Study On Local Corrosion Behavior Of Ferritic Stainless Steel In HCl Electrolytes

As the first step of cold rolling, the effect of pickling process is closely related to the surface integrity of ferritic stainless steel. Common chemical pickling in mixed HNO3-HF is widely applied, but nowadays the application of such HNO3-containing mixed acids is restricted for environmental concerns. Meanwhile, this pickling process is weak in the removal of the chromium-depleted sublayer. Hydrochloric acid pickling with high pickling speed and low pollution becomes the trend of pickling process. However, the existence of chloride ion in HCl-based electrolytes is likely to induce local corrosion such as pitting and intergranular corrosion which restricts its development. The study on local corrosion behavior of ferritc stainless steel in HCl-based electrolytes holds significance for the application of HCl-based pickling process.In this study, the effects of hydrochloric acid, inhibitor and oxidizer on the local corrosion behavior of ferritic steel is investigated based on electrochemical methods and scanning electron microscope. The results were discussed as follows:(1) With increasing hydrochloric acid concentration, the corrosion potential of ferritic stainless steel moved to positive direction and the corrosion current increased which means the acceleration of corrosion behavior. Meanwhile, the corrosion of ferrictic stainless steel in HCl electrolytes was selective which preferentially localized in grain boundary.(2) Inhibitor SDS, SAD and 18C were interface inhibitor and their inhibition efficiency had a threshold. The order of their efficiency was:18C>SAD>SDS.(3) The addition of oxidizer changed the electrode reaction. The cathode reduction of oxidizing agent occured with the hydrogen evolution in HCl electrolytes while the anode dissolved with multivalent metal ions.(4) The addition of oxidizer weakened the selective corrosion of ferritic stainless steel and facilitated uniform corrosion by activating the whole surface.(5) The addition of oxidizer B changed the developing manner of pitting corrosion. The typical V shape pits expanded and developed into hemisphere. The integration of hemisphere pitting corrosion led to the brightening of ferritic stainless steel.