 14 Figure 2.8   Electrostriction effect. ............................................................................................... 15 Figure 2.9   Schematic of reaction profile..................................................................................... 17 Figure 2.10  Effect of pressure on the distance between ions, desolvation and ion pairing - this figure is a representation only and not to scale............................................................................. 17 Figure 2.11  Effect of pressure on the lnKP/K1,atm of water  up to 1000 bar ................................ 22 Figure 2.12  Effect of pressure on the change in potential up to 1000 bar .................................. 24 Figure 2.13 Temperature effects on the dielectric constant of water at constant pressures. Constructed with data from Uematsu et al. [40] and Meyer et al. [42] ....................................... 25 Figure 2.14  Pressure effects on the dielectric constant of water and steam at constant temperatures. Uematsu et al. [40] and Meyer et al. [42]............................................................... 25 Figure 2.15  Temperature effects on density of water at constant pressures. Constructed with data from Haar et al. [39]...................................................................................................................... 26 Figure 2.16  Pressure effects on density of water at constant temperatures. Constructed with data from Haar et al. [39] ..................................................................................................................... 26 Figure 2.17  Temperature effects on viscosity of water at constant Pressures.  Constructed with data from Wagner et al. [86] ......................................................................................................... 27 Figure 2.18  Pressure effects on viscosity of water at constant temperature.  Constructed with data from Wagner et al. [86] ......................................................................................................... 27 Figure 2.19  Temperature effects on ionic product of water at constant pressures. Constructed with data from Marshall et al. [41] and Meyer et al. [42]............................................................. 29  vii Figure 2.20  Pressure effects on ion production of water at constant temperatures. Constructed with data from Marshall  et al. [41] and Meyer  et al. [42]........................................................... 29 Figure 2.21  Effect of temperature and pressure on thermal conductivity of water. Constructed with data from Wagner et al. [86]................................................................................................. 33 Figure 4.1   Flow-loop of the electrochemical cell assembly ....................................................... 38 Figure 4.2   The cell and it components........................................................................................ 39 Figure 4.3   Pressure induced stress distribution of the vessel - no port holes. ............................ 42 Figure 4.4   Thermal stress distribution of the vessel - no port holes. .......................................... 44 Figure 4.5   Superposition of pressure and thermal stresses of the vessel. ................................... 45 Figure 4.6   Overall stress distribution along the major axis of one of the port holes. Thermal gradient was reduced to zero by heating the vessel. ..................................................................... 47 Figure 4.7   Overall stress distribution along the minor axis of one of the port holes. Thermal gradient was reduced to zero by heating the vessel. ..................................................................... 47 Figure 4.8   Overall stress distribution along the major axis of one of the port hole. Thermal gradient effect included................................................................................................................. 48 Figure 4.9   Overall stress distribution along the minor axis of one of port holes. Thermal gradient effect included................................................................................................................. 49 Figure 4.10  Working/Counter electrode assemblies.................................................................... 50 Figure 4.11  Flow-through external pressure-balanced reference electrode................................. 51 Figure 4.12  ILJP – 1 M of Na2SO4 test solution and 0.01 KCl reference solutions .................... 54 Figure 4.13  ILJP – 0.1 M of Na2SO4 test solution and 0.01 KCl reference solutions ................. 54 Figure 4.14  ILJP – 0.01 M of Na2SO4 test solution and 0.01 KCl reference solutions ............... 55 Figure 4.15  Schematic representation of the factors contributing to the TLJP. .......................... 56 Figure 4.16  Schematic representations of benefits of flow-through design ................................ 57 Figure 4.17  Counter current double tube heat exchanger. ........................................................... 58 Figure 5.1   Working/Counter electrode assembly ....................................................................... 62 Figure 5.2   Working/Counter electrode assembly parts............................................................... 62 Figure 5.3   The cell. ..................................................................................................................... 63 Figure 5.4   System laboratory set-ups for tests under pressure ................................................... 65 Figure 5.5   System laboratory set-ups for tests under temperature and pressure – note the pre- heater cell between the pump and the electrochemical cell .......................................................... 66 Figure 6.1   Effect of pressure as a function of time on OCP of alloy 625 in naturally aerated 0.1M Na2SO4 at 25˚C.................................................................................................................... 67  viii Figure 6.2   Effect of pressure as a function of time on OCP of alloy 625 in de-aerated 0.1M Na2SO4 at 25˚C ............................................................................................................................. 68 Figure 6.3   Potential of the 316 SS QRE vs. saturated Ag/AgCl RE at atmospheric pressure and 25˚C. Naturally aerated and deoxygenated environment compared. ............................................ 69 Figure 6.4   QRE vs. SHE OCP potential of alloy 625. Naturally aerated and deoxygenated environment compared (ESHE (mV)=EAg/AgCl (mV) +205mV)...................................................... 69 Figure 6.5   SS 316 QRE  vs. Ag/AgCl potential with standard deviation in oxygenated environment at 25˚C. .................................................................................................................... 70 Figure 6.6   SS 316 QRE vs. Ag/AgCl potential with standard deviation in deoxygenated environment at 25˚C. .................................................................................................................... 70 Figure 6.7    Polarization plots of nickel alloy in 0.1M Na2SO4 solution at 25˚C and different pressures in PSI (Naturally aerated environment) ........................................................................ 72 Figure 6.8    Effect of pressure on the OCP of alloy 625 in 0.1 M Na2SO4 solution at 50˚C ..... 73 Figure 6.9    Effect of pressure on the OCP of alloy 625 in 0.1 M Na2SO4 solution at 100˚C ... 73 Figure 6.10  Effect of pressure on the polarization test of alloy 625 in 0.1 M Na2SO4 solution at 50˚C versus the SS316 QRE at a scan rate of 5mV/s. .................................................................. 74 Figure 6.11  Effect of pressure on the polarization test of alloy 625 in 0.1 M Na2SO4 solution at 100˚C versus the SS316 QRE at a scan rate of 1mV/s ................................................................. 74 Figure 6.12  Diagrammatical representation of mixed potential theory for a simple electrochemical system. ................................................................................................................ 76 Figure 6.13  E-pH diagram for Ni-H2O system at 25˚C. , [Ni] = 1 x 10-6. .................................. 77 Figure 6.14 Plot of expected and observed change in potential with pressure . ........................... 77 Figure 6.15  OCP of alloy 625 and SS 316 versus Ag/AgCl RE.................................................. 80 Figure 6.16  Illustration of the effect of  polarizable QRE on the WE  mixed potential .............. 80 Figure 6.17  Mixed Potential diagram for alloy 625 WE with SS 316 QRE in 0.1 Na2SO4 at pressure at pressure range of 1- 207 bar ...................................................................................... 82 Figure 6.18  Mixed potential diagram for alloy 625 WE with SS 316 in 0.1 Na2SO4 at pressure range of 207-345 bar .................................................................................................................... 83 Figure 6.19  E-pH diagram for Cr-H2O system at 25˚C, Cr] = 1 x 10 -6. ...................................... 84 Figure 6.20  E-pH diagram for Ni-H2O and Cr- H2O System at 25˚C superimposed, [Ni] = 1 x 10-6 [Cr] = 1 x 10-6 ........................................................................................................................ 87 Figure 6.21  E-pH diagram for Cr- H2O system at 50˚C, [Cr] = 1 x 10 -6] ................................... 86 Figure 6.22  E-pH diagram for Cr- H2O stem at 100˚C, [Cr] = 1 x 10 -6....................................... 86  ix Figure 6.23  E-pH diagram for Cr- H2O system at 25˚C and 100˚C compared, Cr] = 1 x 10 -6. ... 87 Figure 6.24  E-pH diagram for Cr-H2Oand Ni-H2O System at 50˚C superimposed, [Cr] = 1 x 10 - 6 [Ni] = 1 x 10-6 ............................................................................................................................. 87 Figure 6.25  E-pH diagram for Cr-H2O Ni-H2O System at 100˚C superimposed, [Cr] = 1 x 10 -6 [Ni] = 1 x 10-6. .............................................................................................................................. 88 Figure C. 1  OCP of alloy 625 in 0.1M Na2SO4 solution at standard condition ........................ 109 Figure C. 2  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 500 PSI  ...................................... 110 Figure C. 3  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 1000 PSI  .................................... 110 Figure C. 4  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 2000 PSI  .................................... 111 Figure C. 5  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 3000 PSI  .................................... 111 Figure C. 6  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 4000 PSI  .................................... 112 Figure C. 7  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 5000 PSI  .................................... 112 Figure D. 1  OCP of alloy 625 in 0.1M Na2SO4 solution at standard condition (deaerated )..... 113 Figure D. 2  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 500 PSI (deaerated)  ................... 114 Figure D. 3  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 1000 PSI (deaerated)................... 114 Figure D. 4  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 2000 PSI (deaerated) .................. 115 Figure D. 5  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 3000 PSI  (deaerated).................. 115 Figure D. 6  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 4000 PSI  (deaerated).................. 116 Figure D. 7  OCP of alloy 625 in 0.1M Na2SO4 at 25˚C and 5000 PSI  (deaerated).................. 116        x LIST OF SYMBOLS AND ABBREVIATIONS   AECL Atomic Energy of Canada Limited CANDU CANada Deuterium Uranium CE Counter Electrode ID Inner Diameter G Free Energy Di Diffusion Coefficient Ea Activation Energy Eo Standard Potential EPBRE External Pressure-Balance Reference Electrode F Faraday’s Constant Fi Force FTEPBRE Flow-Through External Pressure-Balance Reference Electrode h Convective Hear Transfer Coefficient HT/HP High Temperature/High Pressure ILJP Isothermal Liquid Junction Potential ia Anodic Current Density io Exchange Current Density LMTD Log Mean Temperature Difference mc Mass Flow NRCan Natural Resources Canada NSERC Natural Sciences and Engineering Research Council of Canada OD Outer Diameter P Pressure OCP Open Circuit Potential PSI Pounds Per Square Inch PTFE PolyTeraFluoroEthylene PRV Pressure Relief Valve QRE Quasi Reference Electrode r1 Inner Tube Radius r2 Outer Tube Radius  xi r3 Outer Insulator Radius R Gas Constant q Heat Flow SCW Super-Critical Water SCWO Super-Critical Water Oxidation SS Stainless Steel SHE Standard Hydrogen Electrode TA Hot Fluid Inlet Temperature TB Hot Fluid Outlet Temperature tA Cold Fluid Inlet Temperature tB Cold Fluid Outlet Temperature T1 Inner Wall Temperature T2 Outer wall Temperature T3 Ambient Temperature U Heat Transfer Coefficient UNS Unified Numbering System V Volume Vp Volume of Products Vr Volume of Reactants Vi Velocity v Poisson’s Ratio WE Working Electrode Zi Charge on Species !E Change in Potential !G Change in Free Energy !T Change in Temperature !V Change in Volume !V‡  Activation Volume !a Anodic Tafel Constant !c Cathodic Tafel Constant ˚C Degree Celsius "  Viscosity  xii "a Anodic Over-Potential µi Ion Mobility "# (P,T)  Circum. Stress, P= Pressure, T=Thermal       xiii ACKNOWLEDGEMENTS   I acknowledge my supervisor Dr. Edouard Asselin for all his support and guidance from day one of this project.  I am immensely grateful for this opportunity of working with him. I learned a lot. I am also grateful to Dr. Akram Alfantazi,, for offering both moral and academic advice.  I am also grateful for the financial support of Atomic Energy of Canada Limited (AECL), Natural Resources Canada, the Natural Sciences and Engineering Research Council of Canada, (NSERC) and Natural Resources Canada (NRCan)  My gratitude also goes to Mr Ross McLeod, Carl Ng and David Torok, members of UBC Materials Engineering machine shop for the fabrication of this cell. Also not forgetting to mention the help from our Stores Keeper, Mr. Glenn Smith, who made sure that all I needed was ordered as soon as I place the order.  Final my regards also go to my colleagues in the office who were also instrumental in their own ways.  xiv DEDICATION    This thesis is dedicated first to God Almighty for His love, favour and wisdom that has been my corner stone and second, to my late father Mr. Stephen Igboecheonwu Uba, my mother Josephine Uba and my 5 siblings, their constant and unfading encouragement helped me in holding on to my goals and dreams. I am at this position in my life because there was always a back-up strength from them when I needed it.                          1 1. INTRODUCTION    High temperature and pressure technologies are increasingly applied in many processes despite the fact that the corrosion rate of materials associated with these technologies is often high. Corrosion refers to a process involving the deterioration or degradation of metals or alloys. The use of metals and alloys for structural and other industrial applications especially in continuous exposure to water (aqueous media) requires knowledge of the material’s tendency to corrode, which differs from one metal/alloy to another. Corrosion in all its forms is an electrochemical process. Its rate is determined by the stability of ionic or soluble gaseous species, which may be reactants, products or both.  The susceptibility of metals to corrosion is the natural consequence of their temporary existence in metallic form. Contact with water (or moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals give rise to completion of an electrochemical circuit which promotes electrochemical or chemical reactions resulting in species with lower free energy levels. Exposure to gaseous substances such as acid vapors, formaldehyde gas, sulfur containing gases and ammonia gas can also result in corrosion.  When metal atoms are exposed to an environment containing water molecules, they can be oxidized by giving up electrons, becoming positively charged ions. A typical electrochemical reaction of this type is depicted in equations 1.1 – 1.4 [1].  Anodic oxidation:              (1.1)  Cathodic reduction:             (1.2) or      (Acidic solutions)       (1.3) or      (Neutral/Alkaline solutions)    (1.4)  Hence, the electrochemical corrosion circuit consists of two separate sites on a given metal/alloy (cathode and anode between which electronic transport occurs) and a conductive aqueous medium in contact with both sites for ionic transport. The electrons generated by the oxidation of  2 the metal must be consumed by the reduction of another chemical species in accordance with the charge conservation theory (electro-neutrality). The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances.  