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

A predictive methodology has been developed to evaluate the effects of heat treatment on the susceptibility of nickel-base alloys and stainless steels to localized corrosion.  This methodology addresses phenomena due to the precipitation of metal carbides at grain boundaries and the subsequent development of chromium- and molybdenum-depleted grain boundary layers. Also, it takes into account the segregation of alloying elements and precipitation of intermetallic phases. In the first stage of calculations, the model predicts the chromium and molybdenum depletion profiles as a function of temperature and time of thermal exposure and carbon content of the alloy. In the second stage, the repassivation potential of the heat-treated sample is calculated to evaluate the thershold conditions for localized corrosion. The model relates the repassivation potential to the microchemistry of the depleted zone. At the same time, it predicts the repassivation potential as a function of environmental conditions, including temperature and the concentrations of aggressive and inhibitive ions. To validate the methodology, repassivation potentials of heat-treated samples have been have been studied for alloys 600 and 690. Also, the model has been verified using previously obtained experimental data for heat-treated and/or welded samples of alloys 825 and 22.