Predicting IGSCC/IGA susceptibility of Ni-Cr-Fe alloys by modeling of grain boundary chromium depletion

Computer simulations based in part on a semi-empirical model of chromium diffusion in Alloy 600 (Ni-16 Cr-9 Fe) have been performed to predict heat treatment regimes that produce high susceptibility to intergranular stress corrosion cracking (IGSCC) or intergranular attack (IGA). The model is based in part on empirical data to establish boundary conditions for the one dimensional discretized diffusion equation that describes chromium depletion in the presence of semi-continuous grain boundary carbides. The simulations correctly predict the minimum Cr concentration (Cmin) at the grain boundary and also correctly estimate the depletion zone halfwidth for the thermal treatments for which experimental data is available. Once the model was verified by the limited available experimental data, calculations were then extended to much wider temperature and time ranges. Based on these calculations, a time-temperature contour plot is generated for estimating Cmin that predicts regions of high IGSCC/IGA susceptibility. Agreement between the calculated contour plot of Cmin and the limited experimental observations of IGSCC and IGA is good.