Because the corrosion process is electrochemical in nature, electrochemical techniques are required for adequate study.  Moreover, corrosion is a subtle process; often taking place in environments where visual observations are hampered. A delineation of the electrochemical and chemical processes in corrosion in terms of electrochemical and pH values [2] is depicted in Figure 1.1 below.  Figure 1.1   Electrochemical and chemical processes in corrosion.  In some High Temperature and Pressure (HT/HP) processes, water is heated under pressure above its critical point (374.15˚C and 221 bar) to leverage the benefits associated with water’s distinct phase behavior (chemically and physically) at supercritical conditions. However, these benefits are associated with a cost: corrosion. Unfortunately, no metal or alloy is resistant to corrosion in all aqueous conditions between 25°C and 600°C at supercritical pressure [3-6]. HT/HP environments of commercial interest have been seen in the nuclear, hydrometallurgical, mining, petroleum, chemicals, aerospace and transportation industries. Due to the importance of the reliability and serviceability of infrastructure in these industries, there is a requirement for  3 fundamental research in the area of HT/HP electrochemistry and corrosion.  In the laboratory it is possible to create simulated corrosion environments utilizing modern and high-tech monitoring techniques and equipment.  These techniques generate an accelerated and high-resolution picture of the corrosion process.  However, measurements of this nature have been a challenge due to the difficulties associated with designing a suitable electrochemical system for HT/HP conditions.  Canada, through its associated agencies (AECL, NRCan and NSERC), is currently involved in the development of supercritical water-cooled nuclear reactors. This work was undertaken to develop a suitable electrochemical means of measuring corrosion at HT/HP within this context. A review of the relevant literature and the work objectives are presented in Chapters 2 and 3, respectively.  Aspects of the HT/HP electrochemical cell design and preliminary experimentation are presented in Chapters 4 and 5, respectively.    4 2.  LITERATURE REVIEW    This review will begin with a look at published designs of electrochemical apparatae for HT/HP use as described in the literature.  This will be followed by a review of theory and changes in the properties of water at HT/HP.  2. 1       Electrochemical Apparatus The main design components of the system are (a) the cell body which is effectively a pressure vessel (b) the working /counter electrode assemblies and (c) the reference electrode. The cell is the core of the electrochemical apparatus where the reactions take place and are contained. As such, this vessel should have an acceptable reliability for the conditions of interest. These conditions may involve operations at very high temperatures and pressures in a corrosive media. Clearly it is important that the test vessel does not corrode. Hence, materials selection and dimensioning are critical to mitigate against corrosion and failure.   The working electrode is the electrode that is being monitored for corrosion while the counter electrode provides charge to the electrochemical system. This latter electrode is typically made of platinum or graphite to limit its tendency for corrosion and subsequent contamination of the cell environment.  The working/counter electrode assemblies at HT/HP must be designed to seal against pressure but must also transmit electrical charge from the outside of the vessel to the inside.  This poses a significant challenge, as non-conductive materials must be used to isolate the electrode from the vessel body. Typically polymers are used to this end but these are not usually thermally stable at supercritical conditions.  Some designs have relied on ceramic pastes that effectively glue sections of the electrodes together.  However, these designs are appropriate for experimental use but not for long-term industrial use.  Ease of use and assembly are two very important parameters that factor into the electrode design.  The role of the reference electrode (RE) is to provide a fixed potential with respect to which the potential of another electrode can be established. Ideally, the reference electrode must be designed for the purpose of providing a reference potential only.   Unfortunately, due to thermal gradients and junction potentials (as described below) this is not an easy task and reference  5 electrodes for HT/HP use often transmit additional potentials to the electrochemical system. These parasitic contributions must be accounted for. The previous designs of these three major components are reviewed in turn below, starting with the cell body.  2. 1. 1   The Cell Body Nagy et al. [7], developed a flow-through cell, which was fabricated out of titanium and consisted of two parts: a measuring chamber and a pre-heating chamber. The measuring chamber had 4 electrode ports, two of which were positioned across from each other to fulfill the proximity requirement between the working electrode and the Luggin capillary of the reference electrode. A thermocouple and a connector to the effluent were welded onto the measuring chamber.  The measuring chamber was insulated and heated by two heating bands, which were isolated from the cell by sheets of mica. No data on the maximum allowable working pressure and/or temperature were given [7].  Botella et al. [8-9], described two HT/HP cells for corrosion studies. The two cells described were of same design but different dimensions, constructed out of different materials - a nickel based alloy (Inconel 718 UNS: N07718) and titanium alloy (TA6V UNS: R56401). The cell consists of a three-part assembly: (a) a cylindrical tube (cell) (b) a conical cap for the cell body and (c) a clamp for the first two parts. It features 6 openings, two of which are for the inlet and outlet fluid and the remaining four were for the electrodes. Mechanical analysis under these conditions gave a safety factor of 6.5 for the Inconel cell.  The safety factor was not as high in the case of the titanium alloy cell because of different fabrication dimensions.  Macdonald et al. [10-12], developed three different models of HT/HP cells for different potentiometric measurements. Modified diagrams (dimensions not shown) of the designs are presented in Figures 2.1-3 below. One of the designs features a 6-port cell constructed from type 316 Stainless Steel (SS 316) for measurement of pH in aqueous solutions at temperatures between 25ºC and 275ºC (Figure 2. 1).   6  Figure 2.1   Flow-through  cell modified from Macdonald et al.  [10].   In another design (Figure 2.2), the cell featured three ports for redox potential measurement operating at the same temperatures as above and constructed from the same material.   Figure 2.2   Flow-through cell modified from Macdonald et al.   [11].  The third cell shown in Figure 2.3, featured four ports and was constructed from a titanium alloy. The cell is reported to be suitable for temperature and pressure conditions up to at least 450ºC and 400 bar respectively [12].    7  Figure 2.3   Cell assembly modified from Macdonald et al.  [12].   Other reported designs in literature include a cell which integrates optical technology for spectroscopic measurements at 500°C and 1000 bar by Marley et al.  [13], and also a batch (non flow-through) cell constructed out of PTFE, but limited to a working temperature and pressure of 200°C and 3000 bar respectively by Dobson et al. [14].  2. 1. 2   The Working/Counter Electrode (WE/CE) The (WE/CE) holders are for the purpose of mounting the electrode/sample in a steady position. However, for HT/HP closed-system application the electrode holder design should be able to maintain electrical isolation of the lead-electrode connection from any other component of the system – this is to avoid parasitic contributions or error in the measurement. Secondly, the electrode assembly should be suitably sealed to avoid leakage.  Several designs have been published to this end.  Nagy et al. [7], developed a WE/CE combination in a coaxial configuration – sometimes referred to as a Stern-Makrides type electrode [15]. The main body of the electrode was an alumina tube.  8 The button WE was sealed against the tube with a PTFE gasket.  In the coaxial arrangement, the inner member (within the alumina tube) is the lead to the working electrode, the alumina tube serving as the insulator. In their design, the inner lead was spring loaded to guarantee seating of the working electrode sample on the PTFE gasket used, to seal the working electrode sample/alumina tube interface. Connection of the electrode to the cell was through a modified Conax high-pressure gland fitting with adjustable length for close positioning of the working electrode surface to the Luggin tip of the reference. This connective arrangement requires a mechanical back up to avoid gradual slipping of the electrode assembly through the fitting, which relied on a deformed PTFE gland for sealing.  The Agrawal et al. [16] design featured an electrode/electrode holder constructed out of a PTFE- coated steel rod of 6.4 mm diameter.  The rod was threaded at the high temperature end for fastening of the working electrode or counter electrode as necessary.  Mechanical damage to the PTFE coated steel was reduced by slightly chamfering the edge of the rod.  An EGT-type Conax pressure fitting of 3.18 mm and a PTFE seal positioned the cold-side end of the steel rod. The system has been successfully operated between 25ºC and 250ºC and 1 to 100 bar.  The limitation of the temperature to 250ºC was due to the PTFE seals and a gland fitting.  The working electrode holder described more recently by Botella et al. [8-9] featured a PTFE sheathed wire, inserted into a tube. Two counter electrodes of 0.5 mm OD and 1.5 mm OD constructed from platinum were used in the Inconel 718 (UNS: N07718) and TA64 cells, respectively described above.  Brown et al. [17] also developed an electrode/electrode holder of a cylindrical design, which is schematized in Figure 2.4 below. The electrode sample was constructed from platinum of 30 x 50 mm diameter, with a working temperature range of 20-200°C. The working electrode was mounted on an alumina tube and pressure-sealed by lapping with a diamond abrasive paste. An electrical contact to the working electrode was by a stud running through the alumina tube. The tube was inserted in a titanium tube sealed by a circumferential O-ring at the cold end.      9  Figure 2.4   Working electrode and holder assembly modified from Brown et al. [17].   2. 1. 3   The Reference Electrode  A review of some of the reference electrode designs - presented below - for HT/HP applications reflects similarities in methods of operation and design [17-21]. A typical design described by Macdonald et al. [22] and patented in 1979 was an External Pressure-Balanced Reference Electrode (EPBRE). Other examples of EPBREs can be found in literature [10-11, 23]. The thermodynamic representation of the thermocell operating between 25 ºC to 275ºC using 0.005 to 0.505M KCl is:             (2.1)  Thermal gradient, (a major issue if not corrected) causes a change in potential as the electrolyte diffuses toward the end of the junction [7, 9]. This is known as thermal junction potential.  The system proposed by Macdonald et al. featured a mechanism whereby thermal diffusion within the electrode could be maintained at the Soret initial state (i.e along the nonisothermal bridge, a  10 uniform concentration was maintained) [24-25] in spite of variation in the operating conditions (the Soret effect is explained in section 4.5.2 below).  This was apparently achieved by using a flexible (PTFE) material in the inner compartment of the electrode where pressure pulsations from the flow-activating pump were transmitted, preventing the occurrence of thermal diffusion. It was unclear from the explanations in the literature how this resulted in the limitation of thermal diffusion such that the Soret initial state was maintained   In 1983, Danielson [26] designed a flow-through electrode following his work with the U. S. Department of Energy in the early 80s [27].  This reference electrode aimed to solve the problem of thermal diffusion by continual refreshment of the electrodes’ inner compartment solution. The constant refreshing of the inner solution and the adjustable flow rate results in a uniform concentration along the isothermal liquid bridge and maintain the temperature gradient along the non-isothermal liquid bridge in the system.   That is, the inner compartment electrolyte was maintained in the initial or Soret state. Danielson reported that potential stability, which was the target of the design, was met but the working temperature of the system was less than 300ºC, which limited its use to subcritical conditions due to thermal hydrolysis of the active elements at higher temperature.  Danielson’s design was a success but required meticulous calibration against the hydrogen electrode and was also seen to be flow rate dependent.  This same principle was adopted by Lvov et al. [28-29] design, (modified diagram is shown in Figure 2.5) for high subcritical and low supercritical temperatures (374.15oC < T < 400oC), with the aim that highly accurate potentiometric measurements could be made. This high accuracy resulted from an improved stability of the active element of the reference electrode because the reference electrode operated externally from the pressure vessel i.e. at room temperature. Hence, thermal hydrolysis of the active element, which would otherwise cause a parasitic contribution to the measured result. was eliminated [12, 28-29].  11  Figure 2.5    Reference electrode assembly modified from Macdonald et al. [12, 28-29].  A long-life external reference electrode design was also reported in the work of Nagy et al. [7]. Their design was based on a tubular silver electrode and gravity fed filling solution. However, the electrode life was dependent on two factors: the flow rate of the solution and volume of the reservoir.  The developers reported a one-month operation of the system through large temperature cycles with no instability in the measured potential. In another design, they described a low impedance electrode using the same solution as both an internal reference solution and test solution in the case of a Palladium-Membrane electrode; the drawing of the design has been published elsewhere [30].  In 1995, a patent was granted to Jayaweera et al. [31] on their development of a solid state RE for high temperature and pressure application. The tube of this RE was of zirconia or yttria stabilized zirconia. Other electrode systems have also been reported and details can be found in the literature [16, 32].  Out of all the tubular flow-through cell designs reviewed, the design of Macdonald et al. in Figure 2.1 was most closely in line with our requirement, because we could effectively position  12 the electrodes at close proximity to reduce the Ohmic potential drop. The working electrode holder of Brown et al (Figure 2.4) based on the Makrides-Stern principle was adopted both for the counter and the working electrode design, so that we could effectively mount the electrode in the cell body. The operating temperature was a major issue for all WE designs because the PTFE seal is inadequate at temperatures above 250˚C. We will consider the use of a ceramic seal as we go to high temperatures. The choice of the reference electrode was the flow-through design of Macdonald et al. (Figure 2.5) because of the constant refreshing of the solution, which would solve the thermal liquid junction problem, but also reduce the accumulation of corrosion products over time.  Quasi Reference Electrode Another form of RE electrode that is also used in some cases is known as a Quasi Reference Elcetrode or QRE. “Quasi” relates to the fact that they are unable to maintain an invariant potential i.e. they typically exhibit their own mixed potential.  This is due to the fact that QREs are typically metal electrodes such as Pt or Mo.  