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Electrochemical studies of Inconel alloys 617 and 625 in molten PbCl2 : KCl mixture Golozar, Maryam 2013

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    Electrochemical Studies of Inconel Alloys 617 and 625 in Molten PbCl2 ? KCl Mixture  by   MARYAM GOLOZAR B.Sc., Isfahan University of Technology, Iran, 2011 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE  AND POSTDOCTORAL STUDIES (Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   October 2013 ? Maryam Golozar, 2013     ii Abstract  Behavior of Inconel alloys 617 and 625 in PbCl2-KCl molten salt system at three temperatures of 450, 550, and 650?C in air atmosphere and air-15%CO2 atmosphere were compared using different electrochemical experiments. Electrochemical experiments performed included open circuit potential, potentiodynamic polarization, linear polarization, and electrochemical impedance spectroscopy in order to examine the in situ electrochemical behavior of these alloys at high operating temperatures. In addition, Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) were used to examine the cross section of the alloy and characterize chemical composition of the oxide at different temperatures after exposure to molten salt. To obtain phases of the scale formed on the surface of alloys, X-ray diffraction (XRD) was also used. At all temperatures, corrosion resistance for alloy 625 was greater than alloy 617, thus, the corrosion rate for alloy 617 was slightly more than alloy 625. With increasing temperature, the corrosion rate increased. Both alloys formed a Cr2O3 scale. This scale became porous and, as a result, corrosion spices penetrated through the pores. This diffusion of corrosive species was the root cause for corrosion in the two alloys. Other alloying elements that formed oxides and chlorides included Cr, Ni, Mo, Co, and Nb.   iii Preface  Natural Sciences and Engineering Research Council of Canada (NSERC) and Teck Metals Ltd provided financial support for this work. The following conference paper was presented and published from the research work presented in this dissertation. I am the primary author of this paper.  ? Maryam Golozar, Milad Roushanafshar, Akram Alfantazi, ?Corrosion Behaviour of Alloy 617 in PbCl2-KCl Molten Salt System? The European corrosion congress (EUROCORR 2012), Istanbul, Turkey, September 2012, CD proceeding paper # 1639, 12 pages.    iv Table of Contents  Abstract ......................................................................................................................................... ii Preface .......................................................................................................................................... iii Table of Contents......................................................................................................................... iv List of Tables ................................................................................................................................ vi List of Figures ............................................................................................................................. vii List of Symbols...............................................................................................................................x Acknowledgements ..................................................................................................................... xii Dedication................................................................................................................................... xiii 1 Introduction .............................................................................................................................1 2 Literature Review....................................................................................................................3 2.1 Hot Corrosion .....................................................................................................................3 2.1.1 Difference between Hot Corrosion and Oxidation......................................................3 2.1.2 Hot Corrosion Stages and Attacks...............................................................................4 2.2 Salt Mixtures Causing Hot Corrosion...............................................................................10 2.3 Industries Concerned with Hot Corrosion ........................................................................13 2.4 High Temperature Alloys .................................................................................................14 2.5 Nickel-based Super Alloys ...............................................................................................18 2.6 Hot Corrosion in Boilers...................................................................................................20 2.7 Pseudo-Reference Electrode Selection .............................................................................25 3 Objectives ...............................................................................................................................27 4 Experimental Procedures .....................................................................................................28 4.1 Material Preparation .........................................................................................................28 4.2 Experimental Apparatus ...................................................................................................30 4.3 Electrochemical Experiments ...........................................................................................32 4.3.1 Open Circuit Potential...............................................................................................33 4.3.2 Potentiodynamic Polarization ...................................................................................33   v 4.3.3 Linear Polarization Resistance .................................................................................33 4.3.4 Electrochemical Impedance Spectroscopy ................................................................34 4.4 Material Characterization .................................................................................................34 4.4.1 X-Ray Diffraction Phase Analysis .............................................................................34 4.4.2 Scanning Electron Microscopy..................................................................................34 4.4.3 Energy Dispersive X-Ray Spectroscopy ....................................................................35 4.4.4 Differential Thermal Analysis ...................................................................................35 5 Results and Discussion; Air Atmosphere ............................................................................36 5.1 Differential Thermal Analysis ..........................................................................................36 5.2 Open Circuit Potential ......................................................................................................37 5.3 Potentiodynamic Polarization ...........................................................................................40 5.4 Linear Polarization Resistance..........................................................................................43 5.5 Electrochemical Impedance Spectroscopy .......................................................................47 5.6 Summary...........................................................................................................................52 6 Results and Discussion; Air- 15% CO2 Atmosphere..........................................................53 6.1 Open Circuit Potential ......................................................................................................53 6.2 Potentiodynamic Polarization ...........................................................................................56 6.3 Linear Polarization Resistance..........................................................................................58 6.4 Electrochemical Impedance Spectroscopy .......................................................................64 6.5 Summary...........................................................................................................................68 7 Surface Characterization......................................................................................................69 7.1 X-ray Diffraction Phase Analysis .....................................................................................69 7.2 Scanning Electron Microscopy & EDX Spectrpscopy .....................................................76 7.3 Summary...........................................................................................................................87 8 Conclusions ............................................................................................................................88 9 Future Work ..........................................................................................................................89 References.....................................................................................................................................90    vi List of Tables  Table 2-1 Equivalent of symbols used in the circuits (Zeng et al., 2001).....................................10 Table 4-1: Chemical composition of Inconel alloys 617 and 625 (Cramer & Covino Jr., 2005)..29 Table 5-1 Final open circuit potentials (mV) for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air atmosphere..................................................................................................39 Table 5-2 Corrosion current densities in A cm-2 for Inconel alloys 617 and 625 in 450?C, 550?C and 650?C at air atmosphere..................................................................................................41 Table 5-3 Polarization resistance for Inconel alloys 617 and 625 in 450?C, 550?C and 650?C at air atmosphere........................................................................................................................44 Table 5-4 Corrosion rates (mpy) for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air atmosphere........................................................................................................................46 Table 5-5 Electrical equivalent circuit parameters for experiments in air fitted for circuit Rs(R1(Q1(R2Q2))) ...................................................................................................................51 Table 6-1 Final open circuit potentials (mV) for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air-15% CO2 atmosphere .................................................................................55 Table 6-2 Corrosion current densities in A cm-2 for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air-15% CO2 atmosphere .................................................................................58 Table 6-3 Polarization resistance for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air-15% CO2 atmosphere .......................................................................................................58 Table 6-4 Corrosion rates (mpy) for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air-15% CO2 atmosphere .......................................................................................................59 Table 6-5 Electrical equivalent circuit parameters for experiments in air-15% CO2 atmosphere fitted for circuit Rs(R1(Q1(R2Q2))).........................................................................................67 Table 7-1 EDX analysis from three spots near the surface of Inconel alloy 617 in air atmosphere at 550?C exposed to molten salt mixture...............................................................................82    vii List of Figures    Figure 1-1 KIVCET furnace used in Teck- Cominco (Zahrani & Alfantazi, 2013) .......................1 Figure 2-1 A schematic diagram for non-active and active metals corrosion in molten salt: (A) non-active metals, (b) active metals forming a porous scale, (c) active metals forming a protective scale, (d) active metals with localized fast corrosion (Zeng, Wang & Wu, 2001).7 Figure 2-2 Schematic impedance spectrum representing active metals forming porous scale (Zeng, Wang & Wu, 2001). .....................................................................................................8 Figure 2-3 Schematic impedance spectrum representing active metals forming protective scale or localized corrosion of a metal in molten salt	 ?(Zeng, Wang & Wu, 2001)...............................8 Figure 2-4 Equivalent circuit for (a) active metals forming porous scale (Perez et al., 2008; Zeng et al., 2002), (b) active metals forming protective scale (Zeng et al., 2001; Zeng et al., 2002), (c) active metal encountering localized corrosion in molten salt (Zeng et al., 2001; Orazem et al., 2008). ...............................................................................................................9 Figure 2-5 Schematics of the reaction circuit in active oxidation of iron and steels (Mohanty & Shores, 2004). .......................................................................................................................11 Figure 2-6 (a) Porous, cracked oxide scale; (b) deposit/scale with intergranular corrosion in boiler superheater tube; (c) stratified scale from recuperator where fused salts were present; and (d) gross internal fluoridation with oxidation (Elliott, 1989). ........................................13 Figure 2-7 Common process temperatures (Elliott, 1989). ...........................................................15 Figure 2-8 SEM of a cross-section through the low alloyed ferritic steel 13CrMo44 after 1550 h at 440?C (Viklund et al., 2011). ............................................................................................24 Figure 2-9 SEM-imaging showing localized attack on the nickel-base alloys C-2000 (a) and Inconel 625 (b) (Viklund et al., 2011)...................................................................................24 Figure 2-10 Potentiodynamic polarization curves of Inconel alloy 625 in different temperatures (Zahrani & Alfantazi, 2012). .................................................................................................25 Figure 4-1 Schematic of the working electrodes. ..........................................................................28 Figure 4-2 Phase doagram pf PbCl2-KCl (Gabriel & Pelton, 1985) .............................................30 Figure 4-3 a) Schematic of the electrochemical cell b) Vertical tube electric furnace c) Electrochemical cell design ...................................................................................................31 Figure 4-4 Setup of the experiments..............................................................................................32 Figure 5-1 DTA curve of PbCl2-35.38 wt.