Prediction of the Effect of Heat Treatment on the Susceptibility of Fe-Ni-Cr- Mo Alloys to Localized Corrosion

A model has been developed for evaluating the effect of thermal aging on the repassivation potential of austenitic alloys. The model consists of two parts – a grain boundary microchemistry model for calculating the chromium and molybdenum depletion profiles in the vicinity of grain boundaries and an electrochemical model that relates the repassivation potential to the microchemistry and environmental conditions including temperature and solution chemistry. The grain boundary microchemistry model incorporates a thermodynamic para-equilibrium treatment of the formation of carbides and a kinetic treatment of the diffusion of Cr and Mo. With this model, experimental Cr and Mo depletion profiles can be reproduced for sensitized alloys 600, 825, and 316L. The repassivation potential model accounts for the effects of solution chemistry and temperature by considering competitive dissolution, adsorption and oxide formation processes at the interface between the metal and the occluded site solution. Using a previously established relationship between the repassivation potential and alloy composition, a procedure has been established to calculate the observable repassivation potential by integrating its local values that reflect the alloy microchemistry in the depletion zone. The predicted repassivation potentials agree with experimental data for thermally aged alloys 600 and 825 in chloride solutions