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Corrosion Behaviour of High-Carbon High-Molybdenum Stellite Alloys in Amine Environment

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Corrosion Behaviour of High-Carbon High-Molybdenum Stellite Alloys in Amine Environment Corrosion Behaviour of High-Carbon High-Molybdenum Stellite Alloys in Amine Environment by Kafeel Kamal A thesis submitted to the Faculty of Graduate and Postdoctoral Affairs in partial fulfillment of the requirements for the degree of Master of Applied Science in Materials Engineering Carleton University Ottawa, Ontario © 2018, Kafeel Kamal Abstract Stellite alloys are cobalt-based alloys which display exceptional mechanical properties, wear and corrosion resistance owing to its unique chemical compositions. Stellite 6 is the most popular Stellite alloy employed in various applications, one of which is the hardfacing of valve trim components within control valves in boiler feedwater treatment systems where hydrazine and/or amine derivatives are used. Erosion-corrosion caused failure of Stellite 6 hardfacing has been reported from industry in such applications. To find a better replacement of Stellite 6 for improved corrosion resistance in amine environment, 700 series Stellite alloys, including Stellite 706, Stellite 712 and Stellite 720, are proposed, which are high-carbon high-molybdenum Stellite alloys. The corrosion performance of these alloys in morpholine solution with pH 9.5, which simulates the corrosive environment of boiler feedwater service, is evaluated using a series of electrochemical tests such as electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and potentiostatic polarization. Both regular polarization tests (potential up to 1.2 V) and failure polarization tests (potential up to 12 V) are conducted on the alloys. The former identifies general and localized corrosion behaviour, while the latter intends to fail the alloy surface to analyze their limitations. Two different surface conditions (intact and damaged) and two different temperature variables (25°C and 50°C) are also studied to simulate erosion-corrosion behaviour. Stellite 6 alloy is also tested under the same condition for comparison. It is shown that these 700 series Stellite alloys are comparably better than Stellite 6 with respect to oxide film stability, all exhibiting excellent corrosion resistance in morpholine solution with pH 9.5. The protective chromium oxide film on their surfaces is retained well for both regular and failure tests. These alloys behave similarly, however, Stellite 720 containing the highest carbon and molybdenum contents performed better at room temperature and Stellite 706 with the least carbon and molybdenum ii contents performed better at the elevated temperature. Although wear changes the surface condition, the corrosion resistance of the worn alloys is similar to that of intact alloys in the tested solution. The outcomes of this research suggest that 700 series Stellite alloys are good candidates for replacing Stellite 6, where Stellite 720 displayed the best performance under the conditions tested. iii Acknowledgements I would first like to extend my deepest gratitude to my thesis supervisor Professor Rong Liu for giving me the opportunity to do MASc. I sincerely appreciate her confidence in my ability to carry out the research work and thank her for her guidance and patience during the writing stage of my thesis. My sincerest thanks to Professor Glen McRae for mentoring me throughout my entire research period. His continuous guidance and support in this research made it possible to complete my work and gave me great insight on corrosion science. I would also like to thank Steve Truttmann, David Raude and Alex Proctor for their assistance and training on specific equipment usage and material preparation. A special thanks to Kyle Fournier for his guidance and knowledge during my research work. I would like to thank my friends and colleagues, Amin Ghaziaskar, Leigh Corrigall, Peter Walker, Lee Seung Hwan, Shah Khosraviani, Xiaozhou Zhang and Geoff Seatter for keeping me level headed and making these past two years a memorable one. And finally, I would like to thank my family for their encouragement and moral support. iv Table of Contents Abstract .......................................................................................................................................... ii Acknowledgements ...................................................................................................................... iv Table of Contents .......................................................................................................................... v List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Acronyms ........................................................................................................................ xvi 1 Chapter: Introduction ........................................................................................................ 1 1.1 Background and Significance ....................................................................................... 1 1.1.1 Synopsis of Stellite alloys ..................................................................................... 1 1.1.2 Corrosion problems ............................................................................................... 3 1.2 Present Research ........................................................................................................... 5 1.2.1 Objectives .............................................................................................................. 5 1.2.2 Methodologies and tasks ....................................................................................... 5 1.3 Organization of Thesis .................................................................................................. 7 2 Chapter: Literature Review ............................................................................................... 8 2.1 Overview of Stellite Alloys .......................................................................................... 8 2.1.1 Chemical composition and microstructure ............................................................ 8 2.1.2 Effects of alloying elements ................................................................................ 13 2.1.3 Corrosion properties of Stellite alloys ................................................................. 15 2.1.4 Industrial applications ......................................................................................... 16 2.2 Amine Environment ................................................................................................... 18 v 2.3 Previous Research....................................................................................................... 19 2.4 Methods of Corrosion Study....................................................................................... 34 2.4.1 Electrochemical impedance spectroscopy (EIS) ................................................. 34 2.4.2 Equivalent circuit modelling ............................................................................... 38 2.4.3 Potentiodynamic polarization .............................................................................. 44 2.4.4 Hysteresis loop .................................................................................................... 47 2.4.5 Potentiostatic polarization ................................................................................... 48 3 Chapter: Experimental Details ........................................................................................ 49 3.1 Alloy Specimens ......................................................................................................... 49 3.2 Electrolyte Solution .................................................................................................... 53 3.3 Experimental Setup..................................................................................................... 54 3.4 Experimental Procedure ............................................................................................. 56 3.5 Wear Test .................................................................................................................... 57 4 Chapter: Results and Discussion ..................................................................................... 61 4.1 Open Circuit Potential ................................................................................................ 61 4.2 EIS Results ................................................................................................................. 66 4.2.1 Bode plots ............................................................................................................ 66 4.2.2 Equivalent circuit ................................................................................................ 79 4.3 Polarization Results .................................................................................................... 80 4.3.1 Potentiodynamic polarization .............................................................................. 80 vi 4.3.2 Hysteresis loop .................................................................................................... 87 4.4 Corroded Surface ........................................................................................................ 92 4.5 Failure Tests ............................................................................................................
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