CHEMICAL-POTENTIAL DIAGRAMS FOR FE-CR-LI-K-C-O SYSTEM - THERMODYNAMIC ANALYSIS ON REACTION PROFILES BETWEEN ALLOYS AND ALKALI CARBONATES

A chemical thermodynamic analysis has been made on reactions between Fe-Cr alloys and mixed aralia carbonates with a local equilibrium approximation using various kinds of chemical potential diagrams for the Fe-Cr-Li-K-C-O system. The compounds stable in the presence of carbonates have been shown in a log P(O-2) vs. log P(CO2) plot; their stabilities depend on the valence stability of the transition metal ions and the stabilization energy of double oxides. A log {a(Li)/a(M)} vs. log {a(K)/a(M)} diagram for the M-Li-K-C-O (M = Fe, Cr) systems visualizes which double oxides can be formed sequentially between the alkali carbonates and the transition metal oxides. A log {a(Cr)/a(Fe)} vs. log P(O-2) plot for the Fe-Cr-Li-K-C-O system under conditions of a(Li2CO3) = 0.34, a(K2CO3) = 0.14, and P(O-2) = 1 bar can be compared with experimental results on multilayered oxide scale on 310 and 316 stainless steels which were tested as separators in molten carbonate fuel cells. Reaction paths connecting the reaction products in the respective oxide scales formed on 310 and 316 can be represented on chemical potential diagrams. The activities of Cr and Fe in the alloys are the main factor of determining the oxide scales.