Clearly then, the reference potential provided by a QRE is highly dependent on solution chemistry. QRE’s are known to exhibit poorly defined fixed potentials and their potential drifts with time during a series of experiments because of the formation of surface oxides [32a-b]. While their unknown mixed potential can be estimated if the conditions and other parameters of the experiment are accurately known, the potential drift is difficult, if not impossible, to factor out of the measured results. This is a big disadvantage of all QREs over standard REs. However, QREs also offer some advantages: they are easy to design and maintain, there are no liquid junction potential contributions or contamination of the reference solution because they are directly immersed into the test solution, and they operate at high temperatures.  A stainless steel QRE was used for the purposes of testing our electrochemical cell as described in Chapter 5.  It was of interest to employ a QRE to determine its response to changing conditions in solution ORP.  Information of this nature could prove to be useful for monitoring solution chemistry in HT/HP environments with the significant added advantage that QREs are cheap and easy to maintain.   13 2. 2   Solution Chemistry The behavior of solute species is a function of state parameters (temperature and pressure), molecular level interaction (solvation, ion association and cluster formation and destruction) and solution properties (pH, activity coefficient, concentration, number of solute species present) [33]. The two process variables upon which every other fluid property depends are temperature and pressure. It will be convenient to delineate their individual effects before their combined effects.  Obviously, the effect of pressure on water can be studied individually at standard temperature: this has been done in our work presented in Chapter 6.  At high temperature, pressures above atmospheric pressure are required and thus the effects of these two variables are inseparable (pressure increases the boiling point of water).  2. 3   Pressure and Aqueous Solutions The effects of elevated pressure on the thermodynamics and kinetics of inorganic reactions in aqueous solutions with respect to reaction volume, equilibrium and rate constant, and reaction mechanism, has been studied by Horne [34-35] among others [36-38]. However, these effects on the corrosion of metals and alloys are relatively unexplored; largely due to difficulties in obtaining reproducible electrochemical data from pressurized systems, and the unavailability of robust reference electrode assemblies for these conditions. At high pressure, up to 1000 bar, the conductivity of inorganic aqueous solutions increases by approximately 5%. This is because the dielectric constant increases by 5% (from 78 to 82) and the density increases by 3.5% [39-41], see Figures 2.6 and 2.7. Hence, aqueous corrosion processes at high pressure are likely to reflect these changes even though they are relatively small in magnitude.  Some relevant basic theories pertaining to the effect of pressure on the chemistry of aqueous solutions and the associated chemical reactions at ambient temperature (25˚C) and at high pressures are reviewed below followed by the effect of temperature.  2. 3. 1   The Theoretical Effect of Pressure on Reaction Rate Solvent properties such as density, dielectric constant, ionic product and viscosity, vary with pressure. To illustrate, the response of the dielectric constant and ionic product of water with  14 pressure are plotted in Figure 2. 6. Pressure effects on viscosity and density of aqueous solution (water) are also plotted in Figure 2.7 [39-42] below.  Figure 2.6   Variation of dielectric constant and ion product of water with pressure at 25˚C. Constructed with data from Franck et al.  [40-42].   Figure 2.7   Variation of density and viscosity of water with pressure at 25˚C. Constructed with data from Haar et al. [39] and Franck [40].   15 Density, dielectric constant, and ionic product of water increase with pressure between 0-1000 bar, (0-14500 PSI) while viscosity decreases with pressure between 0 – 450 bar (0-6527 PSI), and subsequently increases with pressure [34-36]. An extensive review with descriptive information on the basic principles involved in the high-pressure kinetics of reaction in general, can be found in a review published by Horne [34-36].  When solute particles are introduced into water, a phenomenon known as electrostriction results whereby water experiences contraction, with a resultant decrease in volume. Electrostriction, Figure 2.8, is the macroscopic compressive effect that results due to strong interaction of charged species with a polar medium [22, 34, 43-44]. These charged species excite a Coulombic field and consequently attract the opposite charge of water dipole resulting in a higher density of water in the vicinity of the charged species. The hydrogen bond network in the solvent water then rearranges by breaking and reforming of H-bonds [44-47] in accordance with Le Chatelier’s principle. This rearrangement is fundamental to the solvation process. Similar but higher contraction (decrease in volume) is also observed when pressure is applied to water, because both the local and bulk water structure is affected by pressure [34-35].  Figure 2.8   Electrostriction effect.  The time average for the rearrangement of water structure has been estimated through molecular dynamic studies to be in the order of pico-seconds or less at high pressures [46, 48-56].  The  16 volume change exhibited by weak electrolytes such as sodium sulphate is higher than in strong or highly concentrated electrolytes [34-35, 37], because the ions are less closely packed in the former than in the latter.   In Horne’s review [34-35], it was clear that the negative reaction volume change (difference between partial molar volume of the product and the reactant) exhibited by most electrolytes under pressure results in an increase in conductivity and the rate of chemical reaction. This is because, dissociation of solute species, degree of solvation, oxidizing nature of the solution and collision frequency in aqueous media increase with pressure.  The increase is in part, due to perturbation of the molecular energy level, which lowers the activation energy for the dissociation process given that pressure affects also the bulk water structure [34-35]. Perhaps it is note worthy to mention that the degree of this perturbation depends on the inter-molecular and intra-molecular forces associated with the liquid (solvent). In water, these inter molecular forces mostly consist of hydrogen bonding which is a relatively weak bond.  Several mechanisms for the solvation process or the dissociation of water molecules have been proposed over the years [34-35, 37-38, 46] one of which is the flickering cluster [34-35]. In this theory, the breaking of the H-bond (which can be initiated by the introduction of a solute particle or the application of pressure) results in the dissolution of an entire water cluster, giving room for re-orientation of water molecules before forming a new H-bond around the solute species if present. This process of solvent rearrangement (solvent dynamics) also governs the electron transfer process between aquated ionic/solute species in solution and has been extensively studied 46, 52, 55, 57-69]. Evidently, the solution will have higher concentration of hydrogen ions as pressure increases.  According to transition state theory (also known as activated complex theory) which is applicable to liquids and gases [37, 43 71-72], some reactions are slightly accelerated by pressure owing to the fact that in the transition state, the reacting species are brought closer to one another, cf. Figures 2.9 and 2.10. The theory assumes that migration of ions in liquid is by a series of jumps upon acquiring a certain free energy of activation from one equilibrium position to another. The time-average of the jumps is a function of potential gradient, which lowers the energy of activation in the direction of the field (if an electric filed is present) while that in the opposite direction is increased [43].  17   Figure 2.9   Schematic of reaction profile.   Figure 2.10   Effect of pressure on the distance between ions, desolvation and ion pairing – this figure is a representation only and not to scale.  Thus, the mechanistic information that can be obtained from the effect of pressure on aqueous solutions is based on the fundamental thermodynamic relationship:   18           (2.2)   Where $G, $P and V are the free energy, pressure and volume respectively. The associated effect on corrosion of metals also depends on the properties of the environment and the direction of volume change associated with the activation process, Macdonald et al. [73].  The activated complex theory is also in agreement with Arrhenius theory, which is represented by equation 2.3 [72]:                  (2.3)  The pre-exponential factor A (which is the vicinity of 1013-1019 and 102-109 for opposite charged and liked charged ions, respectively [73) in equation 2.3 is a function of reaction collision frequency (a measure of the number of collisions leading to a chemical reaction). The Boltzmann factor  gives the fraction of colliding pairs, which will possess sufficient energy to lead to a reaction. Given that the concentration is volume dependent, A will increase under pressure, and the activation energy will vary inversely with pressure. Clearly, the spatial factor contribution on the reaction rate due to pressure is also accounted for by the Arrhenius equation because A varies with the collision frequency and there is a close relationship between collision frequency and pressure.  The distance between ions in solution decreases slightly as pressure increases cf. Figure 2.10.  Also the reaction rate equation accounting for the contributions made by the solubility or concentration of oxygen see equation 2.4 [73], suggests an increase in rate because oxygen solubility increases with pressure [74]. Thus, an increase in the oxidizing nature of the solution, which plays an important role on the rate of the corrosion reactions, would be expected.         (2.4)  Where k, CO2 and CH+ are the rate constant, concentration of oxygen and protons (i.e. pH) respectively, m and n are the reaction order. The concentration of oxygen and protons depends,  19 in addition to pressure, to the temperature and the type of solvent used.  For a highly oxidizing solution in contact with a metal electrode, the mixed potential theory in accordance with the principle of conservation of electric charge implies a balance between the oxidizing half-cell reaction and the co-reactant reducing half-cell reaction. Correspondingly, high dissolution rates of the metals in contact with the highly oxidizing solution would be expected, which in turn, will increase the exchange current density due to an increase in the number of electrons generated per unit time. Undoubtedly, a reduced effect would be expected in a deoxygenated environment.  The electrochemical nature of corrosion makes some fundamental parameters such as conductivity of the solution, the free energy of reaction, the equilibrium and rate constant of the chemical and electrochemical reactions important to understanding the corrosion mechanism. Therefore, their response to pressure will also give a sense of the direction of corrosion.  The specific responses of these parameters to pressure are discussed below.  2. 3. 2   Effect of Pressure on Conductivity of Aqueous Solutions The conductivity of water (aqueous solution) varies with pressure [34-35, 75-79] due to the eccentric structural properties of water [38]. As already mentioned above, viscosity, dielectric constant, density, and ionization product etc, change with pressure. Upon increase in pressure, the number of hydrogen bonds progressively decreases with a progressive increase in the concentration of hydrogen ions and other charged species. As discussed above, pressure also enhances this effect by lowering the activation energy of dissociation due to its compressive effect, which results in higher collision frequency.  Owen et al. [44] has reported a 2 to 8 fold increase in the ionization of water and weak electrolyte over a pressure range of 1000 bar.  The Frank-Wen model of the structure of water [34-35, 80-81] can be used to further explain the conductive properties of aqueous solutions or the structural changes in water under pressure. In their model, there are at least two distinct forms of water: (1) a structured region (flickering cluster) of hydrogen-bonded water molecules, which are surrounded by, (2) the free unstructured water.   20        (2.5)                      (2.6)  The breakage of these structured clusters under pressure, equation 2.5, will exact a volume decrease because of the compressive effect. Given that liquid water only has about 10% fewer hydrogen bonds than ice [46], there are still more H-Bonds to be broken increasing the concentration of hydrogen ion in solution which translate to higher conductivity. This phenomenon is also responsible for the decrease in viscosity of water, giving rise to a more fluidized liquid and an increase in the frequency of ionic collision and mobility  2. 3. 3   Effect of Pressure on Reaction and Activation Volume The volume effect of pressure on aqueous solution as mentioned earlier is in agreement with thermodynamic law, which states that the standard volume of reaction is equal to the free energy with respect to the pressure at constant temperature, stated mathematically as [43-44, 83]:           (2.7)   Therefore,            (2. 8)      (2.9)  Where #G˚ P, R, T, Vp and Vr are change in free energy, pressure, gas constant, temperature in Kelvin, partial molar volume of the product and reactant respectively. Thermodynamically, a negative volume change from equation 2.7 will favor a spontaneous reaction because delta G will also be negative. From equation 2.9, reactions that result in a decrease in volume will be aided by an increase in pressure, while a reverse situation will be seen for reactions that result in an increase in volume . Substituting equation 2.8 into 2.7 gives the variation of thermodynamic equilibrium constant with pressure [44]:   21              (2.10)  Likewise the variation of the rate constant with pressure gives the activation volume:          (2.11)  Thus, the volume change is characterized by the equilibrium constant (K), while the volume of activation (the difference between the partial molar volume of the transition and the reactant states), is characterized by the rate constant k. That is, equation 2.10 reflects the effect of pressure on the true thermodynamic equilibrium constant while equation 2.11 reflects the effect of pressure on the rate constant. Integration of equation 2.10 gives [47]:           (2.12)   Where f(P) is a function solely of pressure can be defined as [40, 47]:              (2.13)   The constant b is equal to 9.2 x 10-5 bar -1. Thus the pressure dependency of the equilibrium constant gives:           (2.14)  The value of  ln(KP/K1.atm ) of water up to 1000 bar is plotted in Figure 2.11 below. The value of !V was -21.4 cm3/mol from [40, 47] and was assumed constant. The ratio, ln(KP/K1.atm), increases by 0.7 between 1-1000 bar.  Clearly this is only a small change which should not significantly affect electrochemical measurements.  22  Figure 2.11   Effect of pressure on the ln(KP/K1,atm) of water  up to 1000 bar.  2. 3. 4   Volume Effect of Pressure on Electrode Potential The relationship of the electrode potential with the free energy is defined by:              (2.17)  Thus, the relationship of the electrode potential with the reaction volume change becomes: [84- 85].              (2.18)  where n, F and #E are the number of electrons taking part in the reaction, the Faraday constant (96485C/mol) and the standard potential of the difference between the oxidant half-cell reaction and the reductant half-cell reaction, respectively. For reactions that exhibit negative volume change with pressure, change in potential will be a linear function of pressure.   Applying the “fifty-percent rule” of Marcus theory [58, 62], the free energy of the electrode reaction is one-half that of the homogenous reaction in solution.   23         (2.19)  The electrode equilibrium constant can thus, be represented as:         (2.20)  The cell reaction volume and the volume activation of metal complexes characterize the pressure dependencies of the electrode potential and electrode kinetics, respectively [3]. Likewise, in a manner analogous with equation (2.19) [34-35, 58, 62]:          (2.21)  Ipso facto, the change in volume and activation volume as a result of pressure will vary with the potential of the redox couple. The response of the potential with increasing pressure can be estimated from equation 2.22.   (2.22)   However, the relationship of the electrode potential with pressure can be affected by other associated processes especially desolvation and electrode adsorption at high pressure, depending on the composition of the electrolyte as well as the type of electrode. Desolvation is the removal of solvent components from a material (element or compound etc.), which is normally in solution. In a homogenous phase desolvation is likely while in a heterogeneous phase, surface reaction (adsorption/passivation) may occur too. Considering other effects to be negligible, equation 2.22 (Nernst equation) can be used to quantify the effect of pressure on the potential change. This approximate quantification is plotted in Figure 2.12 and shows that potential change with pressure increases by about 18 mV over a pressure range of 1-1000 bar. The small change in Figure 2.12 reflects the very small changes in potential that we expect to see when only pressure is increased.   24  Figure 2.12   Effect of pressure on the change in potential up to 1000 bar. 2. 4   Temperature and Pressure Effect on Aqueous Solution The existence of water in a single phase beyond the critical point makes it a very attractive medium from a process standpoint. This is related to the fact that the density of water can be tuned between liquid-like and gas-like in the supercritical regime through a judicious application of pressure. In a high temperature and pressure environment, the effects of temperature and pressure are opposite on most properties of water except ionization Figure 2.6-7. These opposite effects are more visible as temperature increases further along the sub and supercritical regimes.  The dielectric constant and density (both, affect the dissociation of inorganic species in water), decrease with temperature and increase with pressure. Due to thermal expansion, an increase in temperature decreases the density of water unlike pressure. As a result, the dielectric constant of water is also expected to vary inversely with temperature. The non-polar nature of water above the critical point is more due to the effect of temperature. Because the density of water varies with pressure (Figure 2.6-7) and hydrogen bond strength varies with density [122]. As such, at supercritical conditions, water has gas-like and liquid-like properties. A comparison of the respective effects of temperature and pressure on dielectric constant up to 500˚C and 1000 bar is depicted in Figure 2.13-14, and their effect on density up to 800˚C and 1000 bar in Figure 2.15- 16 below.  25   Figure 2.13   Temperature effects on the dielectric constant of water at constant pressures. Constructed with data from Uematsu et al. [40] and Meyer et al. [42].  Figure 2.14   Pressure effects on the dielectric constant of water at constant temperatures. Constructed with data from Uematsu et al. [40] and Meyer et al. [42].  26  Figure 2.15   Temperature effects on density of water at constant pressures. Constructed with data from Haar et al. [39].   Figure 2.16   Pressure effects on density of water at constant temperatures. Constructed with data from Haar et al. [39].   27 Viscosity decreases with temperature. However, the introduction of temperature changes the effect of pressure on viscosity from decreasing with pressure in Figure 2.7, to increasing with pressure in Figure 2.17 below.  Figure 2.17   Temperature effects on viscosity of water at constant pressures. Constructed with data from Wagner et al. [86].   28  Figure 2.18   Pressure effects on viscosity of water at constant temperatures.                 . Constructed with data from Wagner et al.  [86]. For any given temperature, an increase in pressure results in an increase in viscosity above 100 bar.  And with temperature increase, the compressive effect of pressure changes from structure breaker to structure former, which limits the drop in the viscosity of water. The effects of temperature and pressure up to 800˚C and 1000 bar, respectively, are compared in Figures 2.17- 18 above.  The ionic product (dissociation constant) on the other hand increases with both temperature and pressure up to the critical point. A sharp fall is observed above the critical temperature for operating pressure below 1000 bar. The effects of temperature up to 800˚C and pressure up to 5000 bar on ionic product are depicted in Figures 2.19-20 below.   29  Figure 2.19   Temperature effects on ionic product of water at constant pressures. Constructed with data from Marshall et al. [41] and Meyer et al. [42].   Figure 2.20   Pressure effects on ion production of water at constant temperatures. Constructed with data from Marshall et al. [41] and Meyer et al. [42].   30 These depicted variations of water properties with temperature and pressure account for the decreased solubility of inorganic species and the increased solubility of organic species above the critical point.  The increased solubility of organic substances at these conditions is the foundation of Super-Critical Water Oxidation (SCWO) technology for organic waste disposal. The process will be discussed further below.  Also the possibility of temperature and pressure tuning of these properties, increases the versatility of water in process technology and further increases the universality of water as a solvent. In addition, quite a good number of chemical reaction and process can be studied in water under the elevated temperature and pressure.  2. 4. 1   Supercritical Water SuperCritical Water (SCW) is water that is progressively compressed from liquid-like to gas-like (dense gas) densities at very high temperature and pressure exceeding that of critical temperature 374.15˚C and pressure 221 bar respectively. It has extraordinary properties differing in great extent from those at normal conditions, which can be regulated by temperature and pressure and forms the basis for groundbreaking innovations in process technology [87-88]. Its properties are similar to a dense gas [22, 87, 89-91] and thus can be identified as a dense gas. These changes are in great proportion attributed to almost complete loss of hydrogen bond of its structure, metamorphosing from polar to non-polar solvent [3, 22, 87, 91-92]. Consequently, a very high miscibility with organic substances and non-polar gases (air, oxygen, and gaseous products) that are immiscible at normal conditions coincides with a very high mass transfer rate [90]. The process variables; pressure and temperature can be regulated in such a way that appreciable solubility of both organic and inorganic compounds/species are achieved. For instance at 500˚C, the ordinary water density of 1 g/cm3 can be generated with a pressure of 1 GPa [3], that is, the solubility of inorganic species in supercritical regime is highly dependent on pressure (and thus density).  SCW is an increasingly relevant reaction medium in the aqueous processing industry especially in the destruction of organic waste and reducing the volume of low-level nuclear waste [3, 87, 90-94]. Some examples of research-intensive processes where supercritical water may be employed include oxidation (SCWO) and as a heat transfer fluid in power generation [22, 92]. Typical operating conditions for SCWO are 450-700˚C and 240-500 bar of temperature and pressure respectively [91, 95]. Other applications include, biomass conversation [96-99]  31 reforming/gasification and synthesis/chemical reactions [3, 94, 99, 101-104], salts separation processes [105], coal cracking [106], analytical applications [107], extraction [101, 108-110], and ceramic processing [111] etc. The growth in application of SCW still however, is faced with the challenges due to lack of experimental data or comprehension of the solution chemistry at this point. Some of these challenges are [112-115]:  1. Excessive corrosion by the mineralization or complete oxidation of hetero- atoms (Cl, O, S, H or P) in the waste stream, to corresponding acids (HCl, H2SO4, H3PO4) or salts in the highly oxidizing high-temperature medium. 2. Low solubility of inorganic salts above the critical point hence scale (precipitation) formation and plugging of the reactor or associated piping system. 3. Lack of experimental data above the critical point due to inadequate experimental tools and 4. Cost evaluation for a possible scale-up is hard to perform due to unpredictable reliability issues  The two important applications of SCW related to this work are SCWO and the use of supercritical water as a heat transfer medium for nuclear reactors.  They are discussed below.  Supercritical Water Oxidation SCWO involves the reaction of an organic compound with oxygen in a single-phase resulting in the formation of CO2 and water. In the SCWO waste destruction technology, the toxic feed, oxidant and waste are heated and pressurized at condition close to 550-650˚C and 230-250 bar. The detoxification to simple CO2 and H2O typically takes less than a minute [91], and the reaction products are detoxified and safe to dispose. In the supercritical region, the solubility of the organic compounds are enhanced giving rise to a single homogenous phase, a basic requirement of the process [93], for high diffusivity and good transport properties [87-88,116- 117]. This is due to the simultaneous liquid-like and gas-like characteristics of the solvent [116]. Unlike in two-phase systems, where the concentration gradient across the phase boundaries is a constraint to the level of hazardous compound destruction, diffusion does not affect the rate of destruction.   32 Hetero-atoms like sulfur, chlorine and phosphorus in the waste stream are converted into their respective mineral acids [112]. Common oxidizing agents include air, oxygen, hydrogen peroxide, and nitric acid [118]. This process is similar to combustion but does not pollute the atmosphere as would open burning (incineration) and involves a much lower operating temperature and low residence time. Also in the incineration or open detonation of hazardous waste, the conversion process is much less efficient with a relatively high cost, and in addition, the by-products are still fairly unsafe or difficult to dispose [118-119].  Some of the by-products of destruction through SCWO are CO, CO2, H2O, N2, N2O, H2, acids, salts and only trace amounts of NOx (depending on the composition of the waste stream). The N2O by-product produced in some occasions is a greenhouse gas but is a much less severe air pollutant than the NOx often generated in incineration.  Furthermore, N2O tends to break down into N2 and O2 in the atmosphere. [119].   The hazardous waste streams that can be treated by this technology are very diversified and include radioactive waste from the nuclear industries, warfare agents and rocket propellant from the military and space science/aerospace industries, waste from pulp and paper industries and from other chemical industries, contaminated soil from oil spillage or agriculture, and municipal solid waste etc. [95, 114-115, 120].  SCW as a Heat Transfer Fluid (Coolant) SCW have a very high thermal efficiency. The estimated efficiencies for SCW-cooled reactors are in the range of 40-45% compared to 32-34% for current Light Water (LW)-cooled reactors [121].  The thermal conductivity increases with pressure, thus, heat transfer deterioration can be reduced by pressure adjustment at very high temperatures, Figure 2. 21. Table 2.1, is a summary of other desirable properties of SCW that contribute to its potential usage as a coolant fluid in both conventional and nuclear power generation plants [3, 94, 121].  33  Figure 2.21   Effect of temperature and pressure on thermal conductivity of water. Constructed with data from Wagner et al. [86].  The single-phase SCW eliminates the integration of handling systems such as steam dryers, steam separators, and re-circulation pumps where multiple phases are involved, thereby minimizing both complexity in design and cost [100, 121].  Also no critical heat flux exists due to the lack of a second phase, eliminating heat transfer regime discontinuities within the reactor core. The high enthalpy of SCW reduces the reactor pumping power and increases the fuel cladding-to-coolant heat transfer coefficient. It is presently being considered as a heat transfer fluid for the Generation IV reactors currently under development in Canada. However, excessive corrosion of the reactor and its components is an issue. For example, corrosion of the nuclear fuel cladding can contaminate the coolant by introducing radioactive fission fragments into it.   34 Table 2.1 Comparison of properties of water at normal and supercritical conditions [86]. Properties Temperature = 25˚C Pressure =1 Bar Temperature = 500˚C Pressure =230 Bar Temperature = 500˚C Pressure = 250 Bar Thermal Conductivity (W m-1K-1) 607.5 x 10-3 94.74 x 10-3 99.02 x 10-3 Specific Heat (kJ kg-1 k-1) 4.19 3.56 3.76 Enthalpy (kJ kg-1 k-1) 104.93 3196.72 3165.92 Entropy (kJ kg-1) 0.37 6.03 5.96  SCW and the Corrosion Issue Corrosion damage at high temperature and pressure is to a large extent related to water chemistry. As a result, the electrochemical potential of the metal or alloys in contact with the aqueous media is key information in formulating or predicting the corrosion behavior of these materials. In a single HT/HP system these harsh aqueous media conditions range from 100-400 bar of operating pressure and between ~250°C (subcritical) to 650ºC (supercritical) of operating temperature [8]. These changes obviously have a tremendous impact on the chemical, physical and thermodynamic properties of the aqueous solutions, which are translated in their electrochemical interaction with the reactor body or components through corrosion. Mitigating this issue requires the capability of monitoring the chemical and electrochemical properties of aqueous solutions at subcritical and supercritical conditions.  Corrosion at sub/supercritical regime of aqueous solution are associated with the solubility of corrosion products, solubility of gases product, dissociation of acids, bases and salts and the stability of protective oxide layer (for metals or alloys that passivate). These factors are delineated to the solution and the material contributions [116], in Table 2.2.   35 Table 2.2 Materials and solution contribution to corrosion in aqueous media [116]. Material Contribution Solution Contribution Alloy Composition pH Material Purity Temperature Surface Condition Electrochemical Potential Heat Treatment Density Position in Activity Series etc Pressure  Composition  Viscosity  Anion  Some of the materials that have been used in SCW applications include stainless steels, nickel base alloys, titanium, tantalum, noble metals or ceramics [122]. And general corrosion, pitting corrosion, inter-granular corrosion and stress corrosion cracking are the various forms of corrosion that have been witnessed [122].  The disparity in the composition of waste streams and hence the associated differences in acids, salts and gases formed, further complicate the selection of suitable materials to mitigate corrosion.  No material is known that can be corrosion resistant to all forms of acids and other products formed under varying supercritical conditions. Illustrating the point further, titanium for instance, is highly resistant to corrosion in contact with oxidizing HCl solution at any temperature but in contact with H2SO4 or H3PO4 solution, its resistance is poor [112-113, 123- 124].  Secondly, in power plants where phosphate are added as pH buffer or in phosphate containing warfare agents  [122], stainless steel also suffers severe corrosion at long service times. And for passivating metals, chloride has been known to attack protective oxide layers in most passivating metals [4, 125].  Thermodynamically, corrosion is expected to increase with temperature due to faster kinetics. This continuum however, deviates beyond the critical point of water. Due to almost complete destruction of hydrogen bonds and lowered dielectric constant (non-polar solvent), SCW loses its ionic solvation capability. As a result, the severity of electrochemical corrosion in supercritical water is less than in subcritical water [4-6]. This means that associated components of the SCW systems like the piping or vessel for preheating or cooling down the fluid will be more  36 vulnerable to corrosion than the reactor itself.  However, depending on the chemical environment and the type of absorbed salts due to high inorganic salt precipitation, there can still be some serious corrosion. The presence of trapped water between the adsorbed salts and the reactor wall can build up a different environment having a different concentration and /or temperature than those of the bulk fluid phase especially if the salt is in molten form [3]. Corrosion of this nature has been seen in SCWO reactors [94].   