%kcl salt mixture. .......................................................36   viii Figure 5-2 Open circuit potential vs. time curves for Inconel alloys 617 and 625 in a) 450?C, b) 550?C and c) 650?C at air atmosphere ..................................................................................38 Figure 5-3 Reproducability of polarization test on Inconel 617 in air atmosphere at 650?C........40 Figure 5-4 Potentiodynamic curves for Inconel alloys 617 and 625 in a) 450?C, b) 550?C and c) 650?C at air atmosphere ........................................................................................................42 Figure 5-5 Linear polarization curves density for Inconel alloys 617 and 625 in 450?C at air atmosphere.............................................................................................................................43 Figure 5-6 Equivalent circuit used for fitting Nyquist diagrams for all experiments (Zeng et al., 2001) ......................................................................................................................................47 Figure 5-7 Nyquist and Bode diagrams for Inconel alloys 617 and 625 in a & b) 450?C, c & d) 550?C and e & f) 650?C at air atmosphere............................................................................48 Figure 5-8 effect of temperature on Nyquist and bode diagrams for Inconel alloys a & b) 625 and c & d) 617 at air atmosphere. ................................................................................................50 Figure 6-1 Open circuit potential vs. time curves for Inconel alloys 617 and 625 in a) 450?C, b) 550?C and c) 650?C at air-15% CO2 atmosphere ..................................................................54 Figure 6-2 Potentiodynamic curves for Inconel alloys 617 and 625 in a) 450?C, b) 550?C and c) 650?C at air-15% CO2 atmosphere ........................................................................................56 Figure 6-3 Effect of temperature on corrosion rate at air and air-15% CO2 atmosphere for Inconel alloys a) 617 and b) 625 ........................................................................................................63 Figure 6-4 Nyquist diagrams for Inconel alloys 617 and 625 in a) 450?C, b) 550?C and c) 650?C at air-15% CO2 atmosphere ...................................................................................................65 Figure 6-5 effect of temperature on nyquist and bode diagrams for inconel alloys a & b) 625 and c & d) 617 at air-15%Co2 atmosphere...................................................................................66 Figure 7-1 X-ray diffraction (XRD) patterns of base alloys 617 and 625.....................................70 Figure 7-2 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 450?C ........................................................71 Figure 7-3 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 550?C ........................................................71 Figure 7-4 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 650?C ........................................................72 Figure 7-5 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 450?C ........................................72   ix Figure 7-6 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 550?C ........................................73 Figure 7-7 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 650?C ........................................73 Figure 7-8 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 625 in air atmosphere at three temperatures of 450?C, 550?C, and 650?C ...75 Figure 7-9 SEM image of cross section of Inconel alloys in air atmosphere at 550?C; a) Inconel alloy 617 interface with oxide scale, b) oxide scale deterioration of Inconel 617, c) internal attach in Inconel 617, d) Inconel alloy 625 interface with oxide scale, e) oxide scale deterioration of Inconel 625, f) internal attack in Inconel 625 ..............................................77 Figure 7-10 EDX analysis from the oxide layer formed on the surface of Inconel alloys a) 617 and b) 625 in air atmosphere at 550?C exposed to molten salt mixture ................................78 Figure 7-11 SEM photomicrographs of Inconel alloy 617 in air atmosphere, at 550? showing the corroded cross section of the exposed alloy to molten chloride at different part of the alloy-oxide interface; a) less damage to the oxide scale, b) more damage to the oxide scale and higher penetration of deposited salt.......................................................................................79 Figure 7-12 a) SEM photomicrographs of Inconel alloy 617 in air atmosphere at 550?C, b) SEM photomicrographs of grain boundaries close to the surface, c) EDX results obtained from point 1, d) EDX results obtained from point 2, e) EDX results obtained from point 3.........80 Figure 7-13 Schematic of the cross section of the alloy when exposed to molten salt .................82 Figure 7-14 a) SEM photomicrographs of Inconel alloy 617 in air atmosphere at 550?C, b) pitting near the surface, mapping results from the pit showing; c) Cl, d) Cr, e) Co, f) Mo, g) Ni, h) Al .................................................................................................................................84 Figure 7-15 a) SEM photomicrograph of Inconel alloy 617 in air atmosphere at 550?C, X-ray mapping of  b) Cr, c) Ni, d) O, e) Si, f) Pb ............................................................................85 Figure 7-16 X-ray mapping of cr, nI and mo of cross section of alloy 617, exposed to molten mixture, at 650?C and 800?C.................................................................................................86     x List of Symbols Ra Anodic charge transfer resistance Rc Cathodic charge transfer resistance Zw Warburg resistance Cdl Double-layer capacitance Cox Oxide capacitance Rt Charge transfer resistance along localized corrosion zone Rox Ion transfer resistance along the slow corrosion zone ? Nickel-base austenitic phase ?' Precipitated Al and Ti R Corrosion rate M Reacted mass T Time A Faraday?s constant N Number of equivalent exchanged electrons a  Atomic weight icorr Current density Rp Polarization resistance M Molecular weight  m  Oxidation state or valence   xi ? Density Ew Equivalent weight Ni Valence of alloying element ?i? fi Mass fraction of alloying element ?i? Ai Atomic mass of alloying element ?i? Rs Salt resistance Rsl Scale resistance Qsl Ion transfer capacitance through the scale Rdl Double layer resistance Qdl Double layer capacitance ?G? Standard Gibbs free energy Kp Equilibrium constant R Gas constant    xii Acknowledgements  I would like to express my sincere gratitude to my supervisor, Professor Akram Alfantazi, for his continuous support and encouragement throughout my M.A.Sc.   I would like to thank the Materials Engineering Machine Shop staff Mr. Ross Mcleod, Mr. Carl Ng, and Mr. David Torok for their technical input, and Mr. Jacob Kabel in the scanning electron microscopy laboratory for helping with accessing the SEM, and also the corrosion group at UBC for their help through the past two years. I acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Teck Metals Ltd. Finally, I wish to thank my parents and brothers for their endless love, encouragement, and, dedication in every stage of my life until this day. I also wish to thank Lynn and Martin for not letting me feel a day away from home with their constant support and kindness.    xiii Dedication   To:   My Fath e r  and  My Moth e r   1 1. Introduction  Teck-Cominco Metals Ltd. (Teck) uses a KIVCET flash lead smelter. The activities done by this company are mining, smelting, and refining. The two parts of the smelter used in Teck are a radiant shaft boiler and a convection boiler to cool the off-gas. A schematic of a KIVCET furnace with its different parts is shown in figure 1-1.  Figure 1-1 KIVCET furnace used in Teck-Cominco (E. M. Zahrani & Alfantazi, 2013)  Originally, the boiler tubes of the smelter were made out of carbon steel, which led to a high rate of corrosion. The boiler tubes are exposed to high concentrations of SO2 and CO2 gases, and a mixture of chloride salts including PbCl2, ZnCl2, TlCl, and CdCl2. Having molten salt in the boiler`s environment causes hot corrosion in the boiler tube walls. Salt mixtures cause    2 corrosion as the result of condensation on the boiler tubes. As the result of condensation on the boiler tubes, low melting temperature phases arise which result in the deterioration of the oxide layer. Serious costs related to this corrosion problem resulted in Teck wanting to develop an understanding of the corrosion behavior of different alloys exposed to KIVCET boiler conditions. As a result of encountering corrosion with a rapid rate in these boilers, Inconel alloy 625 weld overlay was applied in the radiant boilers on the waterwall tubes in order to decrease the corrosion rate. Inconel alloy 625 is a Ni-Cr-Mo solid-solution, strengthened alloy, which was initially studied and developed for turbines for high temperature applications due to its good wear and corrosion resistance (Martin et al., 2003). This alloy can be used in different forms such as bulk, plasma-sprayed coatings or weld-overlay (Perez et al., 2008). Nevertheless, the boilers in Teck still experienced a moderately high rate of corrosion and corrosion attacks, such as pitting, were seen on the weld overlay Inconel 625. Whether Inconel alloy 625 is the best substitute for the carbon steel in the boilers is still a question. Inconel 617 is another nickel-based super alloy that has high mechanical strength and good oxidation resistance, which makes it suitable to be, used in long term applications in high temperatures (Hilpert et al., 1979). Inconel alloy 617 is a Ni-Co-Cr-Mo solid-solution, strengthened alloy (Cramer & Covino Jr., 2005). The difference between these two Inconels is that Inconel 625 contains niobium but 617 contains cobalt which 625 does not have (Perez et al., 2008).   Inconel alloy 617 may be chosen as a candidate to replace Inconel alloy 625 if shows better corrosion resistance in this environment. To better understand the corrosion behavior of Inconel alloy 625 in this environment and to compare these two alloys, this study focused on the electrochemical corrosion aspects of Inconel 625 and 617.    3   2. Literature Review 2.1 Hot Corrosion In environments containing molten salt, hot corrosion is an issue concerning components in contact with the salt. Hot corrosion is an accelerated form of oxidation. For this type of corrosion to take place, the protective oxide layer formed on the surface of the alloy is destroyed as a result of salt deposition (Donachie & Donachie, 2002). Hot corrosion takes place in environments of molten salts such as sulphates, vanadates, sulphate-vanadate mixtures, or chlorides either in gaseous or solid forms, at high temperatures with oxidizing atmospheres (Bornstein et al., 1973; Rapp & Zhang, 1994). The range of the temperature in which hot corrosion takes place depends directly on the alloy composition, the environment, and the salt mixture that the metal is being exposed to (Stringer, 1977).  2.1.1 Difference between Hot Corrosion and Oxidation The main difference between hot corrosion and oxidation is the rate of corrosion, of which the first is much faster, and will result in the reduction of the component?s load-carrying ability, thus causing failure in the system (Eliaz et al., 2002). Hot corrosion involves reactions between a molten salt and an alloy in contact with a corrosive environment. In environments like air, an oxide film is formed on the surface of the metal and in order for corrosion to happen, the transport of metal through the film via solid-state diffusion must occur. In the presence of a molten salt, salt wets the surface of the oxides and is able to penetrate through the pores by capillary action. Molten salt is considered a medium for the transportation of oxidants inward,    4 toward the metal and of dissolved metal ions outward. This transport is much faster than solid-state diffusion (Shores & Mohanty, 2004).   2.1.2 Hot Corrosion Stages and Attacks In most cases, hot corrosion takes place in two stages: initiation and propagation stages. In the initiation stage, the corrosion rate is slow and presence of salt does not have considerable effect on corrosion behavior. In this stage, the protective oxide layer on the surface is damaged through development of cracks along this layer. This stage may happen as a result of erosion, thermal stresses, erosion-corrosion, or chemical reactions. The behavior seen in the propagation stage, however, is different from not having any salt present in the environment. For the propagation stage, various mechanisms including sulphidation?oxidation and salt fluxing may occur. In the case of the salt-fluxing mechanism, the efficiency of the oxide layer protecting the surface is lost when fluxing of this layer happens in the molten salt. This fluxing may occur as the result of two mechanisms. One is the combination of oxides with O2 to form anions, and the other is the decomposition of oxides into the corresponding cations and O2 (Goebel & Pettit, 1970a; Goebel & Pettit, 1970b; Meier, 1989; Sidhu et al., 2005).  Two forms of attack are often seen in hot corrosion: high-temperature, hot corrosion (HTHC), known also as type 1, and low-temperature, hot corrosion (LTHC), known also as type 2 hot corrosion (Donachie & Donachie, 2002). There are different parameters that affect the occurrence of these two attacks: alloy composition and thermo-mechanical condition, contaminant composition and flux rate, temperature and temperature cycles, gas composition and velocity, and erosion processes (Eliaz et al., 2002). Low-temperature, hot corrosion is observed at 650-800?C where a liquid salt phase is only formed because of the dissolution of some corrosion products. LTHC causes pitting    5 corrosion, resulting from the formation of mixtures of Na2SO4 and CoSO4 with low melting temperatures (Eliaz et al., 2002; Meier, 1989; Rapp, 2002).  High-temperature, hot corrosion, however, is observed above the melting point of Na2SO4 at 850-950?C. At the beginning of HTHC, some growth and localized breakdown of the oxide scale layer results in a roughened surface. As the breakdown continues, a rougher surface followed by a decrease in chromium content is achieved. As a result, oxidation of the alloy will continue to a significant depth of the metal. Thus, failure of the component occurs. HTHC initiates when the molten salt accesses directly to the substrate metal. The mechanisms proposed for the propagation stage in HTHC are the sulphidation-oxidation mechanism and the salt fluxing mechanisms (Eliaz et al., 2002; Meier, 1989).  