Elucidating the grain-orientation dependent corrosion rates of austenitic stainless steels

The dependence of oxidation rates on grain orientation has been observed for austenitic alloys suggesting crystallographic control of such processes. To better understand these observations, the active and transpassive oxidation behaviors of an austenitic AISI 316L stainless steel were investigated using vertical scanning interferometry (VSI). The oxidation rates of >100 unique grains were measured and related to their crystallographic orientations (i.e., with respect to the surface normal direction) as identified using electron backscatter diffraction (EBSD). The oxidation rates follow a scaling that is given by: {001} < {101} < {111} for grains undergoing both active and transpassive oxidation. The corrosion tendencies of {001} and {101} grains indicate that the activation energy of dissolution follows a scaling similar to that of the surface energy. However, the high corrosion rates of {111} grains, which feature a surface energy lower than those of the {001} and {101} grains, is attributed to their lower tendency to adsorb passivating species, from solution, that leads to a net reduction in the activation energy of oxidation. These insights provide improved understanding of the crystallographic controls of oxidation processes, and thereby suggest processing pathways for enhancing the oxidation resistance of stainless steels.