Localised corrosion of heat-treated alloys Part II - Predicting grain boundary microchemistry and its effect on repassivation potential

A methodology has been developed for predicting the effect of thermal aging on the repassivation potential of austenitic alloys. The methodology combines two models, 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 paraequilibrium 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 sensitised 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 developed relationship between the repassivation potential and bulk alloy composition, a procedure has been established for calculating the observable repassivation potential of thermally aged alloys by integrating the local E(rp) values that correspond to 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. Additionally, the model has been applied to estimate the repassivation potential of welded samples in which segregation of alloying elements is observed.