Corrosion in molten salt can be explained by the anodic dissolution of metal (ia) and cathodic reduction of oxidants (ic) under a mixed potential. ?The corrosion of metals and alloys in an acidic melt (low oxide ion concentration) seem to be under cathodic control, while in a basic melt (high oxide ion concentration), it is under anodic control? (Nishikata et al., 1991). It should be noted that the reaction causing corrosion does not necessarily control the corrosion rate. In several studies, diffusion of dissolved metal ions was proposed as the rate-controlling step for the phenomenon of corrosion in molten-salt mediums (Espinosa-Medina et al., 2009; Rahmel, 1987). This type of corrosion is called molten salt-induced corrosion. Molten salt-induced corrosion occurs during the initial stage of corrosion when a thin oxide layer is gained (Kawahara, 2007).  In fused salts, all corrosion types such as stress-assisted, galvanic, erosion, and fretting corrosion can be seen. In this type of corrosion, the same concepts concerning aqueous corrosion are involved, for example, the anodic reactions leading to metal dissolution and cathodic    6 reduction of an oxidant. For this reason, electrochemical test methods can be used in molten salts as well (Cramer & Covino, 2003). Having fused salt in the environment when experimenting with hot corrosion acts as an ionic conductor and makes it possible to conduct the investigations using electrochemical techniques (Gao et al., 1990). Different electrochemical methods such as potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy have been used to study molten salt environments. Electrochemical impedance spectroscopy (EIS) provides an understanding of reaction mechanisms and kinetics of corrosion. Corrosion in molten salt environments is similar to aqueous solutions with a much faster rate. In these environments, usually a thick scale is formed on the surface of the sample in contact with molten salt. Since the temperature in a molten salt environment is higher than in an aqueous environment, various corrosion forms may take place such as uniform, localized, or internal corrosion. Depending on the chemical stability of the metal that is used in the molten salt environment, the metals are divided into non-active and active metals. A schematic diagram showing corrosion in these two types of metals is shown in figure 2-1 (Zeng et al., 2001).  High-temperature electrochemical impedance spectroscopy results obtained in various molten salt environments confirm characteristics of a diffusion-controlled reaction (Zeng et al., 2001). In corrosion of active metals in molten salt environments, charge transfer reaction may occur easily and not be rate limiting, which is different from the cases seen in aqueous solutions. Rate limiting processes in these environments might be the oxidant species transportation in the molten salt and the transportation of ions in the oxide scale on the surface of the alloy. When having a porous scale on the surface of the metal, anodic and cathodic charge transfers happen fast and are not rate limiting. The rate-limiting process in this case is the oxidant species transport in the melts. The impedance spectrum for the porous scale in these environments    7 consists of two segments; at high frequencies a semi-circle and at low frequencies a line. A schematic impedance spectrum obtained as a result of having a porous scale can be seen in figure 2-2 (Zeng et al., 2001).   Figure 2-1 A schematic diagram for non-active and active metal corrosion in molten salt: (A) non-active metals, (b) active metals forming a porous scale, (c) active metals forming a protective scale, (d) active metals with localized, fast corrosion (Zeng et al., 2001). The schematic of another impedance spectrum that is obtained in molten salt environments is shown in figure 2-3 (Zeng et al., 2001). This spectrum represents two corrosion mechanisms that take place on the surface of the alloy: active metals that form a protective scale when exposed to molten salt environments or localized corrosion of a metal in molten salt. The impedance spectrum consists of two semi-circles: at high frequencies it contains a small semi-circle and at low frequencies a big semi-circle (Zeng et al., 2001). The corrosion rate in molten salt can be slowed down in cases of active metals forming a protective scale. The rate-limiting process in this case would be the ions transportation in the scale on the surface of the alloy (Zeng et al., 2001).    8  Figure 2-2 Schematic impedance spectrum representing active metals forming a porous scale (Zeng et al., 2001). When partial failure takes place in the protective scale, localized corrosion might occur in the metal. The region attacked by localized corrosion is either covered with a non-protective scale or is in contact with molten salt directly (other parts of the metal may be covered with a scale that protects the surface in that region) (Zeng et al., 2001).   Figure 2-3 Schematic impedance spectrum representing active metals forming a protective scale or localized corrosion of a metal in molten salt (Zeng et al., 2001).  The impedance spectrum in the cases of having protective scale on the surface of the metal or localized corrosion of metal is the same. The equivalent circuits representing these two spectrums; however, are different. In order to choose one of these circuits to discuss the    9 corrosion mechanism that is taking place on the alloy when exposed to molten salt, the cross section of the alloy after exposure to molten salt must be studied using scanning electron microscopy (SEM). Based on whether seeing a protective scale on the surface of the alloy in the SEM results or localized corrosion on the interface of the alloy with salt, the relevant circuit is chosen, and corrosion mechanism on the surface of the alloy is described with details.  The equivalent circuit describing each of these metals and their condition is shown in figure 2-4. The symbols used in each of the circuits are shown in table 2-1.       Figure 2-4 Equivalent circuit for (a) active metals forming a porous scale (Perez et al., 2008; Zeng et al., 2002), (b) active metals forming a protective scale (Zeng et al., 2001; Zeng et al., 2002), (c) active metal encountering localized corrosion in molten salt (Zeng et al., 2001; Orazem et al., 2008).  b a c    10 Table 2-1 Equivalent of symbols used in the circuits (Zeng et al., 2001) Symbol Equivalent Rs Solution resistance W Warburg resistance Rdl Double-layer resistance Cdl Double-layer capacitance Cox Oxide capacitance Rt Charge transfer resistance  Rox Ions transfer resistance along the slow corrosion zone  2.2 Salt Mixtures Causing Hot Corrosion Depending on the application of a high temperature environment, various compositions of salts can be seen. Examples of different salt mixtures are sulphates or chlorides of alkali metals, or a mixture of them. It has been shown that the corrosion rate in the presence of alkali metal chloride is much faster due to the presence of a salt pathway for oxidants to reach the metal. Also in oxidizing conditions, solubility species-containing Cr or Fe are higher in alkali metal chlorides than in alkali metal sulphates (Mohanty & Shores, 2004). When an oxidizing environment also contains contaminants such as sulphur and/or chlorine, the rate of attack and thus the corrosion rate depend on the environment the alloy is in contact with. Corrosion rate of the alloy increases in the following order: ?oxidation only < oxidation + internal sulphidation  < oxidation + internal chlorination < hot corrosion (sulphate salts) < hot corrosion (sulphate + chloride salts)? (Mohanty & Shores, 2004). The most important constituent of the molten salt layer on the alloy after exposure to the molten salt environment, however, is sodium sulphate because of its high thermodynamic stability. Having said that, sodium chloride, heavy metal cations, and carbon could cause severe corrosion damages (Sidhu et al., 2006; Stringer, 1977). Presence of chloride and sulphates in the environment may cause pitting on the alloy in contact with this environment (Montgomery et al., 2011; Zahrani & Alfantazi, 2012). Chlorides are considered the most important species among different salt mixtures, especially when they are    11 combined with alkali metals (Na, K) and heavy metals (Pb, Zn) since they are more reactive in presence of metal, and form metallic chlorides. All common metals have a high solubility when they are in chloride form. Having chloride ions in the electrolytic solution affects the corrosion mechanism that takes place in the system due to the breaking down of the oxide layer that protects the metal surface (the more the chloride content, the easier the shrinkage of the passive region). Also, the increase in the solution temperature results in the susceptibility to both pitting and active dissolution (Neelofar et al., 2008; Bloom, 1963; J?rgen et al., 2004). The process taking place in the presence of alkali metal chlorides is called the ?chlorination-oxidation cycle?. In this process, active oxidation (inward penetration of chlorine into the scale, formation of chlorides at the oxide/metal interface, evaporation of the chlorides and conversion of the evaporating chlorides into oxides (J?rgen et al., 2004 ; Sanchez Pasten & Spiegel, 2006) as shown in figure 2-5 destroys the protective scale, which prevents the re-establishment of a protective scale (Mohanty & Shores, 2004). One environment in which molten chloride mixtures are formed is the waste to energy boilers that are used in waste incineration plants (Perez et al., 2008).   Figure 2-5 Schematics of the reaction circuit in active oxidation of iron and steels (Mohanty & Shores, 2004).   Usually the surfaces in contact with molten salt, on which heat transfer occurs, are the ones suffering from fast corrosion because of the high amounts of potassium and chlorine, which exist in both gas state and deposit form. In cases where the alloy contains Cr, the amount of the    12 Cr in the alloy has an important role, since a Cr2O3 protective scale forms on the surface in the presence of air or oxygen (Li & Spiegel, 2004; Wang & Shu, 2003). Constituents such as sulphur, chlorine, potassium, sodium, zinc, and lead, form salts with a low melting eutectic point; as a result of that, different corrosion attacks like oxidation, sulphidation, chlorination, and hot corrosion may occur. As a result of the interaction taking place between sulphur oxides and metal chlorides (from the deposits), the attacking, mainly by HCl and Cl2, occurs. The chlorine produced is often seen in the middle of the external oxide scale and the substrate or in other parts of the metal substrate in the case of internal chloridation (Pan et al., 2007). As already mentioned, heavy metals like lead and zinc can assist in the formation of low-melting chlorides, especially in waste streams (Baker et al., 2001; Kawahara et al., 2000). Environments containing PbCl2 and KCl increase the corrosion rate of the components in the system. Adding KCl to the salt increases the conductivity of the mixture, and dominates PbCl2 vapor pressure. This mixture is liquid at 450?C in a composition range of 15 to 55% mol KCl. In higher temperatures in the presence of KCl, higher conductivity is achieved (Bloom et al., 1947; Guibert & Plichon, 1978). In the presence of KCl in this environment, the deterioration of protective scales happen, which makes it possible for a direct attack by molten salt (Ma et al., 2009). KCl has a high melting point (above 700?C) but it can form low-temperature eutectics in solution with PbCl2 (Gabriel & Pelton, 1985). Lu et al. (2008) studied the corrosion of five Fe-Cr steels in molten ZnCl2-KCl. All of the specimens formed a porous scale and suffered from accelerated corrosion. The microstructure shows that the Cl-containing species were able to reach the metal matrix. By increasing the Cr content of the steels, the corrosion rate decreased, however, the steel sample that contained the highest amount of Cr, could still not ensure good corrosion resistance in the ZnCl2-KCl deposit. In environments containing chlorine ash deposits, however, the corrosion rate increases even more by several mechanisms (Uusitalo et al., 2004):     13 1. Chlorides lower the first melting temperature of deposits and, as a result, the protective scale might get washed away.  2. The lower partial pressure of chlorine in comparison with deposits containing chloride might cause chlorine attack.  Cross sections of a few alloys in contact with different environments containing molten salt can be seen in figure 2-6. Different corrosion mechanisms are observed for hot corrosion in various environments (Elliott, 1989).   Figure 2-6 (a) Porous, cracked oxide scale; (b) deposit/scale with intergranular corrosion in a boiler superheater tube; (c) stratified scale from recuperator where fused salts were present; and (d) gross internal fluoridation with oxidation (Elliott, 1989).  2.3 Industries Concerned with Hot Corrosion Since the 1940s, hot corrosion has raised component issues in gas turbines, boilers, and industrial waste incinerators. Having said that, hot corrosion was not considered an important !!   14 matter until the late 1960s when gas turbine engines encountered serious corrosion problems in military aircrafts (Rapp, 2002; Sidhu et al., 2005). Other processes involved with high-temperature environments include fossil fuel engines, fuel cells, chemical and petrochemical heat-treating, and power plants (Cramer & Covino Jr., 2005; Gao et al., 1990; Klarstrom & Srivastava, 2005). In chemical reactions, water has always been the most commonly used solvent until the need for solutes like metals and gases appeared. For these cases, molten salt systems play an important role due to the higher amount of metals or gases that can be dissolved in them (Sorell, 1997).  Regarding boilers, as a result of the reaction between salt and the water walls, metal sulphides are formed on the surfaces. Formation of solid particles and the accumulation of them on the surfaces of the water walls will result in components damaging and cause corrosion attacks (Budzinski et al., 1997; Kung & Bakker, 1997). On boiler tubes, molten-salt attack may occur in one of the following two cases: combustion products are formed as a result of having low-melting compounds or in cases of deposit formation of eutectic mixtures with low-melting points. In the operation range of the furnace wall tubes and super heater tubes, many of the metal chloride combinations become molten. In municipal waste incinerators, salt mixtures are often found to contain SnCl2, ZnCl2, and PbCl2 (Otero et al., 1999).   2.4 High Temperature Alloys  In many industries that require a high temperature environment such as materials processing, power generation, and chemical engineering in order to have a better efficiency of fuel conversion and use of it, the right material must be chosen. High-temperature materials can be used in environments with a wide range of mechanical and chemical conditions (Kewther et al., 2001a). High temperature alloys have shown a wide range of applications in many industries    15 such as gas turbine and power generation. High temperatures speed up corrosion degradation thus, choosing the right alloy in these conditions is crucial. Different applications based on their temperature are shown in figure 2-7 (Elliott, 1989).    