Role of chlorides in hot corrosion of a cast Fe–Cr–Ni alloy. Part II: thermochemical model studies

In many high temperature applications severe degradation of alloys is caused by thin deposits of molten salts, especially alkali metal sulfates, alkali metal chlorides and mixtures of these salts. Calculations of multi-component thermochemical equilibria in systems involving (initially) metal/salt/gas as a function of local oxygen activity can help identify the important hot corrosion reactions. Such calculations for four pure salts (KCl, NaCl, Na 2SO 4, and K 2SO 4) and a mixture of these salts in contact with an Fe–20%Cr alloy at 800 °C are presented. The results predict that the compositions of gas, oxide, (sulfide, when sulfate was input) and salt phases depend strongly upon the salt chosen and upon the local oxygen activity. In some cases the equilibrium salt composition is significantly changed by reactions with metal or oxide phases. The calculated results for the salt mixture were compared with experimental data from part I of this paper. The model calculations have led to the identification of two new factors that support faster hot corrosion rates in an alkali chloride + sulfate salt compared to that in alkali sulfate alone. First, alkali chlorides, unlike sulfates, support a continuous salt pathway from ambient to the metal/scale interface, allowing oxidant to be efficiently transported to oxidize metal. Secondly, under oxidizing conditions alkali chlorides have a higher solubility of dissolved Fe- and Cr-containing species than that in alkali sulfates. Both of these factors support higher transport rates, which according to the fluxing theory of hot corrosion will lead to faster corrosion.