Mechanism of single and multiple step pickling of 304 stainless steel in acid electrolytes

Pickling of oxidised 304 stainless steel in single or multiple electrolytes was investigated based on weight loss and corrosion potential measurements. A smooth surface finish was obtained after an extended pickling in HCl, but some external oxide scales remained. On pickling in HF, the external oxide scales were fully removed, but internal oxide scales remained, and intergranular corrosion took place. Multi-step pickling in successive HCl and HF electrolytes allowed achieving a smooth surface finish free of any oxide scales. The sequence and duration of pickling steps in such multi-step pickling greatly influenced the final surface finish. Corrosion potential of oxidised stainless steel in both electrolytes could differ, and that difference leaded to a uniform pickling of stainless steel in HCl and a localised pickling in HF.