Figure 2-7 Common process temperatures (Elliott, 1989).  Materials used in these environments must form a scale to resist the ?excessive metal loss? occurring at high temperature. The most commonly used candidates for high temperature applications are alloy steels and nickel or cobalt-based alloys (Elliott, 1989). Some alloys, designed to be used at high temperature environments, are superalloys based on their basic binary systems of Fe-20%Cr, Ni-20%Cr or Co-30%Cr. These include alloys like Hastealloys, Incoloys, Inconels, Nimonics, and Waspalloy (Elliott, 1989). Among suitable alloys for high temperature purposes, nickel-based superalloys have shown greater resistance to specific types of corrosion that happen at high temperature. Compared to cobalt-based and iron-based alloys,    16 they show greater resistance to oxidation.  Compared to other alloys, they also have higher resistance to carburization and nitridation attack, as well as to environments containing halogen due to lower interstitial atom solubility and formation of halogen compounds with high melting points, respectively (Klarstrom & Srivastava, 2005). Alloys exposed to chlorine-containing molten salt environments are usually damaged from chloride attack. Low-alloy steels encounter a high rate of corrosion in these environments due to the high partial pressure of iron chloride at high temperatures. Nickel-based alloys, however, are more resistant than steel because partial pressure of nickel chloride is significantly lower than partial pressure of iron chlorides, and the Gibbs free energy change of NiCl2 formation is less negative than that of FeCl2 formation (Uusitalo et al., 2004). The amount of the chromium content in the nickel-based superalloys plays an important role on the behavior of these alloys in molten salt environments. Studies show that with an increase in the chromium content of the alloy, the corrosion resistance will increase. However, chromium can act as a solid-solution strengthener. The lower the amount of chromium, the higher the portion of the strengthening phase (Peters et al., 1976). However, superalloys cannot always exhibit the two properties of both high-temperature strength and resistance to corrosion at high-temperature at the same time (Oh et al., 1986). Where a relatively high amount of chromium is seen in molten salt environments, depending on the salts presented, various reactions between the salt and the chromia scale made on the surface may take place. According to Spiegel (1999), three of which could be:    Equation 2-1  Equation 2-2  Equation 2-3    17 All the above reactions will result in the consumption of the alloy?s chromium, the formation of spinel and nickel oxide at the beginning of exposure to molten salt, and the creation of chlorine, which will result in the formation of metal chlor1ides on the surface of the metal, thus causing corrosion with a high rate (Spiegel, 1999).  For the Fe-Cr and Ni-??Cr alloys with low Cr contents, the oxidation resistances improve as a result of spinel-type oxides formation. For the ones having Cr content higher than 20wt%, the oxidation resistance improves more due to the formation of a stable Cr2O3 film (Nishikata et al., 1991). Having said that, over-alloying with Cr may not cause any improvement in the corrosion layer stability towards dissolution in an environment containing molten salt (Tzvetkoff & Gencheva, 2003). One environment in which nickel super alloys are used is one containing NOx. The environment of most boilers is considered to be oxidizing, in which a protective oxide layer is formed on water wall tubes that are made out of steels with grades of 11 or 12. In these boilers, failure is not considered a big issue. In comparison, because of delaying the mixing of fuel and oxygen and, therefore, lack of oxygen in the staged combustion boilers, the atmosphere is considered a reducing one. In such environments, low alloy steels have short lifetimes, due to the formation of corrosive deposits on the water wall tubes. In order to obtain a longer lifetime, a more corrosion-resistant alloy is coated on top of the surface. Nickel-based super alloys such as Inconel 622 and 625 have been studied for this purpose. Even in the case of these alloys, two scales, internal and external scales can be seen using electron microprobe maps. In the case of alloy 622, molybdenum is the main element seen in the scale (Deacon et al., 2007; Li & Spiegel, 2004).     18 2.5 Nickel-based Super Alloys Inconel alloys are examples of high temperature alloys that have shown a wide application in industries including components of gas turbine engines that operate in high temperature (Kewther et al., 2001a). Nickel-based alloys can be used in aircrafts, marine, industrial and vehicular gas turbines, space vehicles, rocket engines, steam power plants, and nuclear reactors (Ezugwu et al., 1998; Meetham, 1991). Also, in applications where both wear resistance and hot corrosion resistance are of importance, nickel-based coatings are used (Sidhu et al., 2006).  Nickel-based superalloys contain the following phases (Sims et al., 1987): 1. Gamma (?), which forms a continuous matrix with FCC crystallographic structure. 2. Gamma prime (???), which are Al and Ti that precipitate in the matrix. Gamma prime phase is rich in aluminum, titanium, and tantalum elements, and the compound is Ni3 (Al, Ta, Ti). The gamma prime is seen as the precipitate phase coherent with matrix (Karunaratne & Reed, 2003).  3. Carbides, which are formed as the result of combination of carbon with reactive elements. Carbides can be seen in M23C6 (?M? is rich in Ni, Nb and Mo (Mukherjee et al., 2006)). and M6C in grain boundary regions.   The two most widely used nickel-based super alloys that are more likely to show resistance to corrosion in high temperature applications are Inconel alloys 617 and 625, due to their unique properties and resistivity to corrosion in these environments.  Inconel 617 has high strength and good oxidation resistance which makes it suitable to be used in long-time applications at high temperatures (Hilpert et al., 1979). Inconel alloy 617 is considered a solid solution, strengthened, Ni-Co-Cr-Mo alloy that has good corrosion resistance, exceptionally high temperature strength, good cyclic oxidation resistance at 1093?C and high    19 creep rupture strength at temperatures from 649?C to 1093?C. It also has a combination of high strength and oxidation resistance at 980?C (Cramer & Covino Jr., 2005; Hosier & Tillack, 1972; Liu et al., 2011; Mankins et al., 1974a; Special, 2005). Excellent properties of this alloy, in high temperature environments, make it a good candidate to be used in thermal systems (Yilbas et al., 2001). High Ni and Cr content in this alloy make the alloy resistance to both reducing and oxidizing environments. The aluminum and chromium contents protect the alloy when exposed to high-temperature oxidation environments. Co and Mo are the causes of solid solution strengthening. These elements also allow the alloy to withstand many wet corrosive environments (Kewther et al., 2001b; Speidel, 1974). Being resistant to oxidation and having high strength makes this alloy a good candidate to be used in high temperature environments such as gas turbine engines (Gonzalez-Rodriguez & Fionova, 1998; Hirose et al., 1998). Heat treatment of this alloy for the purpose of gas turbines and its effect on metallurgical and mechanical properties has also been studied (Kewther et al., 2001). It showed that oxygen-active elements like Al and Ti form a protective oxide layer by their preferential oxidation through their external diffusion which enhanced the corrosion resistance of the alloy (Cho et al., 2008; Cho et al., 2009).  Studies done on microstructure and mechanical properties of Inconel 617 after going through high temperature corrosion showed that the first oxide layer formed on the surface when exposed to molten salt was Cr-oxide. Below this layer, internal oxide and a region with depletion in Cr were also seen. Internal oxide was formed along the grain boundary. As the time of exposure increased, the internal oxide depth increased (Esmaeili et al., 1995; Jo et al., 2007; Yun et al., 1984). The second phases in this alloy are M23C6 (determined to be Cr21Mo2C6) and M6C (determined to be Mo3Cr2(Ni,Co)1C) carbides. M23C6 were mostly seen in the matrix and M6C were mostly observed on grain boundaries. This alloy has an austenitic matrix. Bright particles in    20 the matrix can also be seen which are Ti(C,N) (Mankins et al., 1974; Shah Hosseini et al., 2011; Totemeier et al., 2005). One of the environments in which this alloy is used is the one containing molten salt, an example of which is the heat-treating industry. In such environments, the molten salt dissolves the oxide layer protecting the metal surface from getting corroded away. As a result of that, the oxide is dissolved in the molten salt and corrosion occurs by oxidation of the alloy. The water vapor and oxygen that are in the molten salt increase the corrosion rate (Cramer & Covino Jr., 2005; Klarstrom & Srivastava, 2005). Inconel alloy 617 could also be used for protection against metal dusting under the harshest conditions or it could be used as a filler metal (Baker & Smith,. 2001; Shoemaker  et al., 2008).  Inconel alloy 625 is a Ni-Cr-Mo alloy that shows great resistance to pitting corrosion in marine service. Inconel 625 is a solid-solution, strengthened alloy, which was initially studied and developed for turbines in high temperature applications due to its good wear and corrosion resistance (Cieslak et al., 1988; DuPont, 1996; Martin et al., 2003). However, in waste-to-energy (WTE) boilers, pitting corrosion has been observed on this alloy (Lai, 2004). Even though this alloy has high resistivity to pitting corrosion, it has exhibited failure in some corrosive environments and mechanisms such as waste incineration factories at 400?C (Spiegel, 1999; Zahrani et al., 2010). This alloy can be used in different forms such as bulk, plasma-sprayed coatings, or weld-overlay. It is used mainly for high temperature purposes even though it may not be able to tolerate some environments (Perez et al., 2008; Spiegel, 1999). 2.6 Hot Corrosion in Boilers As already discussed, one environment in which molten-salt attack occurs are boilers. This attack takes place if compounds with low-melting points or eutectic mixtures with low-melting points are formed as combustion products or deposits, respectively. The most common    21 molten salt sources in boilers are metal chlorides (PbCl2 and ZnCl2, for instance). These mixtures of chlorides are in a molten phase in the operating temperature of the furnace wall and superheater tubes (Otero et al., 1999). Compounds containing chlorides, whether in solid or liquid forms, accelerate the corrosion rate; the mechanisms of this corrosion, however, are still being studied. One explanation brought out by Reese and Grabke is called the active oxidation mechanism, which suggests that the rapid attack is due to the formation of gaseous chlorine. Spiegel, however, suggests that rapid corrosion is the result of molten salt per se by dissolving the oxide layer. One kind of alloy that has shown good behavior in these environments is nickel-based superalloys (Otero et al., 1999). Otero et al. (1999) studied corrosion behavior of an IN-800 (which mainly contains Ni, Cr, and Mn) superalloy in a molten mixture of PbCl2-KCl at its eutectic composition at different temperatures. In general terms, corrosion of this alloy in this environment was low and the corrosion rate increased as the temperature increased. Kawahara (2002) studied the corrosion products on SA213-T12 steel and alloy 625 in an actual chemical composition of waste-to-energy boilers environment (containing Pb, Zn, Ca, and K). This study suggested a new corrosion model based on the oxide scale formed on the surfaces of the alloys under high pressure of O2 and Cl2. In this model, reactions of the elements in the alloy were according to the following reactions:  Equation 2-4  Equation 2-5 Perez et al. (2008) studied hot corrosion of nickel-based superalloys 617 and 625 in molten KCl-ZnCl2, which is a mixture seen in boiler superheater tubes and waste incineration plants. They monitored the corrosion process by electrochemical impedance spectroscopy (EIS).    22 They concluded that these two alloys form a protective oxide scale at the beginning of exposure, which altered to a porous scale at longer exposure times. In the initial stages, both Inconel alloys 617 and 625 formed a protective scale on their surfaces. After 24 hours, however, Inconel 617 showed localized corrosion in this molten salt environment due to partial failure in the scale. For Inconel 625, this mechanism was seen after 100 hours. The difference between these two Inconels is that Inconel 625 contains niobium but 617 contains cobalt. The scanning electron microscopy (SEM) analysis confirmed the presence of Cr2O3 layer on the surface of both alloys. The following two reactions explained the formation of Cr2O3 on the surface of the two alloys:  Equation 2-6  Equation 2-7 Chlorine afterwards reacts with the chromium oxide on the surface and forms CrCl3 based on the reaction below; because of the evaporation of CrCl3, this process was thermodynamically favored. This chromium oxide is initially formed on the surface of the alloy as the result of having Cr content of the alloy in contact with the oxygen in the environment in the interface of the alloy with salt.   Equation 2-8 ZnCl2 was consumed at the same time because of its strong evaporation and oxidation in order to form ZnO. ZnCr2O4 spinel was formed afterward as a result of the reaction between ZnO and Cr2O3. Based on a study done by Ishitsuka and Nose (2002) on the solubility of Cr2O3 in molten NaCl-KCl, Cr2O3 films are able to easily dissolve in molten salt. As a result of this dissolution, a hexavalent chromium ion is produced based on the following reaction:    23  Equation 2-9 As the chromium amount near the interface of the alloy-oxide decreased and the Cr2O3 scale stopped protecting the surface of the alloy, other elements formed NiO and MoO3. These scales were found to be porous. In Inconel alloy 625, formation of oxides after the decrease in chromium content near the interface of alloy-oxide occurred through the below reactions:  Equation 2-10  Equation 2-11  Equation 2-12 Viklund et al. (2011) studied the corrosion behavior of some common superheater materials among which were ferritic steel 13CrMo44 and Hastelloy C-2000 and Inconel 625, both of which are nickel-based alloys at different exposure times in an alkali chloride environment. In this study it was concluded that the nickel and molybdenum content of higher-alloyed materials lowered the formation of metal chloride. The oxide scale for these alloys had a thinner thickness (one order of magnitude) compared to lower-alloyed materials. The metal-deposit surface of these alloys can be seen in figure 2-8 and figure 2-9. However, so far no specific study has been dedicated to the study of Inconel alloys 617 and 625 (two nickel-based alloys which are considered to have good corrosion resistance at high temperatures) in a molten PbCl2-KCl mixture, which is a common mixture among mixtures observed in most boiler tubes.    