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Pourbaix Diagrams at Elevated Temperatures ~A Study of Zn and Sn~

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Pourbaix Diagrams at Elevated Temperatures ~A Study of Zn and Sn~ Pourbaix Diagrams at Elevated Temperatures ~A Study of Zn and Sn~ by Olga Palazhchenko A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Materials Science in The Faculty of Energy Systems and Nuclear Science Materials Science University of Ontario Institute of Technology August 2012 © Olga Palazhchenko, 2012 Abstract Metals in industrial settings such as power plants are often subjected to high temperature and pressure aqueous environments, where failure to control corrosion compromises worker and environment safety. For instance, zircaloy (1.2-1.7 wt.% Sn) fuel rods are exposed to aqueous 250-310 oC coolant in CANDU reactors. The Pourbaix (EH-pH) diagram is a plot of electrochemical potential versus pH, which shows the domains of various metal species and by inference, corrosion susceptibility. Elevated temperature data for tin +II and tin +IV species were obtained using solid-aqueous phase equilibria with the respective oxides, in a batch vessel with in-situ pH measurement. Solubilities, determined via spectroscopic techniques, were used to calculate equilibrium constants and the Gibbs energies of Sn complexes for E-pH diagram construction. The 3+ SnOH and SnOH species were incorporated, for the first time, into the 298.15 K and 358.15 K diagrams, with novel G values determined at 358.15 K. Key words: Pourbaix diagrams, EH-pH, elevated temperatures, solubility, equilibrium, metal oxides, hydrolysis, redox potential, pH, thermochemical data, tin, zinc, zircaloy, corrosion, passivity. iii Acknowledgements To start, thank you to my thesis supervisor, Dr. Matthew Kaye, for the tremendous support over the past two years. Thank you for always making me feel that I could indeed accomplish this seemingly insurmountable task. Not only am I grateful for the truly amazing opportunities I have had throughout this project, but also for the gentle pushes toward success, and the words of encouragement. Thank you. I also would not have gotten very far without the guidance from my committee: Dr. Brian Ikeda and Dr. Matthew Thompson. Thank you for keeping me on my toes, but also for the type of advice that one cannot find in any textbook - the kind that encourages optimism when even that seems to be an insurmountable task. Thank you to Dr. Richard Bartholomew and Michael Allison for the countless instrument training sessions on the ICP-OES and flame AAS. As an aside, thank you to Dr. Bartholomew for seeing me through yet another degree. I hope you can forgive that I may have made a permanent dent in the visitors’ seat in your office. I could not have asked for a better mentor. Thank you to Dr. Franco Gaspari for encouraging me to apply to the MSC program. My gratitude to Dr. Ranga Santhanam for all of the assistance and mentoring on the Raman; and making the concept of a PhD sound slightly less terrifying. On that same note, my deepest thanks to the colleagues and friends I have made throughout this experience. I could write a page alone for Ms. (soon to be Dr.) Jennie Eastcott. Thank you for putting up with me - I hardly deserve it. Thank you also to Clayton Jakins for psychoanalyzing me when I needed it the most, and Almey Tse for tackling all of the administrative chaos associated with completing a graduate degree. Thank you to my iv entire office for not evicting me, and a great thank you to the UOIT Graduate Student Council for allowing me to serve two years there. I am honoured to have worked with such bright young people. On a last personal note, there are hardly enough words to express my gratitude to my family. I like to believe that my parents can, finally, stop worrying about me, but I suppose that is rather unlikely. Thank you to my mother for being my foundation during each tumultuous moment of this path. A special thank you goes to the Tremaine research group for all of their advice and shared knowledge. Of course, I am largely indebted to UOIT for providing me with the space, equipment, and funding to carry out my research. Thank you for being my home for six long years. “All we have to decide is what to do with the time that has been given to us.” - J.R.R. Tolkien. v Table of Contents Abstract .............................................................................................................................. iii List of Tables ...................................................................................................................... 1 List of Figures ..................................................................................................................... 3 List of Abbreviations and Symbols..................................................................................... 5 1 INTRODUCTION ....................................................................................................... 7 1.1 Aqueous Corrosion and Passivity ........................................................................... 8 1.2 Elevated Temperature Corrosion Systems ............................................................ 10 1.2.1 Copper and Brass .................................................................................... 10 1.2.2 Zirconium and Zircaloys ......................................................................... 12 1.3 Pourbaix Diagrams................................................................................................ 13 1.3.1 General Construction of a Pourbaix Diagram ......................................... 15 1.3.2 Elevated Temperature Diagrams ............................................................. 19 1.3.3 Challenges with Elevated Temperature Thermodynamic Data ............... 21 1.3.4 Combining Modelling and Experimentation ........................................... 23 1.4 Objectives ............................................................................................................. 25 2 REVIEW OF RELEVANT LITERATURE .............................................................. 26 2.1 Current Thermodynamic Data for Select Aqueous Systems ................................ 26 2.1.1 Speciation and Thermodynamic Data for the Zn-H2O System ............... 26 2.1.2 Speciation and Thermodynamic Data for the Sn-H2O System ............... 27 2.2 Modelling Methods for Elevated Temperature Equilibria .................................... 32 2.2.1 Model Selection ....................................................................................... 33 2.2.2 Estimations within a Model ..................................................................... 34 2.2.3 Species Selection ..................................................................................... 35 2.3 Elevated Temperature Solubility Equilibria ......................................................... 36 vi 2.3.1 Selecting an Apparatus ............................................................................ 37 2.3.2 Establishing Solution pH ......................................................................... 38 2.3.3 Measurement of pH at Elevated Temperatures ....................................... 39 2.3.4 Metal Ion Detection and Speciation Techniques ..................................... 41 3 ZINC AND TIN SYSTEM EQUILIBRIUM MODELS ........................................... 44 3.1 Thermodynamic Equilibrium Constant................................................................. 44 3.1.1 Ionic Strength Correction ........................................................................ 44 3.1.2 Metal Ion Hydrolysis ............................................................................... 46 3.2 Zinc System Equilibrium Model ........................................................................... 47 3.3 Tin(II) System Equilibrium Model ....................................................................... 50 3.4 Tin(IV) System Equilibrium Model...................................................................... 52 4 EXPERIMENTAL METHODS AND MATERIALS ............................................... 54 4.1 Chemicals .............................................................................................................. 54 4.2 Apparatus .............................................................................................................. 54 4.3 Typical Experimental Run and Analysis .............................................................. 58 4.3.1 General Solution Preparation .................................................................. 58 4.3.2 Zinc Oxide Reaction Conditions ............................................................. 60 4.3.3 Tin(II) Oxide Reaction Conditions ......................................................... 60 4.3.4 Tin(IV) Oxide Reaction Conditions ........................................................ 61 4.3.5 Equilibration and Sample Preparation ..................................................... 63 4.3.6 Sample Analysis ...................................................................................... 64 5 RESULTS AND CALCULATIONS ........................................................................ 70 5.1 Zinc Oxide Solubility Results ............................................................................... 70 5.2 Tin (II) Oxide Solubility Results .......................................................................... 72 vii 5.3 Tin (IV) Oxide Solubility Results ......................................................................... 75 5.4 Tin Pourbaix Diagrams ........................................................................................
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