Consequences of selective oxidation during crack-tip corrosion

Nanoscale observations of intergranular stress corrosion crack tips in Fe-Cr-Ni alloys provide evidence for corrosion-induced selective oxidation. Comparatively long-range compositional changes along with structural changes at the tip suggest a significant role of solid-state oxidation and vacancy injection during crack advance. Vacancy injection into the alloy from the oxide/alloy interface causes enhanced diffusion and redistribution of alloy species much faster than allowed by matrix or grain boundary diffusion. The mean free path for vacancy diffusion in a grain boundary influences the role of vacancy injection on solute diffusion, void formation or interfacial advance. The influence of relative solute diffusion rates on altered composition indicated that proportional changes in Fe, Cr and Ni are in accord with those measured during radiation-induced segregation. This suggests that the selective oxidation process at crack tips is controlled by relative diffusion rates as well as by favored kinetics and stability of oxide phases. (author)