24  Figure 2-8 SEM of a cross-section through the low alloyed ferritic steel 13CrMo44 after 1550 hours at 440?C (Viklund et al., 2011).  Figure 2-9 SEM-imaging showing localized attack on the nickel-based alloys C-2000 (a) and Inconel 625 (b) (Viklund et al., 2011). Zahrani and Alfantazi (2012) studied the corrosion behavior of Inconel alloy 625 in a complex mixture of PbSO4-Pb3O4-PbCl2-ZnO at different temperatures. The potentiodynamic polarization cures obtained for Inconel 625 in this salt mixture indicated that, as the temperature increases, the corrosion rate of this alloy increases and the curves shift to the right side of the diagram. The potentiodynamic diagram of this alloy is shown in figure 2-10.    25  Figure 2-10 Potentiodynamic polarization curves of Inconel alloy 625 in different temperatures (Zahrani & Alfantazi, 2012). At high temperatures, the electrical conductivity of molten salt increases and viscosity decreases. This results in faster transportation of the chemical-active species and thus increases corrosion activity. Results confirmed the dissolution of chromium through an active oxidation process as CrO3, Cr2O3, and CrNbO4. Nickel also dissolved in the system as NiO (Zahrani & Alfantazi, 2012). 2.7 Pseudo-Reference Electrode Selection  Selecting the right pseudo-reference electrode in molten salt environments is important due to the harsh environment they are in contact with and high corrosion rates. Platinum has been used in a variety of studies in different molten salt environments. This shows that platinum is a suitable choice as a pseudo-reference electrode in molten salt environments. Some studies that used platinum are reviewed in this part of the thesis.    26 Platinum has been used as a pseudo-reference electrode in a variety of chloride molten salts. In an environment containing a molten salt mixture of PbCl2- 48%KCl, a three-electrode cell system was used by Otero et al. for polarization and EIS studies of 12 CrMoV alloy at three temperatures of 450?C, 500?C, and 550?C. In this three-electrode system, platinum was used both as a counter electrode and a pseudo reference electrode (Otero et al., 1998).  Barraza-Fierro et al. used platinum wire as a pseudo-reference electrode in a three-electrode system in a molten salt mixture of LiCl-55%KCl to study corrosion behavior of Fe- 40%Al at temperatures of 450?C, 550?C, and 600?C. Platinum, in this study, was immersed directly in molten salt (Barraza-Fierro et al., 2012). Platinum has been used in molten salt mixtures not containing chlorides as well. Martinez-Villafane et al. studied electrochemical behavior of SA213-T22 and SA213-TP347H in a molten salt mixture of V2O5- 20%Na2SO4 using a three-electrode cell system. In this study, several three-electrode cell systems were used. However, since platinum contributed to results which were reproducable, it was used as both a counter electrode and a pseudo-reference electrode to study the electrochemical behavior of the mentioned alloys (Mart?nez-Villafa?e  et al., 2003). Espinosa et al. also used a three-electrode system in molten Na2SO4. In this study, two platinum wires were used to determine whether platinum is a suitable choice for this environment or not. The potential difference between the two wires after immersion in molten Na2SO4 with time was monitored, which stayed stable (Espinosa et al., 2003). Gonzalez-Rodriguez et al. studied the electrochemical behavior of a Ni3Al compound in a molten carbonate mixture of LiCO3-48% K2CO3 using a three-electrode system with platinum as a counter and pseudo-reference electrode which were placed in a ceramic tube filled with refractory ceramic cement to avoid exposure of the higher parts of the wires to the salt (Gonzalez-Rodriguez et al., 2009).       27 3. Objectives   The objectives of this thesis are to:  ? Compare electrochemical corrosion behavior of Inconel alloys 625 and 617 in waste heat boiler environments of molten PbCl2- 35.38% KCl.  ? Study electrochemical corrosion behavior of these two alloys in the mentioned salt mixture at temperatures of 450?C, 550?C, and 650?C to find out the effect of the temperature on this comparison. ? Identify the effect of CO2 on the behavior of these Inconel alloys by studying the electrochemical corrosion behavior of them in the same molten salt mixture under two atmospheres of air and air- 15%CO2. ? Compare the corrosion rate of alloys 625 and 617 using various electrochemical methods to understand the corrosion performance of these alloys in waste heat boiler tubes. ? Investigate the electrochemical corrosion mechanism of these two alloys using electrochemical impedance spectroscopy (EIS) in the same salt mixture. ?  Analyze SEM and XRD results obtained from a cross section of the samples exposed to this molten salt environment to confirm the purposed corrosion mechanism.     28  4. Experimental Procedures The purpose of conducting these experiments is to compare the corrosion behavior of Inconel alloys 617 and 625 when exposed to molten PbCl2-35.38% KCl. In this chapter, materials used, their preparation, and electrochemical and characterization methods are discussed.  4.1 Material Preparation To prepare working electrodes, samples were cut in rectangular shapes with dimensions of 13 ? 10 ? 1.5 mm. The nominal chemical compositions of the samples are shown in table 4-1. Samples were finely ground with 240, 320, 600, and 1200 grit silicon carbide paper before testing. To make electrical connections, samples were spot welded to 80Cr-20Ni (wt. %) wires. All electrodes were passed through an alumina tube and sealed with ceramic sealant to prevent exposure of the connections to the molten salt. The schematic of the working electrodes can be seen in figure 4-1.  Figure 4-1 Schematic of the working electrodes.     29 Table 4-1 Chemical composition of Inconel alloys 617 and 625 (Cramer & Covino Jr., 2005). Composition (wt%) Alloying elements 617 625 Ni  52 61 Cr  22  21.5 Co  12.5  -  Nb+Ta - 3.6 Mo  9 9 Ti  0.3 0.2 Fe  1 2.5 Mn  - 0.2  Al  1.2  0.2 C  0.07  0.05 Si 0.5 0.2  To prepare the electrolyte, a salt mixture of PbCl2 with a purity of 99% and KCl, with a purity of 99.4%, was ball-milled for 24 hours. A eutectic composition of the salt was used which contained 35.38 Wt.% KCl. The phase diagram of PbCl2 - KCl and its composition on the diagram is shown in figure 4-2. The chosen composition has the lowest solidification temperature, highest fluidity, and a uniform composition at all parts of the bulk.     30  Figure 4-2 Phase diagram of PbCl2 ? KCl (Gabriel & Pelton, 1985).   4.2 Experimental Apparatus For the purpose of electrochemical corrosion tests, a three-electrode electrochemical cell was made before each experiment. To prepare electrochemical cells, Inconel alloys 617 and 625 were used as working electrodes and two platinum wires with a diameter of 5mm were used as a pseudo-reference electrode and a counter electrode. Both pseudo-reference and counter    31 electrodes were passed through alumina tubes and sealed with a ceramic sealant. The schematic of the electrochemical cell in the furnace can be seen in figure 4-3.    Figure 4-3 a) Schematic of the electrochemical cell b) Vertical tube electric furnace c) Electrochemical cell design The prepared salt was put into a 30 ml alumina crucible, with a melt depth of about 3.5 cm, where the electrodes were fixed and surrounded by the salt. The working electrodes with the height of 1.3 cm were fully immersed in the salt throughout all the experiments. The crucible and the salt used were changed before each experiment. The setup was placed inside a vertical tube    32 electric furnace (VF-1200X) and ramped at 60?C.min-1 up to the operating temperature (450?C, 550?C, and 650?C) under air or air- 15%CO2 exposure. Possible temperature variation in the salt was not considered, since the crucible containing salt was small and did not affect the corrosion rate of the alloys. The complete setup of the experiments is shown in figure 4-4.  Figure 4-4 Setup of the experiments.    4.3 Electrochemical Experiments To study corrosion behavior of Inconel alloys 617 and 625, various electrochemical methods, potentiodynamic polarization, linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS), were done on the samples. To make sure the samples had reached the stable state; open circuit potential (OCP) measurements were done prior to each    33 study. For electrochemical analyses, a Princeton Applied Research (PAR) potentiostat-frequency analyzer model 273 was used. The applied parameters for each of these electrochemical methods are shown here.  4.3.1 Open Circuit Potential Open circuit potential was recorded for 12 hours before each experiment and the potential versus time plots were obtained. All the tests were done on Inconel alloys 617 and 625 at temperatures of 450?C, 550?C, and 650?C, and under two different atmospheres of air and air- 15%CO2.   4.3.2 Potentiodynamic Polarization            Potentiodynamic polarization tests were performed with a potential scan rate of 1 mV.s-1, in a potential range of -0.5V to 2V versus open circuit potential, after the given time of 12 hours in OCP and having the samples reached stability in molten salt. These experiments were used to estimate the corrosion rate of the alloys in each condition.   4.3.3 Linear Polarization Resistance             Linear polarization resistance (LPR) tests were performed on the samples with a potential sweep rate of 0.166 mV.s-1 at ?30 mV versus open circuit potential. The resistance was calculated using the plots obtained afterwards. These experiments were conducted to calculate the corrosion rate and compare them with the rates calculated from potentiodynamic polarization. Since the potential ranges used to calculate corrosion rate in these experiments are closer to corrosion potential, less secondary reactions would take place compared to potentiodynamic polarization and the surface of the samples are closer to the surface obtained    34 after OCP experiments, and thus makes LPR a more precise method for calculating corrosion rate.  4.3.4 Electrochemical Impedance Spectroscopy               Electrochemical impedance spectroscopy measurements were performed to study the corrosion mechanism at the interface of the samples with the electrolyte. EIS measurements were carried out at OCP with the current amplitude of 10 mA. 4.4 Material Characterization The surface and cross-section of the samples were studied after exposure to the salt.  In the following, the metallography of the samples used for this purpose and the methods used are shown in detail.   4.4.1 X-Ray Diffraction Phase Analysis The surface of the exposed samples after washing the salt away were characterized using a Rigaku model MultiFlex X-ray Diffraction (XRD) technique to obtain oxide layer phases. The selected scan speed used for these samples was 2 degrees/min, and 2? was between 2? and 90? to make sure all the possible peaks were obtained. To identify phases, the XRD patterns obtained from the surface of the samples after exposure were compared to the standards compiled by the ?Joint Committee on Poweder Diffraction and Standards? (JCPDS).  4.4.2 Scanning Electron Microscopy To study the edge of the samples where the oxide layer was formed, the cross sections of the samples after exposure were studied using a Hitachi model S3000N VPS Scanning Electron    35 Microscopy (SEM).  The exposed samples were cut from the middle and cold mounted. To prepare the samples for the SEM analysis, they were ground with 600 and 1200 grit silicon carbide paper and polished with 6 and 1 ?m alumina powder. Afterwards, the samples were cleaned in deionized water ultrasonically, rinsed with ethanol, and dried in hot air.   4.4.3 Energy Dispersive X-Ray Spectroscopy For elemental analysis and to characterize chemical composition of the oxide layer Energy Dispersive X-Ray Spectroscopy (EDX) was used. These studies were done on the cross section of the samples so that comparison among the composition of oxide layer, surface of the sample in direct contact with the salt and other parts of the sample not in direct contact would be possible. To better observe this chemical change at different part of the samples, X-ray mapping was also conducted.   4.4.4 Differential Thermal Analysis The melting temperature of the salt mixture which was in contact with the samples was measurred with the aim of the differential thermal analysis (DTA). Melting temperature of the salt mixture was measurred to confirm that the salt was in molten phase in all three temperatures that the experiments were carried in.    36  5. Results and Discussion; Air Atmosphere 5.1 Differential Thermal Analysis The differential thermal analysis (DTA) curve of the prepared salt mixture of PbCl2- 35.38 Wt.% KCl is shown in figure 5-1. A sharp endothermic peak can be seen at 409?C due to the transformation of the salt mixture from solid state to liquid state. This result was expected based on the phase diagram of PbCl2-KCl, showing the eutectic point at this temperature  .   Figure 5-1 DTA curve of PbCl2- 35.38 Wt.% KCl salt mixture     37 5.2 Open Circuit Potential In order to study kinetics and mechanisms of corrosion in Inconel alloy 617 and 625 in a molten salt environment, a steady potential for the working electrode must be achieved.  