Factors Affecting High Pressure and Temperature Environments  For simple HT/HP exposure tests involving either aqueous or non-aqueous phases, the total pressure is usually determined by the sum of the partial pressures of the constituents of the test environment, which will vary with temperature. Below 200˚C it is adequate to use the ideal gas law for the calculation of partial pressure.  Vapor pressures for several compounds used in HT/HP corrosion testing can be found in the technical literature. Unfortunately, above this temperature and close to the critical point of water, gas phases are no longer ideal and equations of state must be used for the calculation of partial pressure.  In some cases, higher test pressures can be obtained by pumping additional gas into the test vessel using a special gas pump. Alternately, hydrostatic pressurization may be employed.  The importance of partial pressure in HT/HP corrosion testing is that the solubility of' the gaseous constituents in the liquid phase are determined by their partial pressure. For a particular mol fraction of gas in the vapor phase, its concentration in the liquid will usually increase with increasing total pressure. This is why the effects of low-level (ppm) corrosive impurities on corrosion (such as oxygen, hydrogen, H2S, and many other species) are often found to be magnified at high pressure and exhibit corrosive effects not commonly found in conventional low-pressure corrosion tests. [126]  This review is the underlining basis of the objectives of this work, which are presented in the next chapter.   37 3      OBJECTIVES    This research is connected with the CANDU Generation IV Reactors currently being developed by Atomic Energy of Canada Limited.  Based on the above discussion and the need for the development of suitable electrochemical technologies for use at high temperature and pressure, the following objectives were set out:  ! Design a high temperature and pressure electrochemical cell, reference, counter and working electrodes with target operating conditions of 500˚C and 5000 PSI to study, electrochemically, the chemistry of water at high temperatures and pressures.  ! To test the stability of a Quasi Reference Electrode (QRE) as a possible alternative for electrochemical measurement in HT/HP.  !Based on the literature information and experimental results, develop and select a suitable material for HT/HP electrochemical sensors to study and monitor corrosion in supercritical water.                      38 4  ELECTROCHEMICAL CELL DESIGN    The electrochemical cell design of this work is shown in Figure 4.1 below. The main design components of the system are: (a) the cell body (b) working /counter electrode assemblies and (c) reference electrode, which will be discussed in turns later in this chapter.    Figure 4.1 Flow-loop of the electrochemical cell assembly.  4. 1  Design Assembly and Operational Principle  The design, presented in the Figure 4.1 above is the designed HT/HP electrochemical cell. The cell is a sealed, static, pressurized test vessel. A HP pump introduces the test solution into the cell with a second HP pump feeding the RE solution at a slight overpressure through an external flow through reference electrode. For safety, a pressure relief valve (PRV) is included to provide relief of any pressure build up. Depending on the specific interest of an experiment, corrosion coupons (or samples which are known as the working electrode) may be placed in the aqueous  39 phase, vapor space or at phase interfaces. The cell is also designed to operate above subcritical water temperatures and pressures where single-phase behaviour is observed.  The purpose of the design is to provide for a means of performing electrochemical experiments such as potentiometry, polarization tests and impedance spectroscopy. Thus ports for housing various electrodes (working, counter and reference) are provided. These components are assembled in a way to achieve electrical insulation from the cell body. The system is expected to operate with a maximum pressure and temperature of 350 bar (5000 PSI) and 500˚C respectively. At the outlet, the test solution passes through a heat exchanger capable of reducing the temperature from 500˚C to 60˚C. A back-pressure regulator at the effluent end of the system is used for the internal pressurization.  4. 2  The Cell Body The cell design featured a 76.2mm (3”) outer diameter and 25.4mm (1”) inner diameter tubular SS 316 cylindrical tube.  SS 316 material was chosen for initial design trials in dilute solutions but may have to be replaced with a Ni or Ti based alloy in the future. The cell houses 6 ports, two of which are for the fluid inlet and outlet, while the rest of the ports are for the connection of thermocouple, reference, working and counter electrodes as shown in Figure 4.2.   Figure 4.2   The cell and it components.   40 4. 2. 1   Structural Analysis  A structural and thermal analysis was performed to assess its structural integrity before fabrication. Analyses of the cell without ports and with ports were done to determine the effects on the cell’s safety factor and are presented below.  Pressure Vessel – Thick-walled Cylinder Cylindrical vessels are commonly used in industry to carry both liquids and gases under pressure. When vessels are pressurized, the material of construction is subjected to pressure loading and thus stresses in all directions. The normal stresses resulting from this pressure are functions of the radius of the element under consideration, the shape of the pressure vessel as well as the applied pressure.  Structural Analysis – Stress The material’s property was one of the variables for the analysis. The dimensions were 38.1mm (1.5”) and 12.7mm (0.5”) for the inner (r1) and outer (r2) radii respectively.  The cell’s working internal pressure and temperature is 350 bar (5000 PSI) and 500˚C, respectively. The above information gives an inner to outer diameter ratio (K) of 3 from equation 4.1[127]:                  (4.1) The three principle stresses calculated were the circumferential (or hoop), !", radial, !r, and the longitudinal (or axial), !z, stresses, equation 4.2 - 4.4 below respectively [127].  These correspond to the three directions or coordinates of the cell. In the analysis, only the internal pressure in the system was considered.                      (4.2)              (4.3)             (4.4)   41 Where r2 is the outer diameter and r is the incremental inner radius. The hoop and radial stresses are functions of r and vary with the thickness. The axial stress is independent of r (i.e., constant through out the wall thickness).  However, these stresses do not act in an uncombined manner. Thus effective stress (or Von-Mises stress), which takes into account the combination of the stresses in different directions gives a more generalized and resultant stress in the system when considering the suitability of the material with respect to the operating load or conditions, and it was calculated by equation 4.5 below [127]:           (4.5)  The summation of the stresses must be less then the yield stress of materials for reliability. Yielding is a phenomenon whereby a material fails to return to the original size or length, after a load is removed. Many mechanical systems are designed with reasonable safety factors to maintain the lifespan of the system, meaning a low effective stress compared to the yield stress.  The calculated values of the three principal stresses and the effective stress for the cell without considering the port holes are plotted in Figure 4.3, with a highest effective stress of 68 MPa for an internal pressure of 350 bar.  42  Figure 4.3 Pressure induced stress distribution of the vessel - no port holes.  Following convention, compressive stresses are negative and tensile stresses are positive, Figure 4.3 shows that the radial direction experiences compression while the circumferential stress experience tension. The stresses in the longitudinal direction remained constant.  Steady State Thermal Stresses  The internal temperature of the cell was rated 500˚C. The cooling and heating of the vessel produces temperature gradients which give rise to thermal stresses as a result of the non-uniform in expansion of the material. For such a high operation temperature, temperature gradient is a major concern. The rate of heat transfer per unit length was determined by equation 4.6 [127]:                  (4.6)  Where k1 is the conductivity of the material, T1 and T3 are the wall temperature and the ambient temperature, respectively. The temperature T2 of the outer wall was determined by equation 4.7 [127]:  43                   (4.7)  A 5 cm layer of insulation has the effect of keeping the outer cell wall temperature, T2, at approximately 490˚C. The radial temperature profile of the cell (that is the temperature distribution as the radius varied at 1mm intervals) was calculated, by equation 4.8 [127]:                  (4.8)  The radial and circumferential stresses as a result of the temperature gradient alone, if the longitudinal stress or strain remain constant, were calculated with equation 4.9 and 4.10 respectively [127]:                (4.9)              (4.10)  Where E, #, v is the Young's Modulus, coefficient of linear expansion and the Poisson’s ratio, respectively. The result is plotted in Figure 4.4 below.  44  Figure 4.4 Thermal stress distribution of the vessel - no port holes.  Because the cross-section of the cell will be subjected to several types of loading (thermal and pressure stresses) simultaneously, the method of superposition was applied to determine the resultant stress distribution caused by the loads. In the superposition, the stress distributions of the individual loads are determined first, and then their distributions are superimposed to determine the resultant stresses. The result is shown in Figure 4.5 below. The effective stress was reduced by 50 percent due to the compressive effect of the thermal stress.  Analysis of the stresses due to the pressure and thermal effect gave reasonable safety factor (approx. 5 for the SS316 with 290 MPa yield strength) for the cell under steady state conditions.    45  Figure 4.5 Superposition of pressure and thermal stresses of the vessel.   Stress Concentration due to Side Holes The existences of the port holes create stress concentration. This is due to irregular cross-section that affects the flow of the stress. Stresses in the radial direction are not affected because radial cross-section was not affected. The stress concentration was estimated from the model of infinite flat plate, which deals with stresses in the x and y directions. These correspond to the longitudinal and the circumferential direction of the cylinder. If Sx and Sy, are the stresses far away from the hole or stress concentration, the stresses near the hole was calculated from equation 4.11 and 4.12, for the major and minor axis respectively [127]. The concentration factor used in the calculation was 3.        end of the major axis       (4.11)       end of the minor axis      (4.12)    46 The major axis lies along the x-axis and the minor axis lies along the y-axis. The Sx and the Sy represent the stresses in the radial and the circumferential directions respectively. To account for the reduced cross-sectional area caused by the ports, the longitudinal direction of the stress was modified from equation 4.4 to equation 4.13 [127]:                (4.13)   where F is the longitudinal force caused by the internal pressure and A is the cross- sectional area of the cell at the midsection. The cross-sectional area was calculated as 4054 mm2, from equation 4.14 below.                 (4.14)  The overall stress distribution was calculated by summing the stresses due to the pressure and thermal effects above, adjusting for the stress concentration due to the port holes with equation 4.15 and 4.16 [127]. The results are shown in Figure 4.6–7.      end  of the majot (z) axis   (4.15)                   end of the minor (#) axis   (4.16)   47  Figure 4.6 Overall stress distribution along the major axis of one of the port holes. Thermal gradient was reduced to zero by heating the vessel.   Figure 4.7 Overall stress distribution along the minor axis of one of the port holes. Thermal gradient was reduced to zero by heating the vessel.   48  Figures 4.6-7 show the principal and effective stresses at the two positions (i.e. end of the major and minor axes) along the circumference of the holes. It should be noted that longitudinal stresses along the major axis and the circumferential stresses along the minor axis were zero. Combination of the effects of the pressure and thermal stresses show some considerable reduction in the overall stress of the system, Figure 4.8-9.  Figure 4.8 Overall stress distribution along the major axis of one of the port holes. Thermal gradient effect included.  49  Figure 4.9 Overall stress distribution along the minor axis of one of the port holes. Thermal gradient effect included.  The tables for the values of the manual calculation can be found in the appendix (Tables B.1-6). 4. 3  Working/Counter Electrode  The Working Electrode  The cylindrical working electrode is drilled and tapped to receive the connecting rod. The working electrode or counter electrode holders are fabricated from stainless steel 316. The diagram is shown in Figure 4.19.  The working electrode of 9.5 mm (0.38”) outer diameter and a height of 7.6 mm (0.30”) was isolated from electrical contact with the cell body via a clearance between the connecting rod housing and the electrode connecting rod (3.05 mm diameter), which is threaded at both ends. One end of the threaded rod holds the cylindrical working electrode specimen, the other end, is used to compress the whole assembly and to create a seal at the hot side. The hot side seal is currently PTFE, which limits operation to a temperature of 250˚C.  A ceramic seal will be required for higher temperature applications.  The size of the working electrode was made as small as possible to achieve an appreciable current density at high temperature because solution conductivity decreases substantially at  50 supercritical conditions as a result of a decrease in ionization, density, and polarity of water. In fact, it has been reported that micro-electrodes may be required for supercritical water conditions [129-130]. The surface area of the electrode is 3 cm2, which is one order of magnitude larger than the 0.25 cm2 reported in [8]. On the other hand, the dynamic flow-through of the system will enhance convective mass transport (Flux), reduce surface fouling effects, increases the limiting current and reduce the thickness of the diffusion layer which in turn may increase the sensitivity of the measurements [131].  Figure 4.10 Working/Counter electrode assemblies.  The driving nut with a parallel thread was used to drive the whole electrode assembly into the cell body and which helps to transmit the force required to seal the electrode assembly against the cell body. The design provides an advantage over Conax packing glands as they rely on frictional forces between a PTFE gland and the electrode for pressure containment.  At high pressures (as low as 35 bar) these glands can result in electrodes “slip” causing leakage or sudden pressure loss and projectile electrodes. This design also has the advantage of providing easy assembly and disassembly, and the probability of parallel threads seizing at high temperatures are lowered than when NPT threads are used.   51 4. 4  Reference Electrode The reference electrode is a critical component of the system. It must be stable and un- polarizable for the duration of the test to accurately measure the potential of the WE.  In view of these requirements, an external cell for the reference electrode connected to the test vessel, using a bridge or conductive solution conduit, was designed to minimize the effect temperature, contamination from the corrosive environment, or a combination of these factors. This type of cell is known as a Wilhelm cell. The system will make use of a chloridized silver wire in contact with dilute KCl. Thus the RE will be Ag/AgCl at 25˚C and system pressure. The Flow-Through External Pressure-Balanced Reference Electrode (FTEPBRE) designed is shown in Figure 4.20.   Figure 4.11 Flow-through external pressure-balanced reference electrode.   