Mechanism of electrolytic pickling of stainless steels in a neutral sodium sulphate solution

Oxide scale that mainly consists of spinel‐type metal oxides is formed on stainless steels when they are heated up during the manufacturing cycle. The removal of the oxide scale and chromium depleted subscale by pickling is one of the most important processes during the production. Electrolytic pickling in neutral sodium sulphate is widely used and provides relatively fast removal of the oxide scale. Despite its vast use, the essence of the electrochemical mechanism of electrolytic pickling is not clear.

Stainless steel EN 1.4301 and EN 1.4404 samples were annealed in a laboratory furnace to produce an oxide scale similar to process conditions. The oxide scale was protective at potentials equal to the passive region but dissolved at anodic potentials above 1050 mV vs. SCE. The dissolution was found to proceed by electrochemical reactions of the scale. Three different stages were discernible in the dissolution process. At the beginning chromium and manganese of the outer layer were preferentially dissolved. When the chromium content in the outer layer decreased, the scale was enriched of iron and dissolution was hindered causing the electrode potential to increase. At the third stage the potential first decreased and then steady state was obtained. The oxide thickness was greatly reduced and the chromium content was lowered below the base metal level. At the steady state the remaining oxide was rich in iron and silicon. Silicon was not dissolved by electrochemical reactions and remained at the surface after prolonged polarisation.