For this purpose, the working electrode potential was measured vs. the reference electrode over time. The graphs for these two alloys are shown in figure 5-2 at different temperatures. As shown in figure 5-2a, at 450?C, there is a sharp drop in potential in the first hour. This drop could be due to the deterioration of the alloy?s initial passive layer caused by chloride attack. For the first 6 hours, monitored potentials for alloy 625 are higher than alloy 617. Also, potential fluctuation was higher for alloy 625 in the first 6 hours, which suggests that the alloy 617 oxide layer stabilized sooner. The final potential of alloy 625 is less than alloy 617. The more negative the potential (active), the less the resistance of alloy towards corrosion. At 550?C, as shown in figure 5-2b, potential fluctuations for both alloys are more than at 450?C, showing that, with increasing temperature, corrosivity of the salt increases. The final potential of alloy 625 is less than alloy 617 as was seen at 450?C as well. Potential fluctuation for alloy 625 stops after 2.5 hours, and for alloy 617, the fluctuations stop after 1 hour. At 650?C, the difference between final potentials of the two alloys is more obvious. Potential fluctuations stop sooner at this temperature (less than 1 hour), and the oxide layer stabilizes faster, which is due to faster kinetics of the reactions between salt and metal at higher temperatures. Fluctuation magnitude, however, is higher due to the higher corrosivity of salt at high temperatures. The potential sharp drop happens a lot faster than at the other two temperatures, due to faster damage to the initial passive layer at high temperatures. Open circuit potentials of each alloy at 650?C were not compared with the other two temperatures due to the application of platinum as a pseudo-reference electrode and unpredictable fluctuation of its potential at different temperatures. Also, the curves shown in    38 figure 5-2 confirm that 12 hours is enough time for the working electrode?s potential to reach the steady state.    Figure 5-2 Open circuit potential vs. time curves for Inconel alloys 617 and 625 in a) 450, b) 550, and c) 650?C at air atmosphere  a b    39  Figure 5-2 Continued Final open circuit potentials of the two alloys in air atmosphere at three temperatures are shown in table 5-1. At all temperatures, final open circuit potentials for alloy 625 are less than alloy 617. As the temperature increases, the open circuit potential difference for the two alloys (?OCP) increases. Table 5-1 Final open circuit potentials (mV) for Inconel alloys 617 and 625 in 450, 550, and 650?C at air atmosphere               temp       alloy 450?C  550?C  650?C  617 -443 ? 22 -256 ? 17 -167 ? 11 625 -448 ? 25 -333 ? 18 -255 ? 15  c    40 5.3 Potentiodynamic Polarization To study the general corrosion behavior of alloys 617 and 625, the potentiodynamic polarization method was used. All electrochemical experiments were conducted at least three times to check the reproducibility of the results. The experiments were reproducible to an acceptable level which is ? 0.1-0.2 of the current densities. As an example, figure 5-3 shows the reproducibility of the polarization test on Inconel 617 in air atmosphere at 650?C.   Figure 5-3 Reproducibility of polarization test on Inconel 617 in air atmosphere at 650?C Results for both alloys at three temperatures are shown in figure 5-4. As can be seen in figure 5-4a, at 450?C, the corrosion potential (Ecorr) of alloy 617 is less than 625. In the case of alloy 617 at 450?C, it seems that at a potential of 0V, an oxide layer was formed but failed after 0.2V causing a sharp increase in current. According to Perez et al. (2008), the oxide layer formed for this alloy is Cr2O3. Chromium oxide forms on the surface of the metal as the result of reduction of oxygen and oxidation of chromium (equation 5-1 and 5-2).  Equation 5-1  Equation 5-2    41 This layer loses its protection ability as a result of salt (PbCl2, KCl) and gases (HCl, Cl2, O2, etc.) penetration through the layer. According to Ishitsuka & Nose (2002), Cr2O3 films dissolve in molten salt easily and form a hexavalent chromium ion. ? Cr2O3+32O2+ 2O2??2CrO42?  Equation 5-3 The anodic current for alloy 617 is also more than alloy 625. At 550?C and 650?C, figure 5-4b and c, the corrosion potential for alloy 625 is slightly less than 617. Corrosion potentials are different from final open circuit potentials, which could be due to changes in reduction reactions in cathodic polarization. This changes the surface in potentiodynamic experiments in comparison with open circuit experiments. No oxide layer is seen for alloy 617 at these temperatures. Also, the anodic current for alloy 617 is more than 625; this difference, however, is not significant. This anodic current could be due to following reactions (Perez et al., 2008). ? 2Cr + 3Cl2?2CrCl3 Equation 5-4 ? 2CrCl3+32O2?Cr2O3+ 6Cl2 Equation 5-5 Corrosion current densities (icorr) for both alloys in three temperatures are shown in table 5-2. At 450?C and 550?C, alloy 617 shows lower icorr. At 650?C alloy 625 shows lower icorr. Table 5-2 Corrosion current densities in A cm-2 for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air atmosphere.        temp alloy 450?C 550?C 650?C 617 (1.50 ? 0.20) ? 10-4 (8.00 ? 0.40) ? 10-5 (3.00 ? 0.20) ? 10-4 625 (3.00 ? 0.30) ? 10-4 (2.00 ? 0.10) ? 10-4 (4.00 ? 0.30) ? 10-5    42       Figure 5-4 Potentiodynamic curves for Inconel alloys 617 and 625 in a) 450, b) 550, and c) 650?C at air atmosphere  a b c    43 5.4 Linear Polarization Resistance To obtain the polarization resistance of Inconel alloys 617 and 625, and to measure their corrosion rates, linear polarization resistance experiments were carried out. A typical curve obtained from these experiments for both alloys in air atmosphere at 450?C is shown in figure 5-5. Slope comparison of the curves indicates that the corrosion rate of alloy 617 is greater than alloy 625. It should be noted that since the surface of alloy after exposure to molten salt is different after various experiments the results obtained from each experiments might be different. Also as mentioned in chapter 4, corrosion rates calculated from LPR curves are closer to the corrosion rate of the alloy when no external current is applied in the system. No scatter is seen in the curve, which shows that the time given for OCP was enough at the beginning of the experiments.   Figure 5-5 Linear polarization curves density for Inconel alloys 617 and 625 in 450?C at air atmosphere      44   Calculated slopes from linear polarization curves, representing polarization resistance (Rp) for both alloys at three temperatures, are shown in table 5-3. Polarization resistance of alloy 625 at all temperatures is greater than alloy 617. Polarization resistance of the two alloys did not show a significant difference by increasing temperature. Table 5-3 Polarization resistance for Inconel alloys 617 and 625 in 450?C, 550?C, and 650?C at air atmosphere         temp alloy 450?C  550?C  650?C  617 74.4 ? 6.5 66.8 ? 4.3 55.5 ? 3.6 625 179.0 ? 12.4 124.0 ? 9.4 117.0 ? 7.8 Rp values, equation 5-6 and equation 5-7 were used to calculate corrosion current densities (Stansbury & Buchanan, 2000).  Equation 5-6  Equation 5-7 ?ox and ?red are anodic and cathodic Tafel slopes, respectively. In general, ?red values start from 60mV to infinity and ?ox values start from 60mV to 120mV. By using equation 5-6, calculated value of B is between 13 and 52 mV (Stansbury & Buchanan, 2000).  Corrosion rate, r, is the reacted mass, m, divided by time, t, and surface area, A, as shown below (Jones, 1996):    45 ? r =mtA=ianF Equation 5-8 Where; F is the Faraday?s constant that has the value of 96,500 (coulombs/equivalent), n, is the number of equivalent exchanged electrons, a, is the atomic weight, and i is the current density which is equals to I/A (I being current in amperes). Equation 5-8 indicates proportionality between the mass being lost in unit area and unit time and current density. Current density is proportional to the corrosion rate in comparison with the current, due to the fact that the same current in a smaller surface area causes a larger corrosion rate. To obtain units of penetration per unit time, equation 5-8 is divided by density, ?, of alloy. Corrosion rate in mils (0.001 in.) per year (mpy) is shown in equation 5-9 (Jones, 1996). Corrosion rates (in mpy) were calculated using the icorr from equation 5-6, and the following equation (Stansbury & Buchanan, 2000):   Equation 5-9 Where, M is the molecular weight (g/mol), m is the oxidation state or valence, ? is density (g/cm3), and icorr is current density (mA/m2). Densities for Inconel alloys 617 and 625 are 8.36 and 8.44 (mg/m3), respectively. For alloys, the penetration rate calculations require having the equivalent weight of the alloy. A/n is the alloy equivalent weight in equation 5-8 that is the average of weights of alloying elements. Alloy equivalent weight calculated from the below equation were used for ? Mm part of the equation (Stansbury & Buchanan, 2000):    46  Equation 5-10 Where, Ew is the Equivalent weight (gm/equivalent), ni is the valence of alloying element ?i? (equivalent/mole), fi is mass fraction of alloying element ?i?, and Ai is the atomic mass of alloying element ?i? (gm/mole). The calculated equivalent weight for Inconel alloy 617 is 27.897 and for Inconel alloy 625 is 30.768. Calculated corrosion rates are shown in table 5-4. Corrosion rates for alloy 617 are greater than alloy 625 in all temperatures. Having higher open circuit potentials for alloy 625 at all temperatures, as was mentioned in part 0, confirms the fact that the corrosion rate for alloy 617 is higher than 625. Having a higher corrosion rate for alloy 617 in comparison with 625 was related to the Co element of alloy 617, which is not seen in alloy 625 by Perez et al. (2008). Table 5-4 Corrosion rates (mpy) for Inconel alloys 617 and 625 in 450, 550 and 650?C at air atmosphere               temp alloy 450?C 550?C 650?C 617 74.1 82.5 99.3 625 30.7 44.4 47.0  5.5 Electrochemical Impedance Spectroscopy  To study corrosion behavior of Inconel alloys 617 and 625 in air atmosphere, electrochemical impedance spectroscopy (EIS) experiments were carried out. The simulating    47 model of EIS is shown in figure 5-6. The simulating model consists of Rs, representing salt resistance between the reference electrode and the working electrode, Qsl, representing capacitance behavior of scale showing deviation from ideal behavior, Rsl representing transfer resistance of ions in the scale, Qdl standing for double layer capacitance which again deviates from ideal behavior, and Rdl standing for the charge transfer resistance through the pores in the porous scale (Zeng et al., 2001). Deviation from ideal behavior is due to surface inconsistencies. Nyquist diagrams and Bode diagrams of phase angle and total impedance magnitude obtained from these experiments with modeled graphs are shown in figure 5-7. Impedance spectra in all conditions are combinations of two capacitance loops. At high frequencies, a small semi-circle and at low frequencies, a big semi-circle can be seen. The first semi-circle represents the scale formed on the surface of the metal and the second semi-circle represents charge transfer through the scale pores (Perez et al., 2008).   Figure 5-6 Equivalent circuit used for fitting Nyquist diagrams for all experiments (Zeng et al., 2001)    48       Figure 5-7 Nyquist and Bode diagrams for Inconel alloys 617 and 625 in a & b) 450, c & d) 550, and e & f) 650?C at air atmosphere a c e b d f    49 Formation of the scale on the surface of the alloy protects the alloy from corrosion attack from the molten salt. However, if partial failure occurs in the scale and the scale cannot reform on that area of substrate, localized fast corrosion takes place in the alloy. The zone that encounters localized corrosion may be in direct contact with molten salt or be covered with a scale that does not protect the surface. Other areas of the surface of the substrate, however, are covered with a more protective scale, thus providing a slow corrosion zone. Charge transfer resistance at areas undergoing slow corrosion at the interface of metal and scale may be neglected when compared with ions transportation resistance in the scale (Zeng et al., 2001). As shown in figure 5-7 at all frequencies, the impedance of alloy 617 is lower than that of alloy 625, which shows that the resistance of Inconel 617 is lower than Inconel 625. A comparison between Nyquist diagrams and Bode diagrams of the total impedance magnitude obtained at three different temperatures is shown in figures 5-8a & b, and figures 5-8c & d for alloys 617 and 625, respectively. It is seen that for Inconel 617, the temperature does not show a significant effect on the impedance.  For Inconel 625 as the temperature increases, impedance decreases. At high temperatures solution resistance, ion transfer resistance and oxide layer resistance decreases. By increasing temperature, corrosion activity increases due to the reduction in the viscosity of the molten salt and also the increase in electrical conductivity of molten salt in contact with the sample (Mohammadi Zahrani & Alfantazi, 2012).      50     Figure 5-8 Effect of temperature on Nyquist and Bode diagrams for Inconel alloys a & b) 625 and c & d) 617 at air atmosphere Electrical equivalent circuit parameters for both alloys fitted for circuit Rs(Q1(R1(R2Q2)) is shown in table 5-5. With increasing temperature, Rs decreases for both alloys at all temperatures except for alloy 617 going from 450?C to 550?C. This decrease is due to the decrease in the molten salt viscosity and increase in its ionic conductivity. Rsl decreases with increase in the temperature. This decrease, however, is not significant, which confirms the consistency in the corrosion rate obtained from the linear polarization resistance curves. In the case of Inconel 617, as the temperature increases, Rdl decreases. For all alloys, Chi squared ? a b c d    51 10?3, which shows a good fitting of Nyquist diagrams with the chosen model as shown in figure 5-7.  Table 5-5 Electrical equivalent circuit parameters for experiments in air fitted for circuit Rs(R1(Q1(R2Q2))) Temp Alloy Rs (? cm2) Rsl (? cm2) Qsl (S secn cm-2) nsl Rdl (? cm2) Qdl (S secn cm-2) ndl Chi squared 617 2.473 10.85 0.064 0.28 260.8 0.0219 0.91 3.66?10-3 450?C 625 5.304 68.94 0.0634 0.51 24 6.644 0.87 2.15 ?10-4 617 3.593 139.6 0.1302 0.67 239.1 0.2487 0.66 5.32?10-4 550?C 625 4.447 44.46 0.0736 0.54 115.2 0.3599 0.84 1.03?10-3 617 1.526 1.456 0.02641 0.13 119.3 0.1867 0.81 4.00?10-3 650?C 625 3.231 20.76 0.2028 0.78 178.9 0.2028 0.62 4.58?10-4            52 5.6 Summary ? The final open circuit of alloy 625 at temperatures of 450?C, 550?C, and 650?C in air atmosphere is less than alloy 617. The alloy 617 oxide layer stabilizes sooner. As the temperature increases, ?OCP between the two alloys and potential fluctuation magnitude increases, and potential fluctuations time decreases due to faster kinetics. Also, a faster potential drop at the beginning is seen due to faster damage to the initial passive layer at high temperatures. ? At 450?C and 550?C, alloy 617 has lower icorr and at 650?C alloy 625 has lower icorr. Ecorr for alloy 617 is less than 625 at all temperatures. At 450?C, in the case of alloy 617 it seems that a layer of Cr2O3 was formed between 0-0.2V and failed afterwards. The anodic current is also more for 617 compared with 625. At 550?C and 650?C, the anodic current for alloy 625 is slightly more than 617. ? At all temperatures, Rp for alloy 625 was greater than alloy 617. Also, the corrosion rate for alloy 617 was more than 625.  ? All impedance spectra consisted of two capacitance loops. The first semi-circle represents the oxide scale and the second semi-circle represents the double layer. For both alloys, as the temperature increases, impedance decreases. Electrical equivalent circuit parameters for both alloys was fitted with circuit Rs(Q1(R1(R2Q2)).     