Experimental Study Of Ferritic Stainless Steel In Multi-step Pickling Process

During the manufacturing process, the ferritic stainless steel undergoes hot-rolling or heat treatment, and the material is coated with complicated and compact oxidation layer and Cr-depleted layer which have to be removed entirely through the pickling process. Therefore, the effect of pickling process is closely related to the surface integrity of ferritic stainless steel.In this study, the effect of two pickling process (the H2SO4-HCl pickling process and the H2SO4-(HNO3+HF) mixed acid pickling process) on the surface quality of 430 ferritic stainless steel were studied through SEM, surface roughness and electrochemical analysis. The results were discussed as following:(1) In terms of pickled surface quality, the H2SO4-HCI pickling process is superior to the H2SO4-(HNO3+HF) mixed acid pickling process for that it can remove the oxide layer deeply and smooth the substrates.(2) For the H2SO4 pre-pickling system, as the concentration of sulfuric acid increases and the temperature rises, the weight loss rate of stainless steel increases which leads to fewer residue of oxides layer with some local corrosion on the surface of base metal.(3) For the H2SO4-HCl multiple step pickling process, the hydrochloric acid final pickling process can removal the residues of oxide layer and eliminate the intergranular attack gained from the H2SO4 pre-pickling process and the surface is microstructurally smooth. However, the pitting corrosion will occur on the surface if local corrosion is severe after pre-pickling.(4) The weight loss rate of stainless steel in hydrochloric acid pickling system is much higher than that in the (HNO3+HF) mixed acids pickling system which is beneficial for the removal of Cr-depleted layer and elimination of the spots on the cold-rolled plates. Meanwhile, the surface roughness of stainless steel pickled in hydrochloric acid pickling system is much lower than that in the (HNO3+HF) mixed acids pickling system.(5) The reasonable parameters for the H2SO4-HCl pickling process is as following: pickled in 26%H2SO4 at 80℃ for 42s and then pickled in 4%HCl-3%oxidizer at 30℃ for 50s. The pitting potential of pickled stainless steel reaches 0.143V with the surface roughness of 2.55μm.