Chromium depletion at 2-dimensional features during the selective oxidation of a 20Cr25Ni austenitic steel

2-dimensional finite-difference calculations have been undertaken to examine the depletion of chromium at specimen edges and ribs in a 20Cr-25Ni-Nb steel which is being selectively oxidized. The calculations have been performed for parabolic oxidation kinetics and for temperatures in the range 800-1200 degrees C. It is found that under re-entrant features, e.g. interior edges, the extent of depletion is less than under a planar surface but the converse holds for exterior edges. For this latter case, it is shown how the initial severe depletion can fill in with time by the supply of chromium from the bulk of the specimen. This process, however, results in the propagation of the "edge effect" away from the corner with kinetics that scale as (Dt)(1/2). Rectangular ribs contain both types of geometric features but it is found that the two exterior edges forming the rib tip lead to extensive depletion in this region which gradually propagates along the length of the rib. Similar calculations for a trapezoidal rib are also given and, in this case, are compared with SEM/EDX chromium concentration measurements for samples oxidized at 1000 degrees C in a CO2/1%CO environment.