| The pickling process, which is aimed at the removal of oxide layers and Cr-depleted layer and the passivation of the base metal, is closely related to the surface integrity. The HNO3-HF mixed acids pickling process, the most widely used process for stainless steel, has severe environment concerns for the nitrite effluent and is weak in the removal of Cr-depleted layer. Therefore, the hydrochloric acid pickling process which can fix the problems mentioned above is an environment-friendly pickling process for stainless steel with broad application prospect.On the basis of the investigation of the electrochemical corrosion behavior of ferritic stainless steel in the H2SO4-HCl mixed acids solutions, this paper developed a multiple step pickling process and proposed the reasonable pre-pickling system and final pickling system. Based on that, theoretical and experimental study of ferritic stainless steel in the oxidized H2SO4-HCl mixed acids solutions was performed. The conclusions of the study were summarized as following:(1) The corrosion and passivation behavior of stainless steel is controlled by the redox potential and the ion composition of the solutions.(2) The multiple pickling process can remove the oxide layers and the Cr-depleted layer deeply as well as passivate the substrates and a smooth surface of stainless steel with high corrosion resistance can be achieved.(3) The pickled surface is smoothly brightening with few residues of oxide layer and no pitting and intergranular corrosion is found.(4) After the final pickling process, the stainless steel is passivated with a bright surface without residue of oxides or local corrosion. And the pitting resistance is better than that of stainless steel pickled in HNO3-HF mixed acids. |
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