Pickling behavior of AISI 304 stainless steel in sulfuric and hydrochloric acid solutions

Oxide scales as well as a Cr-depleted layer, which grows between the oxide scale and base metal, are formed on AISI 304 stainless steel surface during high temperature processing. Pickling is an important process which includes mechanical and chemical operations to remove oxide scales, Cr-depleted layers, and to recover the surface passivity. The multi-step pickling is commonly used because of its higher efficiency than a single step pickling. In this study, the multi-step pickling of AISI 304 stainless steel in HCl solution was investigated instead of H 2 SO 4 solution for the first step of pickling. HF+HNO 3 mixed acid is traditionally used in the second step of pickling. The pickling mechanism of HCl and H 2 SO 4 was discussed based on weight loss and the pickled surface qualities. It was found that the first step pickling efficiency directly affected the surface qualities of the final pickled sample. HCl solution showed much lower pickling efficiency than H 2 SO 4 solution. This resulted in a high concentration of remaining oxide scales and intergranular attack at the Cr-depleted layer, which cannot be completely removed in the second pickling step. Increasing of HCl concentration and electrolytic current did not improve the pickling efficiency. The addition of a small amount of H 2 O 2 , which is a strong oxidizing agent, significantly improves the pickling efficiency of HCl. A smooth surface without any oxide scale and free of intergranular attack could be obtained.