Its construction features chloridizing a piece of silver wire in hydrochloric acid, and connected to a second cell pressure vessel known as the  “Wilhelm” cell by a Conax fitting to operate at ambient temperature but at overall system pressure. The cold- side mounting of the reference electrode prevents thermal hydrolysis, dissolution and complex formation of the electro-active element – in this case the Ag/AgCl couple – when operating at temperatures above 275°C [132-  52 134].  Furthermore, the thermodynamic data for the reference solution is readily available at ambient temperatures such that the reference potential can be obtained with high accuracy.  The solution is injected through a metering pump into the cell at a low but positive flow-rate. This prevents contamination of the electrode solution; however, a small amount of reference solution is introduced by this method into the main pressure vessel. This can be a problem in batch autoclave testing. Connection to the hot region is made via a non-isothermal bridge tightened at both end of the hot and cold side of the cell.  The measured potential at the working temperature can be easily converted to the standard hydrogen scale by calculation or via calibration data.  4. 5  Challenges of the Reference Electrode  There are two notable problems seen in all FTEPBRE that make accurate potentiometry measurements difficult.  First, an Isothermal Liquid Junction Potential (ILJP) factored in the potential reading. Second, and most importantly, Thermal Liquid Junction Potential (TLJP) established as a result of thermal diffusion of ionic species in the RE solution also factored in the measured potential. The TLJP originates from temperature gradients in the system.   4. 5. 1   Isothermal Liquid Junction Potential EILJP. Liquid junction potential is related to single ion activities in solution; it is calculable, negligible in most cases and slightly changed as the test solution is altered. When the ionic conductance of all of the relevant ions is known the liquid junction potentials may be estimated by use of the Henderson equation. The ILJP is introduced in an electrochemical system through contact of two different solutions [134-138] as a result of mass transport interactions. These interactions take place at the electrolyte boundary where differences in mobility and concentration gradient of ions result in diffusional movement. Thus the measured potentials must be calibrated against the ILJP to extract the true electrode potential.  Estimation of ILJP. In the estimation of the ILJP, irreversible transfer of electrons is normally assumed because the timescale of the experiment does not allow for equilibration of ionic species  53 and the establishment of a steady-state potential [135].  This estimation requires knowledge of the individual ionic conductivity involved and their activity coefficients because the ILJP is generated by the differences in these parameters.  The contribution of the ILJP to the measured cell potential can be represented mathematically thus [135]:              (4.17)  where ECell, ENernst and EIJLP are the overall cell potential, Nernst potential of the couple and liquid junction potential, respectively.  The liquid junction potential of the system was estimated by equation 4.18 (Henderson equation) [135] for three different concentrations of the reference (filling) solution and the results are plotted in Figures 4.21-23 below.          (4.18)   Where zi, is the charge on species i, %i, the mobility of the species, Ci, the molar concentration and the superscripts 1 and 2 are for the reference and test solution, respectively. From the graphs, there is clearly a significant dependence of the ILJP on the concentration difference between reference and test solutions.   54   Figure 4.12 ILJP – 1 M of Na2SO4 test solution and 0.01 KCl reference solutions.    Figure 4.13 ILJP – 0.1 M of Na2SO4 test solution and 0.01 KCl reference solutions.    55  Figure 4.14 ILJP – 0.01 M of Na2SO4 test solution and 0.01 KCl reference solutions.   4. 5. 2   Thermal Liquid Junction Potential ETLJP In 1856, Ludwig discovered thermal diffusion in a liquid, a process by which the TLJP is established [22]. This phenomenon was further examined by Soret and sometimes called the Soret effect [139]. The thermal liquid junction potential, also known as the thermal diffusion potential, is another hidden component of the measured cell potential. Its influence on equilibrium measurement (static and dynamic) was experimented upon and verified by Eastman et al. [140]. The temperature gradient causes a local increase in the concentration of electrolyte at the cold region of the non- isothermal electrolyte bridge as the system moves toward achieving steady state. The increase in concentration is driven by the high ionic mobility at the hot side of the electrolyte bridge.  The main contribution to the TLJP is the heat of transport by the ionic species and its evaluation involves knowing the entropy of transport, conductivity and activity coefficient of the respective ionic species [23-26, 141-143]. The magnitude of the TLJP is generally higher than that of the ILJP and is a function of the temperature, pressure, electrolyte and liquid junction composition [135]. The process is schematically represented in Figure 4.24.  Thermal liquid junction = combination of heat and ionic flux    56  Figure 4.15 Schematic representation of the factors contributing to the TLJP.   Macdonald et al. [10-12, 23] have published calculated values of the TLJP for a KCl reference solution in the temperature range of 25°C-347°C using linear irreversible thermodynamics [144- 147] for (EPBRE). The upper temperature limit for TLJP calculations was further increased to 400°C by Lvov et al. [29, 148-150] for NaCl + HCl reference solution.  This solution was chosen because thermodynamic (transport number, dissociation/ionization constant) data for KCl were not available at the required temperatures.  The flow design of this work has the advantage of keeping this effect constant (at Soret initial state) by constant refreshing of the reference solution. Other benefits include continuous flushing of corrosion products to decrease its effect on the measured potential and, cell wall adsorption of electro-active element over time were greatly reduced. A comparison of the static and flow- through design highlighting these benefits are schematically shown in Figure 4.25 below.    57  Figure 4.16 Schematic representations of benefits of flow-through design.  Other parasitic potentials that were also eliminated or reduced in the FTEBRE design include, a streaming potential, which is a potential difference driven by a pressure gradient. This was minimized by the pumping the reference solution as the same system pressure. Second, a thermoelectric potential, which occurs when an electron conductor is in a non-isothermal condition, this was negligible by the use of a flow through design because the composition of the non-isothermal liquid junction is maintained at a well-defined concentration [29, 150].  4. 6  Heat Exchanger  Heat exchanger can be seen as any device that transfers heat (enthalpy) from one fluid to the other. There are two main categories of heat exchanger: the direct contact and the indirect contact.  In the direct, there is no intervening surface between the fluids while in the indirect case (regenerators), the transfer of heat between two fluids, or between a surface and a fluid takes place. A counter current heat exchanger was designed to reduce the temperature of the cell’s effluent fluid.   58  Figure 4.17 Counter current double tube heat exchanger.  The dimensions of the tubes, both inner and outer shell, were constrained by the dimensions of the design. The heat removal demand was set by a requirement to reduce the temperature of the discharged fluid from 500˚C to 60˚C.  4. 6. 1   Structural Analysis  Overall Heat-transfer Coefficient  The processes of calculating the dimensions of the heat exchanger to achieve the cooling requirement were as follows: the heat flow was calculated by equation 4.19 [128,151]:              (4.19)  The overall heat-transfer coefficient U is defined by the relationship in equation4.20 [128, 151]:                (4.20)  From equation 4.21 the mass flow of the cold fluid was calculated:               (4.21)   59 The inner and outer overall heat coefficient was calculated by equation 4.22 and 4.23 respectively, [128, 151]              (4.22)                  (4.23)   Where, hi, ho, Ui, Uo, ri, ro are the inner and outer convective heat transfer coefficient, inner and outer overall heat transfer coefficient, inner and outer radius respectively. Thus the heat flow must satisfy the relationship of equation 4.24           (.4.24)  where               (4.25)   The average effective temperature difference otherwise known as the Log Mean Temperature Difference (LMTD) was determined by equation 4.26 [128, 151]:             (4.26)  Substituting for the LMTD, the heat flow becomes:  60                (4.27)  The required length calculated from equation 4.25 was 600 mm tube length for a 10 mm inner diameter and 13 mm outer diameter hollow tube. The spreadsheet of the values can be found in appendix B.7.  4.7        Other Component of the Design  Other parts of the design that were ordered from outside vendor/source include:  • Fittings by Swagelock® • Back Pressure Regulator by Swagelock® • Pressure relief valve by Swagelock® • Thermocouple and Temperature meter by Omega® Engineering • Band heaters and controllers by Omega® Engineering • High pressure Pump by Eldex Laboratories Inc        61 5  EXPERIMENTAL   5. 1  Test Materials and Solution  The tests were carried out with a two-electrode setup consisting of an alloy 625 Working Electrode (WE), and SS 316 Counter/Quasi Reference Electrode (CE/QRE).  This is in contrast to the conventional three-electrode cell. The chemical analyses of the two electrodes are summarized in tables 5.1 and 5.2 as determined by EDX.  Table 5.1 Alloy 625 chemical compositions in weight percent. Alloy Composition Percentage (%) UNS Nominal (%) Nickel (Ni) 62.76 58 min Chromium (Cr) 21.81 20-23 Molybdenum (Mo 7.35 8-10 Iron (Fe) 3.97 5 max. Niobium (Nb) 2.7 3.15-4.15 Ti 0.3 0.4 max  Table 5. 2  SS 316 alloy chemical compositions in weight percent. Alloy Composition Percentage (%) UNS Nominal (%) Iron (Fe) 68.54 65.40 min Chromium (Cr) 17.10 16-18 Nickel (Ni) 10.35 10-14 Manganese (Mn) 1.13 2 max Molybdemum (Mo) 1.72 2-3   Both electrodes were of the same design.  They were machined from stock to a dimension of 0.30” x .38” (7.62 mm x 9.52 mm). Their holders were fabricated from SS 316 material as described above (Section 4.3) and shown in Figures 5.1-2 below.   62  Figure 5.1 Working/Counter electrode assembly   Figure 5.2 Working/Counter electrode assembly parts.  The two electrode holders, each tightly holding the WE and the CE/QRE, were screwed into their respective ports in the main cell with a hex bolt, with PTFE gasket for sealing purposes as shown in Figure 5.3 below.  63  Figure 5.3 The cell.  The main cell as constructed had 4 ports, two vertical ports were for mounting the WE and CE/QRE holder and the two horizontal ports for the inlet and outlet fluid. The port for the thermocouple in the original design was eliminated and rather swaged at the outlet port with a Swagelok® union tee to measure the temperature of the outgoing fluid. The assembly, Figure 5.3 was housed inside a box constructed with 12.7 mm (0.5 inch) thick steel plates, for further maximization of safety at high operating temperature and pressure. The inlet port was connected to a high-pressure pump rated at 345 bar (5000 PSI) pressure at a set flow rate of 7 mL/min by a 6.35 mm (0.25 inch) Swagelock® union fitting.  The outlet port at the opposite end was connected to a 414 bar (6000 PSI) rated back-pressure regulator from Swagelock® and a filter to remove any corrosion product or solid impurity that could otherwise damage the regulator or at least affect its sensitivity. A relief valve to relieve any excess pressure was also installed before the back-pressure regulator, see Figure 5. 4. Detailed information on the dimensions and the whole electrochemical setup are presented in the design section of this thesis and has been published in our earlier paper [152]. The test solution was 0.1M sodium sulphate (Na2SO4) prepared from reagent grade salt and de-ionized water. The solution pH was 5.7.   64 5. 2  OCP Measurements and Conditions The OCP tests were carried out in the following sequence at 25˚C, 50˚C and 100˚C: atmospheric pressure, 34.5 bar (500 PSI), 69 bar (1000 PSI), 138 bar (2000 PSI), 207 bar (3000 PSI), 276 bar (4000 PSI) and 345 bar (5000 PSI) in both naturally aerated (atmospheric oxygen) and deoxygenated environments.  The pressures for the 50˚C and 100˚C tests were limited to 207 bar. De-oxygenation was carried out by sparging argon gas through the test solution at least 30 minutes before and continuously throughout the duration of each test.  A minimum of four tests were carried out for a given condition, each lasting 3 hours which was the time chosen for the terminal OCP for the ambient temperature tests at 7.5 mL/min solution flow rate. At the end of each individual test, the sample was dismantled, polished with 1200-grit SiC paper and allowed to dry before commencement of the next test.  The mean and the standard deviation for the tests in each sequence were calculated. This was followed by potentiodynamic tests, polarizing the electrode between -250 mV and 500 mV versus the OCP at the scan rate of 1 mV/s to determine the effect of pressure on corrosion rate of alloy 625. In some of the high temperature tests, the potential range and the scan rate were varied as will be seen in the results and discussion below.  5. 2. 1   Apparatus for OCP Tests under Pressure  In the setup for the pressurized electrochemical tests at ambient temperature, the QRE was connected to a saturated Ag/AgCl reference electrode  (0.205V versus SHE) located outside the test cell at ambient temperature and pressure to measure the stability of the QRE as a function of time and cell pressure. The set-up is shown in Figure 5.4 below.   65  Figure 5.4 System laboratory setup for tests under pressure.  5. 2. 2   Apparatus for OCP Tests under Pressure and Temperature  The temperatures tests were carried out in naturally aerated conditions only. The setup was modified (figure 5.5) to incorporate, a pre-heater chamber in a 540˚C rated HBZ-2X88 cylindrical Band Heater connected to solid state relay both from Omega®. The solution was pre- heated up to 90 percent of the required temperature prior to flowing into the main cell. A heat tape was wound around the main cell to compensate or prevents excessive heat loss that would cause an unacceptable temperature gradient through the cell. Super OMEGACLAD® mineral insulated high temperature K-Type thermocouples were mounted on the pre-heater and to the main cell by a Swagelock® union tee. The thermocouples for the pre-heater and the band heater were connected to a ramp/soak controller with a digital display from Omega®. The thermocouple for the main cell was connected to another digital display temperature meter with integrated 2-selectable alarm system from Omega® that automatically activates upon deviation from the set temperature. Also integrated in the modification was the heat exchanger designed and fabricated to cool the effluent fluid.  66  Figure 5.5 System laboratory setups for tests under temperature and pressure – note the pre-heater cell between the pump and the electrochemical cell.  67 6.  Results and Discussion  6. 1  Results 6. 1. 1    High Pressure Experiments  The OCP of alloy 625 in 0.1M Na2SO4 decreased with an increase in the pressure up to 207 bar (3000 PSI). Beyond this point, increasing pressure resulted in an increase in the OCP. This behaviour was observed both for the naturally aerated and the de-aerated condition and is plotted in Figures 6.1 and 6.2 below, respectively. A slightly higher potential for the naturally aerated conditions was observed compared to the corresponding de-aerated conditions. The individual OCP measurements and their standard deviations of approximately 9 mV (highest recorded) for naturally aerated conditions and 12 mV (highest recorded) for deoxygenated conditions can be found in appendix C and D respectively.  Figure 6. 1 Effect of pressure as a function of time on OCP of alloy 625 in naturally aerated 0.1M Na2SO4 at 25˚C.    68  Figure 6.2 Effect of pressure as a function of time on OCP of alloy 625 in de-aerated 0.1M Na2SO4 at 25˚C.  The potential of the QRE versus the Ag/AgCl RE at atmospheric pressure was also recorded and showed an increase with pressure up to 207 bar and a decrease with pressure above 207 bar. This is shown in Figure 6.3 and the corresponding value versus the Standard Hydrogen Electrode (SHE) is shown in figure 6.4 for both the naturally aerated and de-aerated conditions. The potentials were slightly higher for the naturally aerated conditions. The potential change of the QRE over a pressure range of 1-345 bar was approximately 30 mV.  