Phase Stability Diagrams for High Temperature Corrosion Processes

Abstract

Corrosion phenomena of metals by fused salts depend on chemical composition of the melt and environmental conditions of the system. Detail knowledge of chemistry and thermodynamic of aggressive species formed during the corrosion process is essential for a better understanding of materials degradation exposed to high temperature. When there is a lack of kinetic data for the corrosion processes, an alternative to understand the thermodynamic behavior of chemical species is to utilize phase stability diagrams. Nowadays, there are several specialized software programs to calculate phase stability diagrams. These programs are based on thermodynamics of chemical reactions. Using a thermodynamic data base allows the calculation of different types of phase diagrams. However, sometimes it is difficult to have access to such data bases. In this work, an alternative way to calculate phase stability diagrams is presented. The work is exemplified in the Na-V-S-O and Al-Na-V-S-O systems. This system was chosen because vanadium salts is one of the more aggressive system for all engineering alloys, especially in those processes where fossil fuels are used.

Understanding the oxidation behavior of a ZrB2–MoSi2 composite at ultra-high temperatures

“…Given the lack of kinetic data for the corrosion processes, an alternative approach to understand the thermodynamic behavior of chemical species is to utilize phase stability diagrams. In addition, an increase of the temperature, and thereby the reaction rates, causes diagrams solely based on thermodynamic considerations to be more relevant [35].…”

High Temperature Corrosion of Nickel in NaVO3-V2O5Melts

“…Because the Na 2 O reduces the acidity of the molten salts, the increase in the corrosion rate observed by increasing the V concentration is consistent with the reactions mentioned above, since at higher V content the proportion of Na 2 O formed is reduced. Based on the above reactions, in addition to the equilibrium, 2NaVO 3 = V 2 O 5 + Na 2 O, the phase stability diagram shown in Figure 12 was constructed, according to the procedure indicated elsewhere [25].…”

High Temperature Corrosion of Inconel 600 in NaCl-KCl Molten Salts

“…Considering that Cl 2 and O 2 are the main species of the NaCl-KCl-air system that contribute to the degradation of materials in waste incinerators environments, the construction of phase stability diagrams of these species with the main elements of the alloy is a useful tool for understanding the corrosion behavior at high temperature in this environment. Figure 8 shows the phase stability diagrams for the Fe-Cr-Ni-Cl-O system at 700, 800, and 900 ∘ C. The procedure to generate the diagrams is described elsewhere [25]. Table 2 shows the set of chemical reactions used for their calculation.…”