53  6. Results and Discussion; Air- 15% CO2 Atmosphere 6.1 Open Circuit Potential Open circuit potential measurements in air-15% CO2 atmosphere on both Inconel alloys 617 and 625 at three temperatures were done to make sure a steady potential is reached before running other electrochemical experiments (figure 6-1). At 450?C, as shown in figure 6-1a, there is a sharp potential drop at the beginning of potential monitoring which represents the damage in the initial passive layer. Potentials for both alloys do not have a significant difference at this temperature. Potential fluctuation is also higher for alloy 625 in the first 5 hours, suggesting that alloy 617?s oxide layer stabilizes sooner than that of 625. At 550?C, potential fluctuations for both alloys are more than at 450?C. This shows that as temperature increases, salt corrosivity increases too, as can be seen in figure 6-1b. Potential fluctuation for both alloys stop after about 5 hours. At 650?C, as shown in figure 6-1c, the final potential difference between the two alloys is more obvious compared with the potential difference seen in the other two temperatures. For both alloys, the potential fluctuations stop sooner and a stabilized oxide layer is achieved faster because of the faster kinetics at this temperature. Fluctuation magnitude is higher at this temperature. Similar to air atmosphere, in air-15% CO2 atmosphere open circuit potentials were not compared with each other at various temperatures. Curves in figure 6-1 confirm that the given time to each alloy was enough at different temperatures to reach a stable potential.      54   Figure 6-1 Open circuit potential vs. time curves for Inconel alloys 617 and 625 in a) 450, b) 550, and c) 650?C at air-15% CO2 atmosphere a b    55    Figure 6-1 Continued The final open circuit potentials for alloys 617 and 625 in air-15% CO2 atmosphere at temperatures of 450, 550, and 650?C are shown in table 6-1. At 650?C, the open circuit potential difference for the two alloys is higher compared with the other two temperatures. As the temperature increases, potential difference increases too. Table 6-1 Final open circuit potentials (mV) for Inconel alloys 617 and 625 in 450, 550, and 650?C at air-15% CO2 atmosphere          temp alloy 450?C 550?C 650?C 617 -516 ? 35 -169 ? 12 -223 ? 18 625 -461 ? 28 -181 ? 13 -368 ? 25  c    56 6.2 Potentiodynamic Polarization The potentiodynamic polarization curves for Inconel alloys 625 and 617 in air-15% CO2 atmosphere, at temperatures of 450, 550, and 650?C are shown in figure 6-2 to study the general behavior of the two alloys. As shown in figure 6-2a, at 450?C the corrosion potential of the two alloys do not show a significant difference. The anodic current for alloy 625 is more than alloy 617. This difference is not significant. For alloy 617 there is a small scattering in the potentiodynamic curve, which could be due to oxide layer instability. At 550?C and 650?C, figure 6-2b and figure 6-2c, respectively, corrosion potentials do not show significant difference, similar to that of 450?C. Corrosion potentials are different from final open circuit potentials which could be due to changes in the surfaces of the samples in the potentiodynamic experiments in comparison with open circuit experiments. The anodic current for alloy 617 is more than alloy 625. As the temperature increases, the anodic current difference between the two alloys increases as well. The anodic current could be due to the same reactions as equation 5-2 and equation 5-3.  Figure 6-2 Potentiodynamic curves for Inconel alloys 617 and 625 in a) 450, b) 550, and c) 650?C at air-15% CO2 atmosphere a    57   Figure 6-2 Continued  The corrosion current densities (icorr) for alloy 625 and 617 at three temperatures at air-15% CO2 atmosphere are shown in table 6-2. There is no difference between the corrosion b c    58 currents for the two alloys at 450 and 650?C. At 550?C, the corrosion rate for alloy 617 is slightly lower than alloy 625. Table 6-2 Corrosion current densities in A cm-2 for Inconel alloys 617 and 625 in 450, 550, and 650?C at air-15% CO2 atmosphere         temp alloy 450?C 550?C 650?C 617 (8.00 ? 0.60) ? 10-5 (1.00 ? 0.10) ? 10-4 (2.00 ? 0.20) ? 10-4 625 (8.00 ? 0.50) ? 10-5 (3.00 ? 0.30) ? 10-4 (2.00 ? 0.20) ? 10-4  6.3 Linear Polarization Resistance Linear polarization experiments were done to measure the polarization resistance and corrosion rates of Inconel alloys 617 and 625. A typical curve obtained from these experiments is shown in chapter 5 (figure 5-4. Measured polarization resistances for both alloys at air-15% CO2 atmosphere from linear polarization curves are shown in table 6-3. As the temperature increases, polarization resistance decreases a little. Table 6-3 Polarization resistance for Inconel alloys 617 and 625 in 450, 550, and 650?C at air-15% CO2 atmosphere         temp alloy 450?C 550?C 650?C 617 62.9 ? 4.6 61.3 ? 4.3 48.1 ? 3.7 625 123.0 ? 9.1 14.5 ? 1.2 11.5 ?0.9    59 The corrosion rate for alloys 617 and 625 in three temperatures at air-15% CO2 atmosphere were calculated using icorr calculated from equation 5-6 and equation 5-9, and are shown in table 6-4. As the temperature increases, the corrosion rate for both alloys increases.  Table 6-4 Corrosion rates (mpy) for Inconel alloys 617 and 625 in 450, 550, and 650?C at air-15% CO2 atmosphere            temp alloy 450?C 550?C 650?C 617 87.7 89.9 114.6 625 45.0 379.7 479.9 The partial pressure of oxygen at temperatures of 450?C, 550?C, and 650?C was calculated in air-15% CO2 atmosphere to confirm the increase in the corrosion rate as a result of increasing temperature. To do so, the reaction below was considered (Gaskell, 2008): ? CO2?CO+12O2 Equation 6-1  Standard Gibbs free energy change for this reaction was calculated using the two reactions below: ? C +12O2?CO Equation 6-2  Equation 6-3 Standard Gibbs free energy change in joules (J) for these two reactions is shown below, respectively (Gaskell, 2008):  Equation 6-4    60  Equation 6-5 Summation of standard Gibbs free energy changes of reactions 6-2 and 6-3:  Equation 6-6 Standard Gibbs free energy changes at three temperatures of 450?C, 550?C, and 650?C (723?K, 823?K, and 923?K) using the above equation were calculated and the values are:     The equilibrium constant for this reaction (Kp) for the three temperatures was calculated using the equation below:   ? Kp= exp(??G?RT) Equation 6-7 where R is the gas constant and is equal to? 8.314(Jmol.K). Calculated Kp for 450?C, 550?C, and 650?C are:        61 As a result of the decomposition of 15%CO2 gas in an air-15%CO2 environment from the stoichiometry of the reaction shown by equation 6-1, ? x  moles of Co and ?? x  moles of O2 are formed as a result of decomposition of ? x  moles of CO2. At any constant temperature, any reacting mixture contains 0.15-? x  moles of CO2, ? x  moles of CO, and, 0.85?0.21+?? x  moles of O2:  ? CO2 ? ? ? CO  + ? 12O2 Initially 0.15  0  0.85?0.21 Upon reaction 0.15?? x   ? x   0.85?0.21+? 12x  Total number of moles in this particular gas system is as follows:   Equation 6-8 The value 0.6715 is related to N2 part of the environment. As ? pi=ninTP  Equation 6-9 Following pressures can be calculated as: ? PCO=x0.9995 +12x Equation 6-10     62  Equation 6-11    Equation 6-12 where ? PCO=  Partial pressure of CO ? PO2= Partial pressure of O2 ? PCO2=  Partial pressure of CO2 In this gas mixture, the equilibrium constant for the reaction is calculated using: ? Kp=PCOPO212PCO2 Equation 6-13 Using the above calculations we have:  Equation 6-14     63 In which Pt=1atm. Using this equation and the calculated Kp from equation 7-4, value of? x  for temperatures of 450?C, 550?C, and 650?C can be calculated. These values are:    In this gas mixture, partial pressure of O2 can be calculated using equation 6-11. From the calculated ? x  at each temperature and considering P = 1, partial pressure at each temperature is as below:   ? PO2,450?C= PO2,550?C= PO2,650?C= 0.178atm  Equation 6-15 As can be seen, partial pressure of O2 does not change as temperature increases. The increase in the corrosion rate, therefore, is due to the increase in temperature as shown in figure 6-3.  Figure 6-3 Effect of temperature on corrosion rate at air and air-15% CO2 atmosphere for Inconel alloys a) 617 and b) 625 a    64  Figure 6-3 Continued Figure 6-3 shows that corrosion rates in air-15% CO2 atmosphere are more than the corrosion rates in air atmosphere. The effect of having CO2 in the atmosphere; however, is not known yet, and further study must be done.  6.4 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) experiments were carried out to study the corrosion behavior of Inconel alloys 617 and 625 in molten salt in air-15% CO2 atmosphere. Nyquist and Bode diagrams for these two alloys at three different temperatures are shown in figure 6-4. All impedance spectra consisted of two capacitance loops. A small semi-circle and a big semi-circle are seen at high and low frequencies, respectively. A semi-circle at high frequencies represents the scale and the one at low frequencies represents ion transfer. The larger radius for the second capacitance loop is due to the fact that transportation of ions is the rate-determining step of corrosion in both Inconel alloys.  b    65       Figure 6-4 Nyquist diagrams for Inconel alloys 617 and 625 in a) 450, b) 550, and c) 650?C at air-15% CO2 atmosphere a c e b d f    66 Figure 6-5 shows the effect of temperature in the Nyquist and Bode diagrams of both Inconel alloys 617 and 625 in air-15% CO2 atmosphere. In the case of Inconel alloy 617, resistance at 650?C is lower compared with 450?C and 550?C. For Inconel 625, temperature does not have a significant effect on the resistance of the alloy.       Figure 6-5 Effect of temperature on Nyquist and Bode diagrams for Inconel alloys a & b) 625 and c & d) 617 at air-15% CO2 atmosphere All EIS profiles for different temperatures and the two alloys in air-15% CO2 atmosphere were fitted by the same circuit as in air atmosphere, Rs(R1(Q1(R2Q2))), as shown in figure 6-4. a b c d    67 Parameters obtained from an electrical equivalent circuit are shown in table 6-5. As the temperature increases Rs, representing salt resistance, decreases due to an increase in ionic conductivity. R1, representing oxide layer resistance, decreases with increasing temperature as well, however, this change is not significant. Values for ndl are close to 0.5, showing ion transfer resistance. In air-15% CO2 atmosphere in the case of Inconel 625, Rdl values decrease as temperature increases. The same trend can be seen for Inconel 617 except from 550?C to 650?C.  Table 6-5 Electrical equivalent circuit parameters for experiments in air-15% CO2 atmosphere fitted for circuit Rs(R1(Q1(R2Q2))) Temp Alloy Rs (? cm2) Rsl (? cm2) Qsl (S secn cm-2) nsl Rdl (? cm2) Qdl (S secn cm-2) ndl Chi squared 617 3.891 30.86 0.072 0.41 105.6 2.948 0.48 1.19?10-3 450?C 625 4.533 25.84 0.05547 0.58 153.8 0.7104 0.81 4.5 ?10-3 617 3.612 22.56 0.0488 0.43 95.3 1.0764 0.47 3.10?10-4 550?C 625 3.75 25.3 0.09309 0.93 92.54 0.3823 0.51 2.74?10-3 617 3.369 16.8 0.03845 0.87 187.2 0.1407 0.46 1.08?10-3 650?C 625 3.424 22.74 0.01886 0.72 60.28 0.1592 0.59 1.5?10-3           68 6.5 Summary ? In air-15% CO2 atmosphere at 650?C, the open circuit potential difference for the two alloys is higher compared with the other two temperatures. As the temperature increases, potential difference increases too. The potential fluctuation is higher for alloy 625 than for alloy 617. As the temperature increases, fluctuation magnitudes increase, however, these fluctuations stop sooner at higher temperatures due to faster kinetics.  ? The corrosion rate for alloy 617 at 550?C is slightly less than for alloy 625. At the other two temperatures, however, no significant difference is seen. Both alloys have close corrosion potentials in comparison with each other at all temperatures. Also, the anodic current for alloy 617 is slightly more than alloy 625. ? As the temperature increases, polarization resistance decreases by a little. The corrosion rate for both alloys increases as well. Since the oxygen partial pressure stays the same at all temperatures, the increase in corrosion rate is due to the increase in temperature only. ? All EIS profiles were fitted by the same circuit as air atmosphere, Rs(Q1(R1(R2Q2)). All impedance spectra consisted of two capacitance loops in the air-15% CO2 atmosphere, representing the oxide scale at high frequencies and double layer at low frequencies.        69  7. Surface Characterization   The surface characterization of the exposed alloys to molten PbCl2- KCl were done using X-ray diffraction phase analysis and scanning electron microscopy (SEM) in order to obtain information about the corrosion products and mechanisms that took place in this environment. 7.1 X-ray Diffraction Phase Analysis In order to study corrosion products on the surface of the samples after exposure to molten salt at different temperatures as well as the oxide layer formed on the surface, X-ray diffraction (XRD) phase analysis was used. Figure 7-1 shows XRD patterns of as-received Inconel alloys 617 and 625. Both alloys show the same XRD pattern. Patterns show peaks from Ni(Cr2O4) and Cr2O3 phases. This Cr2O3 phase is different from the Cr2O3 scale that covers the surface of the alloys after exposure to molten salt. As the EIS results showed, the Cr2O3 scale covering the surface after exposure is porous and caused high rate of corrosion on the alloy, high values of resistance calculated from EIS models confirms that. The Cr2O3 of the as-received alloys; however, is a protective scale which breaks in the initial stage of exposure to salt, the scale deterioration was seen in the OCP curves. Figure 7- and figure 7-4 show the XRD patterns of the corrosion products formed on the surface of both Inconel alloys 617 and 625 at temperatures of 450?C, 550?C, and 650?C.     70  Figure 7-1 X-ray diffraction (XRD) patterns of base alloys 617 and 625 In the XRD patterns for both alloys, Cr2O3, NiO, MoO3, Cr3O were detectable in the oxide layer. In the case of Inconel alloy 625 NbCrO4 was also detected in the oxide layer. For both alloys, corrosion products CrCl3 and also part of the original salt: PbCl2 was observed in the pattern. XRD patterns for these two alloys after being exposed to molten salt under air-15% CO2 atmosphere are shown in figure 7-5, figure 7-6, and figure 7-7. The same phases detected in air-atmosphere were seen in this atmosphere as well. Detection of Cr2O3 in XRD patterns confirms the occurrence of reactions shown in equation 5-2 and equation 5-3. As chromium was forming Cr2O3 to protect the surface of the metal, chlorine and chromium oxide react with each other and to form CrCl3. Due to evaporation of CrCl3 at high temperature, this  reaction was thermodynamically favored (Perez et al., 2008). ? 