The standard deviation of the QRE versus the Ag/AgCl RE from the number of tests carried out at a given condition is plotted and shown in Figure 6.5 for naturally aerated, and Figure 6.6 for de-aerated conditions. The standard deviations were approximately 8 mV and 9 mV for the respective cases.        69  Figure 6.3 Potential of the 316 SS QRE vs. saturated Ag/AgCl RE at atmospheric pressure and 25˚C. Naturally aerated and deoxygenated environment compared.   Figure  6.4 QRE vs. SHE OCP potential of alloy 625. Naturally aerated and deoxygenated environment compared (ESHE (mV)=EAg/AgCl (mV) +205mV).   70   Figure  6.5 SS 316 QRE  vs. Ag/AgCl potential with standard deviation in naturally aerated environment at 25˚C.  Figure  6.6 SS 316 QRE vs. Ag/AgCl potential with standard deviation in de-oxygenated environment at 25˚C.    71 A summary of these results at 3-hour period is tabulated in Table 6.1 below.  Table 6. 1 OCP versus QRE and Ag/AgCl at 298K for a alloy 625 electrode in 0.1M Na2SO4 solution at various pressures after 3 hours. Pressure (298K) atm (PSI) E vs. QRE (mV)  E vs. QRE (mV) D* Stand. Devia. (mV)  Stand. Devia. (mV) D* E(298K) vs. Ag/AgCl (mV)  E(298K) vs. Ag/AgCl (mV) D* Stand. Devia. (mV)  Stand. Devia (mV) D* STP 22.50 29.50 8.87(4) 1.77 (4) - - - - 34(500) 16.50 24.75 8.20 (4) 3.96 (4) -44.33 -52.00 3.40 6.22 68(1000) 10.00 18.80 2.55 (4) 6.49 (5) -39.33 -43.33 5.44 3.30 136 (2000) 2.25 10.90 5.85 (4) 3.80 (5) -37.50 -38.25 7.12 8.55 207(3000) -3.10 -0.75 4.60 (5) 2.95 (4) -19.60 -21.60 2.69 2.75 272 (4000) 1.50 5.88 5.56 (4) 3.51 (4) -30.60 -32.80 5.25 3.43 345(5000) 9.80 17.00 5.98 (5) 12.81 (5) -40.25 -43.75 6.36 2.36 Number in bracket represents number of data points, D* (De-aerated)  The result of the potentiodynamic tests conducted in a naturally aerated condition showed an increase in the Critical Passivating Current (CPC) up to 207 bar (3000 PSI) and a decrease was observed afterward at higher pressure up to 345 bar (5000 PSI) as depicted in Figure 6.7 below. The CPC is the peak current observed prior to passivation.  In the present case, since the alloy 625 spontaneously passivates, the CPC is essentially identical to the passive current density. The lines of highest and lowest OCP observed at STP and 207 bar  (3000 PSI) respectively, also depicted in Figure 59, show that all the open circuit potentials reported above were in the passive region of the alloy 625 working electrode.  72  Figure 6.7 Polarization plots of alloy 625 in 0.1M Na2SO4 solution at 25˚C and different pressures in PSI (Naturally aerated environment).  6. 1. 2    Temperature and Pressure Experiments The OCP of alloy 625 versus the 316 SS QRE followed the same trend of a decrease with pressure at 50˚C and 100˚C as shown in Figures 6.8 and 6.9 below. From the graph an increase in OCP with temperature at constant pressure is also observed.  73  Figure 6.8 Effect of pressure on the OCP of alloy 625 in 0.1 M Na2SO4 solution at 50˚C.    Figure 6.9 Effect of pressure on the OCP of alloy 625 in 0.1 M Na2SO4 solution at 100˚C.   The polarization tests of -300 mV to 500 mV versus OCP conducted at 50ºC, Figure 6.10 and 100ºC, Figure 6.11 also showed an increase in the passive current density with pressure. Similarly, the passive current density increases with temperature at constant pressure.  74  Figure 6.10 Effect of pressure on the polarization test of alloy 625 in 0.1 M Na2SO4 solution at 50˚C versus the SS316 QRE at a scan rate of 1mV/s. (Naturally aerated).    Figure 6.11 Effect of pressure on the polarization test of alloy 625 in 0.1 M Na2SO4 solution at 100˚C versus the SS316 QRE at a scan rate of 5mV/s (Naturally aerated).    75 6. 2 Discussion  6. 2. 1    Results of Tests at Pressure below 207 Bar The decrease in the OCP of alloy 625 with pressure in the naturally aerated and de-aerated conditions Figures 6.1 and 6.2, respectively, below 207 bar  (3000 PSI) deviated from the linear function of E (potential) with pressure predicted by equation 2.18.  According to this equation, the compressive effect of pressure, which would result in a negative volume change for water dissociation and metal dissolution reactions, should give an increase in potential. As highlighted previously, and shown in Figures 2.6 and 2.7 above, the effect of pressure on the properties of water (dielectric, density, ion-product) are such that an increase in the oxidizing nature (hydrogen ion or oxygen solubility) of the aqueous solution would be expected. Consequently, oxidation of the electrode in such an environment would be increased as pressure increases, leading to an increase in the potential of the electrode in response to the changes in the solution (environment). However, this was not observed in the results as shown in the OCP curves of Figures 6.1-2 and 6.8-9 above. The OCPs were observed to be decreasing with pressure up to 207 bar while the corrosion current densities (passive or otherwise) from the polarization tests were increasing at the same pressure range.  According to mixed potential theory, schematically represented in Figure 6.12 below, a decrease in potential should indicate a decrease in current and vice versa in agreement with the principle of charge conservation. Mixed potential is a combination of the half-cell electrode potential for the anodic and cathodic reactions because the two half-cell electrode potentials cannot coexist separately on an electrically conductive surface but must change potential or polarize to a common intermediate value known as (Ecorr).  76  Figure 6.12 Diagrammatical representation of mixed potential theory for a simple electrochemical system.  According to the mixed potential diagram above, the mixed potential (Ecorr or OCP as measured here) is expected to increase from Ecorr to E’corr as the environment becomes more oxidizing with a corresponding increase in the current density from icorr to i’corr for a typical anodic and cathodic reaction:  Cathodic reaction:      E˚= 1.23 versus SHE            (6.1)                  (6.2) Anodic reaction:                 (6.3)   77 The cathodic and anodic half-cell electrode potentials are polarized to Ecorr . The rate of cathodic and anodic reaction at this point is also equal and corresponds to corrosion current, icorr, equation 6.4.                    (6.4)  The measured OCP values were all within the passive region of the polarization tests (Figure 6.7) and within the water stability region as presented thermodynamically in the Pourbaix diagram, Figure 6.13, below for a pH 5.7 – the pH of the sulphate solution used in the tests. The Pourbaix diagrams were plotted with HSC software.   Figure 6.13 E-pH diagram for Ni-H2O system at 25˚C, [Ni] = 1 x 10 -6 .  As such, there is neither hydrogen nor oxygen evolution in the system, leaving equation 6.1 as the only predominant cathodic half-reaction where dissolved oxygen and hydrogen ions are reduced to water. Both the solubility of oxygen and the acidity of the solution increase with pressure as mentioned previously. Correspondingly, the potential is expected to increase according to equation 6.2. It is noteworthy that the effect of pressure on the concentration of  78 oxygen in the solution might be fairly negligible because the tests were conducted in a closed system with no overhead air. But that notwithstanding, pressure can contribute to the amount of oxygen taking part in the reaction as more of oxygen is dissolved with pressure.  Alloy 625 is a Ni-Cr-Mo alloy which passivates through the formation of a Cr oxide [153-154] over a broad range of pH. The anodic potential associated with formation of the Cr2O3 can be calculated thus:  ! E Cr /Cr2O3 = E°Cr /Cr2O3 " RT nF ln H 2 O[ ] 3 Cr[ ] 2 H +[ ] 6 Cr 2 O 3[ ]      (6.5) Under conditions of standard state equation 6.5 becomes:  ! E Cr /Cr2O3 = E°Cr /Cr2O3 " RT nF ln 1 H +[ ] 6 = RT nF ln H +[ ] 6    (6.6) The change in potential at the alloy 625 working electrode is then directly proportional to and ! H +[ ] . Since the activation energy contribution to electrode potential is small between 1- 700 bar [74] then the only factor of concern is the activity of the proton.  Assuming we have a measured OCP, E1, for alloy 625 at 1 bar and an OCP at 207 bar, E2 (EP), then the change in OCP (related to the change in ) with P will be given by:  ! E P "E 1 # $E # RT F ln H +[ ] P " RT F ln H +[ ] 1         (6.7)  ! = RT F ln H +[ ] P H +[ ] 1          (6.8)  All other factors remaining constant, any change in pH associated with pressure is then directly proportional to the change in the ion-product of water with pressure (cf. equation 2.22):   ! E P "E 1 # $E # RT F ln (K W ) P (K W ) 1          (6.9)         &E ' 3.2 mV (data for ln[(KW)P/(KW)1] is from ref. [5])   79 A comparison of theoretical "E and measured "E is plotted in Figure 6.14 below. Clearly our measured potentials deviated from the expected result.  It is explained below that this is caused by a drift in the mixed potential on the QRE.  Figure 6.14 Comparison of measured versus theoretical "E as a function of pressure.  Based on mixed potential theory, the results as regards the observed increase in corrosion current of the potentiondynamic (polarization tests) tests below 207 bar (3000 PSI) are predictable. However, the direction of the OCPs as pressure changed remained questionable for both cases of ambient and high temperature tests. An investigation into the observed potential behavior below 207 bar shows that the SS 316 alloy used as the QRE was less stable electrochemically than the alloy 625 WE. This is due to the fact that, upon measurement of their respective open circuit potential, with a standard Ag/AgCl RE (Figure 6.15), the potential of the SS316 was slightly lower than that of alloy 625, which also reflects the magnitude of the changes observed in the tests. Thus, the tendency to corrode or dissolve was slightly higher for the SS 316, because it has more active potential electrochemically than the alloy 625 leading to a slightly higher potential change of the QRE than the WE with pressure. This was further proven by the average potential change with time of 3.6 mV/min. and 4.6 mV/min. observed for alloy 625 and SS 316, respectively, in the OCP tests of Figure 6.15. Consequently, this parasitic reaction occurring at  80 the QRE was also part of the measured OCP and responsible for the observed change in the direction of the OCP results.  Figure 6.15 OCP of alloy 625 and SS 316 versus Ag/AgCl RE.   This parasitic contribution is represented schematically in Figure 6.16 below.   Figure 6.16 Illustration of the effect of polarizable QRE on the WE mixed potential.  81  It follows that as presurre increases from 1 bar  to a value P ( 207 bar,  the change in potential of the QRE (!E316) is greater than the correpsonding change in the potential of the WE (!E625) cf. equation (6.4 – 6.9) and Figure 6.16. This is because the SS316 is electrochemically more active than alloy 625 (Figure 6.15), and polarizes more than alloy 625. The resulting polarization causes a larger increase in the potential of the SS316 QRE. At 1 bar, the measured OCP, E1 (Ecorr in Figure 6.17) is the potential difference between the QRE and WE. At pressures above 1 bar but below 207 bar, the measured potential difference E2 (E ’ corr in Figure 6.17) was less than E1, reflecting the contribution from the change in potential of the QRE. If the SS316 QRE potential were to have remained constant irrespective of the change in pressure, the measured potential would be E1 plus any change in the potential of the WE  i.e. ( E1 + !E625), which should give a potential increase with pressure according to theory. However, this was not the case but, rather, for every increment of pressure the QRE mixed potential changed to  a new value which became the new reference potential. Therefore this behavior was factored into the measured potential at the WE and responsible for the observed decrease in OCP with pressuren between 1- 207 bar (Figure 6.16 -6.17).      !E 316 = EP(316) – E1 (316)                    (6.11)      !E 625 = EP(625) – E1 (625)                   (6.12)       !E 316 > !EP625                   (6.13)  and       E1 > E2                          (6.14)  Where     E1 = E625 - E316 = Ecorr @ 1                        (6.15)    E2 = E625 – E316 = E’corr @ (P) where  0>(P)( 207  = E’corr                            (6.16)   Accordingly, the mixed potential diagram of Figures 6.12 is modified in Figure 6.16. The modified diagram accounts for the instability of the QRE and represents the test situation  82 observed for the OCP and polarization tests as pressure increased, i.e. a decrease in the OCP and increase in the corrosion current as pressure increased from 1-207 bar.   Figure 6.17 Mixed Potential diagram for alloy 625 WE with SS 316 QRE in 0.1 Na2SO4 at pressure at pressure range of 1- 207 bar.  In the temperature tests of up to 100˚C and pressure up to 207 bar, the same pattern in the OCP and corrosion current as above was observed. However, the corrosion currents were slightly higher than in the latter case, Figure 6.10 -11.  6. 2. 2    Results of Test at Pressures above 207 Bar The OCP increase observed above 207 bar (3000 PSI) was attributed to a reduction in the rate of dissolution of the QRE as a result of the formation of a stable and protective oxide layer on its surface. Stainless steel and nickel-based alloys are strong passivating alloys due to their chromium content.  It follows that, the instability (potential drift) of the QRE or its contribution to the measured OCP of the WE was high at pressures below 207 bar. But as the protective passive layer continued to build up on the SS 316 surface, the rate of potential drift of the QRE decreased above 207 bar.  Consequently, the QRE potential drift effect on the OCP above 207 bar decreased resulting in an increase in the OCP of the WE. However, the OCP at the highest  83 tested pressure of 345 bar was still less than the OCP measured at standard pressure. From a mixed potential perspective, the situation is similar to that presented in Figure 6.17, but in a reversed direction as shown in Figure 6.18. The reverse shows a decrease in corrosion current as corrosion potential increases corresponding to the observed results for the 207-345 bar pressure range.  Figure 6.18 Mixed Potential diagram for alloy 625 WE with SS 316 QRE in 0.1 Na2SO4 at pressure range of 207-345 bar.  The major concern for this work was assessing the stability of the stainless steel QRE, not the alloy 625 WE, because the accuracy of any future measurements in a two-electrode setup depends on the QRE.  In a moderately oxidizing environment the formation of a passive film on the surface of the SS316 alloy helps to limit further dissolution or rate of potential change of the alloy. This imparted stability reduces the parasitic contribution of the QRE potential change on the measured OCP of the WE as described in section 6.2.1. It follows that, as pressure increases above 207 bar, the oxidizing nature of the homogenous phase increases, and for an oxygen containing medium, the solubility of oxygen also increases; which enhances the passive layer stability.   Studies on surface chemistry and chemical composition of passive films [153, 154] have shown that chromium oxide is the predominant compound passive films are composed of. The high  84 chromium content of passive films for chromium containing alloys is also verified thermodynamically in the Pourbaix diagrams below. Passive layer formation or its stability is highly dependent on the pH of the environment and the operating potential. Nickel for instance forms a stable passive layer, however its presence in both alloys is not responsible for the passivation behavior observed in the test as can bee seen in Figure 6.13 and 6.19 which are superimposed in Figure 6.20 compared below in Figures 64, 68-69.  In the Ni-H20 system at pH 5.7 and at a potential above -0.5 V versus SHE, Figure 6.13, the nickel ion is active and its oxide is not present. But for the Cr-H20 system, at the same condition Figure 6.19, the chromium oxide is stable even at much lower pH and potential, in agreement with results in the literature [153]. The superimposition of the two systems is shown in Figure 6.20, with predominately chromium and no nickel present in the oxidized state.    Figure 6.19 E-pH diagram for Cr-H2O system at 25˚C, [Cr] = 1 x 10 -6 .  85  Figure 6.20 E-pH diagram for the Ni-H2O and Cr-H2O systems at 25˚C superimposed, [Ni] = 1 x 10 -6 , [Cr] = 1 x 10 -6 .  