Nanoporous Oxide Formation and Corrosion Behavior of Stainless Steel in Chloride and Borated Media

Anodization of 304 stainless steel in ammonium-fluoride-based electrolytes produced thick (~3 µm) Fe-Cr-O-F oxide films whose chemistry and corrosion behavior were examined before and after post-anodization heat treatment. XPS and XRD revealed mixed-valence Fe2+/Fe3+ oxides with Cr3+ incorporation in the as-formed layer, which was highly hydrated and fluoride-rich. Heat treatment at 450 ˚C induced defluorination, dehydration, and crystallization into a dense spinel of Fe3O4-Fe2O3. Electrochemical testing in borated water (2 ppm Li, 1000 ppm B) and 3.5 wt.% NaCl showed that corrosion response depends strongly on oxide composition and formation voltage. The heat-treated oxide exhibited the lower current density (~0.08 µA cm-2) and higher resistance (~7 × 105 Ω cm2) in borated water, whereas in chloride solution, performance varied non-monotonically with anodizing voltage due to competing effects of porosity, cracking, and localized breakdown resistance. These results demonstrate that anodization markedly alters the Fe-Cr oxide chemistry, and corrosion behavior is governed by fluoride content, hydration, and film integrity.