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 cycling voltammetry in 1 mol dm(-3) NaOH solution. The anodic voltammetric peak of Fe(OH)(2) oxidation was used as a measure of the oxide growth as a function of potential limits, sweep rates and the number of potential cycles. The oxide film stability was monitored by the use of rotating ring-disc electrode. During potential cycling in alkaline solution, it was found that after 30 cycles, 1.4% of the anodic charge was due to iron dissolution from the 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 stability of hydrous oxide film in acid solution is proposed. The model, based on voltammetric, rotating ring-disc electrode and X-ray photoelectron spectroscopy measurements, implies that after hydrous oxide growth by potential cycling 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 polymeric hydrated oxide model for the passivity of iron and stainless steels and the percolation model for the passivity of stainless steels.