Depth-composition profiles of iron-base alloy surfaces anodically oxidized in concentrated NaOH solution

The depth-composition profiles of Fe-30Cr alloy, Fe-Ni alloys, SUS 304 and 316 austenitic stainless steels anodically oxidized at room temperature in deaerated 10.7 mol.kg-1 NaOH solution were measured by simultaneous use of Auger electron spectroscopy and argon ion sputteretching techniques. A rather thick film (9-28 nm) with a chromium-depletion zo ne grows on Fe-30Cr alloy during short polarization time (5-1 5 min) in the transpassive region where chromium dissolves into solution. The effective thickness of the zone, delta (12-25nm), increases with increasing polarization time. Assuming that th e steady state of selective dissolution of chromium was established, the recession velocity, u (10-10-10-9cm.s-1) of the film/substrate interface during transpassive dissolution was estimated, and further the interdiffusion coefficient, D-tilde (10-15cm2.s-1), which was extremely large as a value at room temperatur e, was obtained from the approximate relation, delta asymptotically equals D-tilde/u. Nickel-enrichment ranging was obser ved for a series of Fe-Ni alloys anodically oxidized in the passive, transpassive or secondary passive potential region of nickel. Particularly, nickel enrichment is significantly enhanced on the uppermost surface and at the film/substrate interface, which may be attributed to anodic deposition of nickel once dissolved and to selective dissolution of iron, respectively. The thickness (2-4 nm) of the anodic oxide film formed on a series of Fe-Ni alloys decreases with increasing buk nickel content, indicating the improvement of corrosion resistance due to nickel-enrichment. The effective thickness of nickel enrichment layer, delta (6-10 nm), in the substrate exhibits both the potential-and alloy composition-dependence, suggesting the importance of interdiffusion process in the vicinity of the alloy surface for anodic oxidation of a series of Fe-Ni alloys. (J.P.N.)