2CrO3+ 3Cl2(g )?2CrCl3(g )?+3O2(g ) Equation 7-1    71   Figure 7-2 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 450?C  Figure 7-3 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 550?C    72   Figure 7-4 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air atmosphere at 650?C  Figure 7-5 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 450?C    73  Figure 7-6 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 550?C  Figure 7-7 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 617 and 625 in air-15%CO2 atmosphere at 650?C   74 During the corrosion process, PbCl2 was consumed due to evaporation and also its oxidation to form PbO. Afterwards, PbO reacted with the Cr2O3 formed in the surface and formed PbCrO4 spinel. Reactions resulting in the production of PbCrO4 are shown below: ? PbCl2+12O2(g )?PbO+Cl2(g ) Equation 7-2 ? PbO(s)+Cr2O3(s)?PbCrO4+Cr  Equation 7-3 The protective properties of the Cr2O3 layer were lost at some parts of the metal surface as a result of the decrease in chromium content near the interface of metal and oxide. Chromium decrease occurred due to CrCl3 evaporation (equation 7-1), chromium dissolution as CrO42- (equation 5-1), and PbCrO4 formation (equation 7-2 and equation 7-3) (Ishitsuka & Nose , 2002; Perez et al., 2008). As a result of the chromium oxide losing its protection against corrosion, other elements in the alloy formed oxides such as NiO and MoO3. The layer containing these phases, however, is porous and cannot protect the surface effectively. This results in a high corrosion rate. In the case of Inconel alloy 625, when chromium content decreases, other elements in the alloy forms oxides such as NiO, MoO3, and NbCrO4. Reactions are shown below (Perez et al., 2008): ? Mo+32O2?MoO3 Equation 7-4 ? 2Ni +O2?2NiO Equation 7-5 ? Cr2O3+ Nb2O5?2NbCrO4 Equation 7-6 According to Kawashara (Kawahara et al., 2000), reactions that take place in the corrosion process of Inconel 625 in molten chlorides are as below:    75 ? 2Cr + 3Cl2?2CrCl3 Equation 7-7 ? 2Ni +12O2?NiO Equation 7-8 A comparison between the XRD patterns obtained from the surface of Inconel alloy 625 after exposure to molten salt at three temperatures is shown in figure 7-8. It shows that as the temperature increases the intensity of Cr2O3 peaks increases as well.  Figure 7-8 X-ray diffraction (XRD) patterns of corrosion products formed on the surface of Inconel alloys 625 in air atmosphere at temperatures of 450?C, 550?C, and 650?C 76  7.2 Scanning Electron Microscopy & EDX Spectrpscopy   Figure 7-9 a and b shows the cross section of Inconel alloys 617 and 625 after exposure to molten chlorides at a magnification of 50?m, backscattered electron (BSE). This figure shows the deposited salt on the surface of the two alloys, the interface of these two alloys with the oxide scale, and the matrix of them. Figure 7-9 illustrates the corrosion behavior at the interface between the samples and the oxide scale. Deterioration of the oxide scale at the interface with the alloy, which resulted in the internal attack on the grain boundaries, is due to the difference of the thermal expansion coefficient between the oxide layer and the matrix at high temperature that can be seen in figure 7-9 b and e (Esmaeili et al., 1995; Jo et al., 2007). Deterioration starts at inhomogeneous areas, in this case, at the grain boundaries, where the oxide layer is believed to be weaker. Internal attack started from the interface of the alloy and the oxide scale and continued to the inside of the alloy through the grain boundaries as shown in figure 7-9, c and f. The EDX results of the outer layer of both Inconel alloys 617 and 625 after exposure to molten salt are shown in figure 7-10, which confirms the presence of Cr-oxide in this scale. The spots from which these results were obtained are marked with green crosses in figure 7-9, a and d. Having a Cr2O3 layer on the surface of the alloy is the main reason for resistance to corrosion in these two alloys (Kewther et al., 2001b). Localized corrosion in the SEM imaging confirms the suitability of the circuit that was used to explain the data extracted from EIS results.     77   Figure 7-9 SEM image of the cross sections of the Inconel alloys in air atmosphere at 550?C; a) Inconel alloy 617 interface with oxide scale, b) oxide scale deterioration of Inconel 617, c) internal attach in Inconel 617, d) Inconel alloy 625 interface with oxide scale, e) oxide scale deterioration of Inconel 625, f) internal attack in Inconel 625  78   Figure 7-10 EDX analysis from the oxide layer formed on the surface of Inconel alloys a) 617 and b) 625 in air atmosphere at 550?C exposed to molten salt mixture Figure 7-11 shows two different areas of the cross sections of Inconel alloy 617 in air atmosphere after exposure to molten salt at a magnification of 20?m, backscattered electron (BSE). This figure illustrates that the Cr2O3 layer protected the alloy before the molten phases of  79 PbCl2 and KCl and also gas components such as Cl2 and O2 penetrated through the defects in this layer and resulted in the loss of corrosion protection at some parts of the materials (Perez et al., 2008). As the comparison of figure 7-11a and b shows, that localized corrosion taking place due to the cracks and oxide layer failure at all the surfaces of the alloy exposed to the salt did not take place uniformly. In some areas, the oxide scale lost its protection more than the other parts; salt penetrated more into the alloy and resulted in more damage to that area of the alloy, figure 7-11b.   Figure 7-11 SEM photomicrographs of Inconel alloy 617 in air atmosphere, at 550? showing the corroded cross section of the exposed alloy to molten chloride at a different part of the alloy-oxide interface; a) less damage to the oxide scale, b) more damage to the oxide scale and higher penetration of deposited salt Figure 7-12a shows the SEM image obtained from the cross section of Inconel alloy 617 after exposure to molten salt at magnification of 300?m, backscattered electron (BSE), and figure 7-12b shows grain boundaries close to the surface of the alloy at a magnification of 30?m. Figure 7-12c, d, and e illustrates the EDX results obtained from three spots close to the interface;  80 grain boundary indicated as 1, an area of a grain close to grain boundary indicated as 2, and one of the black spots indicated as 3. Table 7-1 shows the EDX analysis of these three spots.      Figure 7-12 a) SEM photomicrographs of Inconel alloy 617 in air atmosphere at 550?C, b) SEM photomicrographs of grain boundaries close to the surface, c) EDX results obtained from point 1, d) EDX results obtained from point 2, e) EDX results obtained from point 3 Regarding the SEM micrograph shown in figure 7-12, intergranular corrosion of the substrate is clearly visible.  In addition, localized pitting inside the grains is also observed. According to literature (Fontana, 1987), the intergranular corrosion could be due to various phenomena. These include the higher energy state associated with the nature of grain boundaries. The atoms arrangement in the grains forms a crystal structure. Atoms in the grain boundaries, however, are not arranged in a specific order. This disorder in the atoms placement results in a higher energy level in the grain boundaries in comparison with inside the grains. Grain  81 boundaries are high-energy areas and are thus more chemically active than the grains. Hence, grain boundaries are usually attacked slightly more rapidly than grain faces when exposed to a corrosive environment. In the microstructures, the grain boundaries appear darker because they have been more severely attacked than the grains. Intergranular corrosion can also be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain boundary areas. For instance, depletion of chromium in the grain-boundary regions results in intergranular corrosion of stainless steels. Referring to the spot analyses taken from point 1 (at grain boundary) and point 2 (adjacent to the grain boundary), it can be seen that there has been a depletion of chromium adjacent to the grain boundary of this alloy. It is believed that, since chromium has the lowest atomic radius and also due to its high tendency to react with oxygen in comparison with the other alloying elements at high temperature, chromium atoms in the vicinity of grains have moved to the high energy area of the grain boundaries, where the possibility of chromium oxide formation is more likely to happen. Regarding the dispersed pitted areas seen within the grains, various phenomena could be attributed. These include: localized breakdown of the oxide scale and thus galvanic cell formation between the oxide scale (cathode) and the bare substrate (anode). Another phenomenon could be the presence of inclusions and impurities within the solid solution grains. It is believed that the oxide scale formed on the substrate is weaker in nonhomogeneous areas such as the grain boundaries or inclusion sites. Therefore, breakdown of the oxide scale is more likely at these areas, setting up a galvanic cell (cathode is passive film and anode is the bare area) and causing localized corrosion such as intergranular and pitting ones. Figure 7-13 shows a schematic of the cross section of the samples when exposed to molten salt. This figure illustrates oxygen and Cr path through the pores and grain boundaries, which results in intergranular corrosion in the alloy.  82 Table 7-1 EDX analysis from three spots near the surface of Inconel alloy 617 in air atmosphere at 550?C exposed to molten salt mixture         Wt% Element Spot 1 Spot 2 Spot 3 Minimum Maximum Average C 8.06 4.07 8.88 4.07 8.88 7.01 O 5.78 3.71 3.12 3.12 5.78 4.20 Al 0.90 1.15 0.50 0.50 1.15 0.85 Si 0.21 0.22 0.00 0.00 0.22 0.14 Cl 2.09 1.08 1.27 1.08 2.09 1.48 K 0.19 0.10 0.17 0.10 0.19 0.15 Ti 0.21 0.23 0.33 0.21 0.33 0.26 Cr 23.50 20.48 23.72 20.48 23.72 22.56 Fe 0.77 0.93 0.97 0.77 0.97 0.89 Co 9.56 11.42 10.59 9.56 11.42 10.52 Ni 40.26 48.95 43.78 40.26 48.95 44.33 Mo 8.47 7.63 6.65 6.65 8.47 7.58 Spot 3 shows almost the same weight percentages as spot 1. This could be the EDX result obtained from the corrosion products in the pits. The phenomenon is discussed in figure 7-14.  Figure 7-13. Schematic of the cross section of the alloy when exposed to molten salt  83 Figure 7-14a, shows the cross section of Inconel alloy 617 after exposure to salt with a magnitude of 20?m. Mapping analysis from an area close to the interface of the alloy with the salt showing pitting is shown in figure 7-14. The pit mapping suggests a corrosion product qualitative analysis (Kewther et al., 2001b). The chloride found in the pit is from the deposited salt on the surface of the sample. Mapping results from the pit show that the corrosion products in it  have high amounts of Cr and lower amounts of Ni and Co. Amount of Mo in this part is more than the other surfaces of the sample. However, Cr is the major element that was detected in the pit. Figure 7-15a, shows the cross section of Inconel alloy 617 after exposure to salt with a magnitude of 20?m, secondary electron mode. X-ray mapping results of this part of the exposed sample show the contribution of alloying elements of Cr, Ni, O, Si, and Pb. Presence of Cr and O in the interface of the substrate and molten salt and lack of Ni in this part confirm the presence of Cr oxide in the interface as was discussed in the XRD results. High amounts of Pb in this part are due to the deposited salt on the surface of the sample. There are two areas in the substrate/ molten salt interface that contains pure Si, as can be seen in figure 7-15e. Mapping results of O in this part show that there is no amount of O in these two areas. Thus, the possibility of having Si oxide is eliminated. The reason for having pure Si in this interface is not known. X-ray mapping done on the samples exposed to molten salt at higher temperatures, 800?C, shows the Cr-rich and poor in Cr areas close to the surface more significantly than in lower temperatures. X-ray mapping of Cr, Ni, and Mo of the cross section of Inconel 617 is shown in figure 7-16 (Golozar et al., 2012).     84  Figure 7-14 a) SEM photomicrographs of Inconel alloy 617 in air atmosphere at 550?C, b) pitting near the surface, mapping results from the pit showing; c) Cl, d) Cr, e) Co, f) Mo, g) Ni, and h) Al          85        Figure 7-15 a) SEM photomicrograph of Inconel alloy 617 in air atmosphere at 550?C, X-ray mapping of  b) Cr, c) Ni, d) O, e) Si, and f) Pb           86      Figure 7-16 X-ray mapping of Cr, Ni, and Mo of cross section of alloy 617, exposed to molten mixture, at 650?C and 800?C                    87 7.3 Summary ? Both Inconel alloys 617 and 625 formed a protective scale of Cr2O3 that protected the surface before molten salt penetrated through the pores that were made in the scale after a certain time. ? Cr2O3 lost its protection due to chromium evaporation and also its dissolution as CrCl3 and CrO42-. ? Penetration of the salt resulted in oxidation and chlorination of Cr, Ni, Mo, and Co for both alloys, as well as Nb in the case of alloy 625.  88  8. Conclusions ? At all temperatures, the corrosion rate for alloy 617 was slightly more than 625, which was due to the presence of Co which increases the corrosion rate in alloy 617 and the presence of Nb, which decreases the corrosion rate in alloy 625.  ? As the temperature increases, the corrosion rate for both alloys increases. Since oxygen partial pressure stays the same at all temperatures in the presence of CO2, the increase in the corrosion rate is due to the change in temperature only. ? Both alloys formed a Cr2O3 oxide layer on their surfaces. However, this layer became porous and resulted in the penetration of salt through the pores. This caused oxidation and chlorination of other elements of the alloy such as Cr, Ni, Mo, Co, and Nb. Diffusion of corrosive species was the root cause for corrosion in the two alloys.  ? Both alloys exhibited almost the same corrosion resistance. None of the oxides formed on the surface of the alloys, Ni and Mo oxides could sufficiently protect the surface of these two alloys due to the harsh corrosive environment they were in contact with.    89  9. Future Work  ? Carrying out the long-time immersion tests in order to investigate corrosion characteristics of these alloys in real sevice. ? Carrying out the in-situ tests: using coupons of Inconel alloys 617, installing in the real corrosive environment (field tests) and measuring the weight loss/weight gain after pre-determined exposing time intervals. SEM, EDS and XRD tests must also be done on these specimens inorder to investigate the corrosion mechanism and phenomena in action.   ? Comparing the corrosion rate of Inconel alloys 617 and 625 with other alloys such as Inconel 622 and 686. These two alloys contain higher concentrations of Mo. They also contain W instead of Nb.  90 References  Amin N, Amin M, M,. 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