Electrolytic pickling of stainless steel studied by electrochemical polarisation and DC resistance measurements combined with surface analysis

A tightly adhering oxide scale is formed on stainless steels when they are annealed. The removal of the oxide scale and chromium-depleted subscale is one of the most important processes during the stainless steel production. Electrolytic pickling in neutral sodium sulphate is widely used for oxide scale removal. This study describes the different stages of the oxide scale removal on Polarit 725 (EN 1.4301) stainless steel in sodium sulphate solution. A mechanism of the scale dissolution is also proposed. The dissolution is proposed to proceed by the electrochemical reactions of the scale in three successive stages. At the beginning of the pickling process chromium and manganese of the outer oxide layer were preferentially dissolved. When the chromium content of the outer layer decreased, the scale was enriched of iron. The electrode potential was then increased and the scale thickness greatly reduced. Finally a steady state was obtained and a thin oxide layer, rich in iron and silicon, was left on the surface. Silicon could not be removed by the electrolytic pickling and post-treatment in nitric-hydrofluoric acid is required. (C) 2001 Elsevier Science Ltd. All rights reserved.