Surface modification of stainless steel-304 electrode. 1. Voltammetric, rotating ring-disc electrode and XPS studies

Hydrous oxide film was grown on stainless steel-304 under conditions of cyc ling voltammetry in 1 mol dm(-3) NaOH solution. The anodic voltammetric pea k of Fe(OH)(2) oxidation was used as a measure of the oxide growth as a fun ction of potential limits, sweep rates and the number of potential cycles. The oxide film stability was monitored by the use of rotating ring-disc ele ctrode. During potential cycling in alkaline solution, it was found that af ter 30 cycles, 1.4% of the anodic charge was due to iron dissolution from t he oxide film. The selective dissolution of iron and nickel from the oxide film, previously grown in alkaline solution, was monitored at the potential of open circuit in an acid solution of pH 2.7. A model for enhanced stabil ity of hydrous oxide film in acid solution is proposed. The model, based on voltammetric, rotating ring-disc electrode and X-ray photoelectron spectro scopy measurements, implies that after hydrous oxide growth by potential cy cling in an alkaline solution, the oxide placed in an acid solution remains enriched in chromium due to selective dissolution of nickel and iron. The Cr-O-Cr chains stabilize the modified layer and contribute to the enhanced stability against corrosion. This model unifies several previously proposed models: the model for a hydrous oxide growth by potential cycling, the pol ymeric hydrated oxide model for the passivity of iron and stainless steels and the percolation model for the passivity of stainless steels.