HIGH-TEMPERATURE CORROSION STUDY OF ALLOYS IN MOLTEN MgCl2-KCl EUTECTIC SALT by YUXIANG PENG RAMANA G. REDDY, COMMITTEE CHAIR RUIGANG WANG SUBHADRA GUPTA SHREYAS S. RAO MANOJ K. MAHAPATRA A DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Metallurgical and Materials Engineering in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2019 Copyright Yuxiang Peng 2019 ALL RIGHTS RESERVED ABSTRACT The MgCl2-KCl molten salt has been proposed as one of the heat transfer fluids (HTFs) in the solar energy system because of its potential thermal properties. This research investigates the corrosion behavior and protection strategies of Haynes 230 (H230), Incoloy 800H (800H), and Stainless Steel 316 (SS316) in MgCl2-KCl molten salt at high temperature. First of all, the corrosion rates of different alloys are determined by performing the long- term static corrosion experiments. The morphologies of the surface and corrosion depths are examined by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Secondly, the effect of Ni on the corrosion behavior of the alloys is studied in this research. When the alloy is connected with Ni, the corrosion rate is higher than that without connection in the same condition. Thirdly, the electrochemical tests of three alloys are also investigated in present research work. Tafel curves are plotted to calculate the corrosion potentials and corrosion rates of three alloys at different temperatures. Fourthly, two methods are proposed and studied to protect the alloys from severe corrosion in the MgCl2-KCl molten salt: 1) adding a sacrificial anode into the salt, and 2) coating a high-corrosion-resistance alloy. Mn, Zn, and Zr are chosen as the sacrificial anode materials to protect the alloys from further corrosion. The result showed that additions of inhibitor into the molten salts decrease the corrosion rates of the tested alloy. Pure Ni, Al2O3, Ni-Al alloy, and Cr-Fe-Al alloy are chosen to coat the tested alloy. Long-term corrosion experiments, as well as electrochemical tests of alloys with and without different coatings, are studied in present work. The results demonstrated that all ii these coatings decreased the corrosion rates of the tested alloys. Finally, the diffusion models are constructed to predict the distribution of the Cr in the different alloys after corrosion. And the results are comparable to the experimental data. In conclusion, H230 showed the highest corrosion resistance in the MgCl2-KCl salt proved by long-term corrosion test and electrochemical tests. Different inhibitors and coatings all improved the corrosion resistance of alloys in the molten salt. iii DEDICATION This dissertation is dedicated to my parents, girlfriend, friends, and everyone who helped and guided me throughout my stay and research at the university campus while pursuing my graduation goal. iv ACKNOWLEDGEMENTS I take this opportunity to thank my Ph.D. advisor, parents, and friends who have offered me invaluable help during my research work while pursuing Ph.D. Without their constant encouragement or patience, I would not have accomplished the proposed research work to achieve my goal. Firstly, I would like to offer my sincere thanks and gratitude to my advisor Prof. Ramana G. Reddy. Without his extraordinary patience, a wide range of knowledge, and consistent encouragement, I would not have been able to finish my research work. With my whole heart, I would also like to thank the group members, who worked with me directly or indirectly during my Ph. D. course: Haoxing Yang, Malli Bogala, Yudong Li, Jacob Young, Muhammad Imam, and Pravin Shinde. Their involvement and support really helped me deal with the problems in my experiments. I would like mention my special thanks to Dr. Pravin Shinde for regularly assisting not only for my experimental works but also for revising this dissertation, the research papers, and having fruitful discussions. I would like to thank my parents as well for their continued support and sacrifice, without whom I would not have come thus far, and achieved what I am today. They owe me a lot. They have always been my inspiration and supported me not only economically but also with emotional comfort, both unconditionally, despite of having the distance between us. They are always a harbor of the little boat like me, guiding the way home. I would like to thank my girlfriend, Shi Zhe. With her hand, my sorrows were lifted; with v her wine, my cup is full; with her candle, my life will never be dark. Finally, I thank the Department of Energy, National Science Foundation, ACIPCO for providing financial support for my research. My thanks are also due to the Department of Metallurgical and Materials Engineering in The University of Alabama for giving me this unique opportunity to pursue higher education. I cannot emphasize enough how much I love this place. I will treasure the happiness I experienced here for the rest of my life. vi CONTENTS ABSTRACT .................................................................................................................................... ii DEDICATION ............................................................................................................................... iv ACKNOWLEDGEMENTS ............................................................................................................ v LIST OF TABLES ......................................................................................................................... xi LIST OF FIGURES ..................................................................................................................... xiv CHAPTER 1: INTRODUCTION ................................................................................................... 1 CHAPTER 2: LITERATURE REVIEW ........................................................................................ 7 2.1 History of molten salts .......................................................................................................... 7 2.2 Thermal properties of MgCl2-KCl molten salts ................................................................... 8 2.2.1 Melting point of MgCl2-KCl and compared with other molten salts ............................ 8 2.2.2 Density of MgCl2-KCl and compared with other halide molten salts ......................... 11 2.2.3 Heat capacity of MgCl2-KCl and compared with other halide molten salts ................ 12 2.2.4 Thermal conductivity of MgCl2-KCl and compared with other halide molten salts ... 13 2.2.5 Viscosity of MgCl2-KCl and compared with other halide salts ................................... 15 2.3. Corrosion of alloys in fluoride and chloride salts .............................................................. 16 2.3.1. Corrosion mechanism of alloy in chloride salts .......................................................... 16 2.3.2. History of corrosion of structural alloys in molten halide salts. ................................. 18 vii 2.3.3. Alumina-forming alloys and coatings. ........................................................................ 22 2.3.4. Electrochemistry in halide molten salts. ..................................................................... 23 CHAPTER 3: RESEARCH OBJECTIVE .................................................................................... 27 CHAPTER 4: EXPERIMENTAL................................................................................................. 29 4.1. Materials and salt preparation ............................................................................................ 29 4.2. Corrosion tests of alloys in MgCl2-KCl salts ..................................................................... 30 4.2.1 High-temperature corrosion cell .................................................................................. 30 4.2.2. Long-term static corrosion experiments ..................................................................... 31 4.2.3. Electrochemical study of alloys in MgCl2-KCl .......................................................... 33 4.3. Ni-plating in aqueous solution ........................................................................................... 37 4.3.1. Preparation of the solution .......................................................................................... 37 4.3.2. Preparation of the electrodes ....................................................................................... 38 4.3.3. Parameters set in the electrodeposition experiment .................................................... 40 4.4. Alumina-plating in ionic liquid.......................................................................................... 40 4.4.1. Materials preparation for electrodeposition of Al ....................................................... 40 4.4.2. Preparation of the electrodes ....................................................................................... 41 4.5. Sample cleaning and salts remove ..................................................................................... 43 CHAPTER 5: RESULTS AND DISCUSSION ........................................................................... 44 5.1. Long-term static corrosion experiments ............................................................................ 44 5.1.1. The corrosion rate of H230, 800H, and SS316 calculated using the weight-loss method ..............................................................................................................