An increase in temperature increases the domain of stability for chromium and nickel oxides in the oxidized state but more significantly for chromium, even in more acidic environment, Figure 6.21-22. Superimposition of E-pH of chromium and nickel at high temperature are shown in Figures 6.23-25 below. Chromium is highly responsible for corrosion resistant of alloys in acidic and high temperature application.   86  Figure 6.21 E-pH diagram for the Cr- H2O system at 50˚C, [Cr] = 1 x 10 -6 .    Figure 6.22 E-pH diagram for the Cr- H2O system at 100˚C, [Cr] = 1 x 10 -6 .   87  Figure 6.23 E-pH diagram for Cr- H2O system at 25˚C and 100˚C compared, [Cr] = 1 x 10 -6 .   Figure 6.24 E-pH diagram for the Cr-H2O and Ni-H2O system at 50˚C superimposed, [Cr] = 1 x 10 -6  [Ni] = 1 x 10 -6 .  88    Figure 6.25 E-pH diagram for the Cr-H2O and Ni-H2O systems at 100˚C superimposed, [Cr] = 1 x 10 -6  [Ni] = 1 x 10 -6 .  6. 2. 3    Quasi Reference Electrode  The QRE employed in the two electrode-method used in this work was simple, requires little maintenance and is affordable. The measured OCP versus the QRE for the individual tests were fairly reproducible. The standard deviation was not more than 13 mV even for the most unstable situation as tabulated in Table 6.1. This means that the changes or the drift in the QRE potential at constant pressure were fairly constant and repeatable. The observed OCP test followed a linear pattern with pressure range of 1-207 bar and 207-345 bar. This was also observed when the QRE potential drift was measured against a standard Ag/AgCl (Figures 6.3-6.6), suggesting that if other parameters of the test are accurately known, the contribution of the QRE can be determined with acceptable level of accuracy and can therefore be useful in a range of applications.  The polarization tests above were also reproducible, and fairly stable with pressure.   89 However, the QRE was responsible for observed error in the test. The QRE potential drift resulted in a decrease in OCP with pressure, which did not agree with theory. However, due to the establishment of its own mixed potential it is not recommended to be used in corrosive environments such as those seen in a SCWO reactor, for example.  The essence of using the stainless steel QRE is because is a cheap materials and if some stability or steady pattern of potential drift is observed, then it would served as a very affordable alternative with higher operating temperature for electrochemical measurements.  90 7.      CONCLUSION AND RECOMMENDATIONS   7. 1  Conclusions  This work was a first attempt at determining the stability of an electrochemical system under HT/HP conditions and while work remains to be done the system (one of only 3 currently known) was able to generate relevant results.  This work was divided into: 1. The design of Flow-through electrochemical system and its components rated up to 500˚C (723K) and 345 bar (5000 PSI) temperature and pressure respectively. 2. Testing of the system by conducting simple electrochemical tests in aqueous media under pressure, and a combination of pressure and temperature to verify the reliability of the system.  The structural analysis of the pressure vessel, gave a acceptable safety factor of above two for the rated 345 bar operation, which is increase to by up to 45 percentage in the temperature situation.  The pressure and temperature electrochemical test of up to 345 bar and 150°C respectively, employing the two-electrode methods of testing was fairly stable, but the accuracy was affected by the instable of the SS316 used as the QRE. The potential was changing at a faster rate than the specimen been tested. The magnitude of the changes were however very small, because of the very little potential difference of the two electrodes. However instability was easily assessed and corrected to give adequate information on the effect of pressure and temperature on the corrosion of alloy 625.  The corrosion rate of alloy 625 increases with pressure as a result of negative volume change of reaction with pressure which, increasing the volumetric concentration of the aqueous solution and enhancing the collision frequency and the rate of the reaction. The current density, a measure of corrosion rate in metal was found to increase with pressure. The increase however is very small, less than 1 order of magnitude for pressure 1-207 bar. The corrosion current in naturally  91 aerated conditions was slightly higher than the de-aerated ones due to the linear function of oxygen partial pressure with pressure. Also from the measured potentials, which are within the water region thermodynamically, the cathodic reaction is predominately oxygen reduction. Temperature also increases corrosion current of alloy 625.  The increase in corrosion current of alloy 625 in sulfate media were reduced by the formation of passive layer on the surface of both metals which was attributed to the presence of chromium as one of the alloying element thermodynamically stable at the given conditions. At high temperatures the stability of the oxidized state of chromium increases even in acid media.  7. 2  Recommendations  Since the corrosion resistant of nickel-based alloys is highly attributed to their chromium content, responsible for the discrete stable metal carbide particles formation at the grain boundary even at very high temperatures, chromium-based alloys with very high chromium content should be recommended for construction of QREs.  In a two-electrode method of electrochemical testing, the materials for the construction of the QRE should be less electrochemically active than the material been tested (WE) to avoid unacceptable error in the data collected. 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Macdonald, “Estimation of the thermal liquid junction potential of an external pressure balanced reference electrode”, J. Electroanalyst. Chem., 403 (1996) 25-30.  [150]  S.N. Lvov, H. Gao, D. Kouznetsov, I. Balachov, and D.D. Macdonald, “Potentiometric pH measurements in high subcritical and supercritical aqueous solutions”, Fluid Phase Equilibria 150–151, 1998.  [151]  J. H. Lienhard IV / J. H. Lienhard V, A heat transfer Textbook 3rd ed., pp. 99-120 (2006) Phlogiston Press.  [152]  C. G. Ubah, and E. Asselin “High pressure and temperature electrochemical cell design for corrosion research: Part I”, ECS Transactions, 19 (29) 3-20 (2009).  [153]  J. H. Qiu – “Passivity and its breakdown on stainless steel alloys”, Surf. Interface Anal. 2002; 33: 830 – 833.  [154]  E. Asselin, A. Alfantazi and S. Rogak, “Passive and transpassive films formed on the nickel alloy in ammoniacal solution” surf. Interface Anal. 2009, 41, 489-495.      103  APPENDIX A: Parts List             Table A.1  Parts list Part Name Qty Manufactured at UBC Machine Shop Ordered The Cell 2 X Electrode 2 X X Electrode Connection Rod 2 X Cardboard Washer 2 X Nut 2 X  Electrode Body 2 X Driving Nut (Parrallel Thread) 4 X PTFE Guide 2  X 1/8 Swagelok Fittings 3  X 1/8 Conax Fitting 1  X , Union Elbow 5 Min.  X , Union Tee 2  X , /3/8  Union Tee 2 Min.  X ,  Hallow Tube 52inch length Min. 1/8  Hollow Tube 10inch Length min.  X 3/8 hallow tube 26 inch  length min. X Containers 3  X Pressure relief valve 1  X Pressure Control Valve 1  X High Pressure Pumps 2  X Glass fiber Insulator 1 x  20inch layer  X Coupling tubes 2 X Thermocouple 1  X Mounting screws 4  X  The total system estimated cost is between 45,000 to 50,000 Dollars (CDN) but only 7% of the cost was spent as most of the parts were manufactured in-house  104 APPENDIX B: Material’s Stress and Heat Exchanger Analyses  Table B.1.  Material’s properties and values. Parameters Units Stainless S316 Poisson's Ratios  0.3 Density kg/m^3 7778 Coe. Of Expansion (m/m.K) x10^-6  16 Thermal Conductivity k (W/m-k)   15.9 Yield Strength MPa 290 Tensile Strength MPa 579 Shear Strength (Torsion) MPa 167 Shear Modulus Gpa 73 Young's Modulus GPa 200   Table B.2 Pressure induced stress distribution of the vessel. Outer Radius (ro) (mm) Inner Redius (ri) (mm) Circum. Stress # (MPa) Radial Stress (r) (MPa) Long Strees (z) (MPa) Effective Stress (MPa) 38.10 12.70 43.75 -35.00 4.38 68.20 38.10 13.70 38.21 -29.46 4.38 58.61 38.10 14.70 33.76 -25.01 4.38 50.90 38.10 15.70 30.14 -21.39 4.38 44.63 38.10 16.70 27.15 -18.40 4.38 39.44 38.10 17.70 24.65 -15.90 4.38 35.11 38.10 18.70 22.54 -13.79 4.38 31.46 38.10 19.70 20.74 -11.99 4.38 28.34 38.10 20.70 19.20 -10.45 4.38 25.67 38.10 21.70 17.86 -9.11 4.38 23.36 38.10 22.70 16.70 -7.95 4.38 21.35 38.10 23.70 15.68 -6.93 4.38 19.58 38.10 24.70 14.78 -6.03 4.38 18.03 38.10 25.70 13.99 -5.24 4.38 16.65 38.10 26.70 13.28 -4.53 4.38 15.43 38.10 27.70 12.65 -3.90 4.38 14.34 38.10 28.70 12.09 -3.34 4.38 13.35 38.10 29.70 11.57 -2.82 4.38 12.47 38.10 30.70 11.11 -2.36 4.38 11.67 38.10 31.70 10.69 -1.94 4.38 10.95 38.10 32.70 10.31 -1.56 4.38 10.29 38.10 33.70 9.97 -1.22 4.38 9.69 38.10 34.70 9.65 -0.90 4.38 9.14 38.10 35.70 9.36 -0.61 4.38 8.63 38.10 36.70 9.09 -0.34 4.38 8.17 38.10 37.70 8.84 -0.09 4.38 7.74   105 . Table B.3 Thermal stress distribution of the vessel. Outer Radius (ro) (mm) Inner Redius (ri) (mm Radial temp. Thermal Cir. Stress (MPa) Thermal Redial. Stress (MPa) 38.10 12.70 500.00 -30.04 0.00 38.10 13.70 499.32 -24.93 -1.99 38.10 14.70 498.68 -20.63 -3.38 38.10 15.70 498.09 -16.95 -4.35 38.10 16.70 497.53 -13.76 -4.99 38.10 17.70 497.01 -10.96 -5.40 38.10 18.70 496.51 -8.48 -5.62 38.10 19.70 496.04 -6.25 -5.69 38.10 20.70 495.60 -4.24 -5.66 38.10 21.70 495.17 -2.42 -5.54 38.10 22.70 494.76 -0.74 -5.36 38.10 23.70 494.38 0.80 -5.12 38.10 24.70 494.00 2.22 -4.85 38.10 25.70 493.65 3.55 -4.54 38.10 26.70 493.30 4.79 -4.21 38.10 27.70 492.97 5.95 -3.85 38.10 28.70 492.65 7.05 -3.48 38.10 29.70 492.34 8.08 -3.11 38.10 30.70 492.04 9.06 -2.72 38.10 31.70 491.75 9.99 -2.33 38.10 32.70 491.47 10.87 -1.93 38.10 33.70 491.20 11.71 -1.53 38.10 34.70 490.94 12.52 -1.13 38.10 35.70 490.68 13.29 -0.73 38.10 36.70 490.43 14.03 -0.34 38.10 37.70 490.19 14.74 0.06    106  Table B.4 Superposition of the pressure and thermal stress of the vessel. Outer Radius (ro) (mm) Inner Redius (ri) (mm Circum. Stress # (MPa) Radial - r Stress (r) (MPa) Long. Stress (z) (MPa) Effective Stress (MPa) 38.10 12.70 13.71 -35.00 4.38 44.78 38.10 13.70 13.28 -31.45 4.38 41.01 38.10 14.70 13.14 -28.40 4.38 37.92 38.10 15.70 13.19 -25.74 4.38 35.35 38.10 16.70 13.39 -23.39 4.38 33.20 38.10 17.70 13.69 -21.29 4.38 31.38 38.10 18.70 14.06 -19.40 4.38 29.82 38.10 19.70 14.49 -17.68 4.38 28.49 38.10 20.70 14.95 -16.11 4.38 27.35 38.10 21.70 15.45 -14.66 4.38 26.37 38.10 22.70 15.96 -13.31 4.38 25.53 38.10 23.70 16.48 -12.06 4.38 24.81 38.10 24.70 17.01 -10.88 4.38 24.19 38.10 25.70 17.54 -9.78 4.38 23.66 38.10 26.70 18.07 -8.74 4.38 23.22 38.10 27.70 18.60 -7.75 4.38 22.85 38.10 28.70 19.13 -6.82 4.38 22.54 38.10 29.70 19.65 -5.93 4.38 22.30 38.10 30.70 20.17 -5.08 4.38 22.10 38.10 31.70 20.68 -4.27 4.38 21.95 38.10 32.70 21.18 -3.49 4.38 21.83 38.10 33.70 21.68 -2.75 4.38 21.76 38.10 34.70 22.17 -2.03 4.38 21.72 38.10 35.70 22.65 -1.34 4.38 21.70 38.10 36.70 23.12 -0.68 4.38 21.72 38.10 37.70 23.59 -0.03 4.38 21.75       107  Table B.5 Overall stress distribution - major and minor axis (thermal effects excluded). Cirum. Stress (MPa) Radial Stress (MPa) Longi stress (MPa) Effective Stress (MPa Outer Radius (ro) (mm) Inner Radius (ri) (mm Major Axis Minor Axis Major Axis Minor Axis Major Axis Minor Axis Major Axis Minor Axis 38.10 12.70 126.88 0 -35.00 -35.00 0 -30.63 147.52 33.03 38.10 13.70 110.26 0 -29.46 -29.46 0 -25.09 127.57 27.54 38.10 14.70 96.92 0 -25.01 -25.01 0 -20.64 111.55 23.14 38.10 15.70 86.04 0 -21.39 -21.39 0 -17.01 98.50 19.57 38.10 16.70 77.07 0 -18.40 -18.40 0 -14.02 87.72 16.65 38.10 17.70 69.56 0 -15.90 -15.90 0 -11.52 78.73 14.22 38.10 18.70 63.23 0 -13.79 -13.79 0 -9.41 71.14 12.20 38.10 19.70 57.84 0 -11.99 -11.99 0 -7.61 64.68 10.51 38.10 20.70 53.21 0 -10.45 -10.45 0 -6.07 59.13 9.09 38.10 21.70 49.21 0 -9.11 -9.11 0 -4.74 54.34 7.89 38.10 22.70 45.72 0 -7.95 -7.95 0 -3.57 50.17 6.90 38.10 23.70 42.67 0 -6.93 -6.93 0 -2.56 46.52 6.07 38.10 24.70 39.98 0 -6.03 -6.03 0 -1.66 43.31 5.40 38.10 25.70 37.60 0 -5.24 -5.24 0 -0.87 40.47 4.87 38.10 26.70 35.48 0 -4.53 -4.53 0 -0.16 37.95 4.46 38.10 27.70 33.58 0 -3.90 -3.90 0 0.47 35.69 4.16 38.10 28.70 31.88 0 -3.34 -3.34 0 1.04 33.67 3.96 38.10 29.70 30.35 0 -2.82 -2.82 0 1.55 31.86 3.84 38.10 30.70 28.96 0 -2.36 -2.36 0 2.01 30.22 3.79 38.10 31.70 27.71 0 -1.94 -1.94 0 2.43 28.73 3.80 38.10 32.70 26.57 0 -1.56 -1.56 0 2.81 27.38 3.84 38.10 33.70 25.53 0 -1.22 -1.22 0 3.16 26.16 3.91 38.10 34.70 24.57 0 -0.90 -0.90 0 3.48 25.03 4.00 38.10 35.70 23.70 0 -0.61 -0.61 0 3.77 24.01 4.10 38.10 36.70 22.90 0 -0.34 -0.34 0 4.03 23.07 4.22 38.10 37.70 22.15 0 -0.09 -0.09 0 4.28 22.20 4.33    108  Table B.6 Overall stress distribution - major and minor axis (thermal effects included). Cirum. Stress (MPa) Radial Stress (MPa) Longi stress (MPa) Effective Stress (MPa Outer Radius (ro) (mm) Inner Radius (ri) (mm Major Axis Minor Axis Major Axis Minor Axis Major Axis Minor Axis Major Axis Minor Axis 38.10 12.70 36.76 0 -35.00 -35.00 -0.59 0 62.15 34.71 38.10 13.70 35.47 0 -31.45 -31.45 -0.16 0 57.99 31.37 38.10 14.70 35.03 0 -28.40 -28.40 -0.01 0 55.03 28.39 38.10 15.70 35.19 0 -25.74 -25.74 -0.06 0 52.98 25.71 38.10 16.70 35.78 0 -23.39 -23.39 -0.26 0 51.62 23.26 38.10 17.70 36.68 0 -21.29 -21.29 -0.56 0 50.79 21.02 38.10 18.70 37.81 0 -19.40 -19.40 -0.94 0 50.39 18.95 38.10 19.70 39.09 0 -17.68 -17.68 -1.36 0 50.32 17.04 38.10 20.70 40.49 0 -16.11 -16.11 -1.83 0 50.50 15.28 38.10 21.70 41.96 0 -14.66 -14.66 -2.32 0 50.90 13.65 38.10 22.70 43.50 0 -13.31 -13.31 -2.83 0 51.46 12.14 38.10 23.70 45.06 0 -12.06 -12.06 -3.35 0 52.15 10.78 38.10 24.70 46.65 0 -10.88 -10.88 -3.88 0 52.94 9.55 38.10 25.70 48.25 0 -9.78 -9.78 -4.42 0 53.81 8.48 38.10 26.70 49.85 0 -8.74 -8.74 -4.95 0 54.74 7.59 38.10 27.70 51.44 0 -7.75 -7.75 -5.48 0 55.72 6.90 38.10 28.70 53.02 0 -6.82 -6.82 -6.01 0 56.74 6.45 38.10 29.70 54.59 0 -5.93 -5.93 -6.53 0 57.78 6.25 38.10 30.70 56.14 0 -5.08 -5.08 -7.05 0 58.84 6.30 38.10 31.70 57.67 0 -4.27 -4.27 -7.56 0 59.92 6.56 38.10 32.70 59.18 0 -3.49 -3.49 -8.06 0 61.00 7.00 38.10 33.70 60.66 0 -2.75 -2.75 -8.55 0 62.08 7.56 38.10 34.70 62.13 0 -2.03 -2.03 -9.04 0 63.17 8.22 38.10 35.70 63.57 0 -1.34 -1.34 -9.52 0 64.25 8.93 38.10 36.70 64.99 0 -0.68 -0.68 -10.00 0 65.33 9.68 38.10 37.70 66.39 0 -0.03 -0.03 -10.46 0 66.40 10.45    109  Table B.7 Calculated values for the heat exchanger. Parameter Value inner tube radius ri (m) 0.0055 Outer tube radius ro(m) 0.0064 T3 Ambient temp. (ºC) 25 T4 - Hot Inlet temp. (ºC) 500 T5 - Hot Outlet temp. (ºC) 60 t4 - Cold Inlet temp. (ºC) 16 t5 - Cold Outlet temp. (ºC) 70 Specific Heat (NH3/H20 Solu.) (KJ/kg.C) 11.5 Specific Heat (H20 Solu.) (KJ/kg.C) 4.18 Conv. Heat transfer coe. - hi (W/m^2.ºC). 3400 Conv. Heat transfer coe. - ho (W/m^2.ºC). 5000 Flow rate Hot Fluid (kg/s) 0.00083 Flow rate Cold Fluid (kg/s) 0.019 Heat Flow (q) (KW) 4.20 LMTD - !Tm (ºC) 169.33 Ui 2136.36 Uo 1835.93 Length of tube based on Ui (m) 0.67 Length of tube based on Uo (m) 0.67 !                  110 APPENDIX C:  Plot of Standard Deviation of OCP of Alloy 625C Versus SS 316 QRE (Naturally Aerated) ! !   Figure C.1  OCP of alloy 625 in 0.1M Na2SO4 solution at standard condition.    111  Figure C.2   OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 500 PSI.    Figure C.3 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 1000 PSI.    112   Figure C.4 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 2000 PSI.   Figure C.5 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 3000 PSI.    113  Figure C.6 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 4000 PSI.   Figure C.7 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 5000 PSI.    114 APPENDIX D:  Plot of Standard Deviation of OCP of Alloy 625C Versus SS 316 QRE (Deaerated)   Figure D.1  OCP of alloy 625 in 0.1M Na2SO4 solution at standard condition (deaerated).  115  Figure D.2  OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 500 PSI (deaerated).  Figure D.3  OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 1000 PSI (deaerated).  116  Figure D.4  OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 2000 PSI (deaerated).  Figure D.5 OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚Ç and 3000 PSI (deaerated).  117  Figure D.6  OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 4000 PSI (deaerated).  Figure D.7  OCP of alloy 625 in 0.1M Na2SO4 solution at 25˚C and 5000 PSI (deaerated).  118 APPENDIX E: Drawings    119   120   121   122   123  124  125  126  

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