Fundamental Study On The New HCl-based Pickling Process For Ferritic Stainless Steel

Surface integrity, one of the most important indexes of stainless steel sheets, is closely related to rational pickling process. According to the industrial processes currently applied, stainless steel pickling is normally almost exclusively based on the use of a nitric-hydrofluoric acid mixture, which involves extremely serious pollution of NOx and leads to a rough pickled surface. In this dissertation, on the basis of the previous research, the new HCl-based pickling process for ferritic stainless steel was developed to solve the existing problems.In this study, the effects of hydrochloric acid and temperature on the HCl-based pickling system were systematically investigated, and then the effect mechanism of oxidizer and stabilizer was discussed through the weight loss test, SEM-EDS analysis and the electrochemical analyzer. On this basis, the new HCl-based pickling process was developed and compared with the HNO3-HF mixed acid pickling process. The main research contents and results obtained in this paper are as follow:(1) The HCl-based pickling process is effective and feasible for ferritic stainless steel since it can completely avoid the NOx pollution brought by the HNO3-HF mixed acid pickling process and improve the surface integrity obviously.(2) The rational parameters of HCl-based pickling process for ferritic stainless steel are as follow:The composition of the pickling solution contains 10%-15% hydrochloric acid, 1%-2% oxidizer with stabilizers consisting of the mixture of D and f. The appropriate pickling temperature is 30-50?.(3) In the sole hydrochloric acid pickling system, the pickled stainless steel resulted in a less smooth surface with residue oxides and local corrosion. With oxidizer added, the oxide layer can be removed completely and the pickled surface becomes smoother.(4) With the increase of temperature, concentrations of hydrochloric acid and oxidizer, the charge transfer resistance of ferritic stainless steel in HCl-based solutions decreased and the corrosion current increased according to the electrochemical measurements. Correspondingly, the weight loss rate increased which accompanied with a smoother pickled surface without oxides residue.(5) The pickling effect significantly depends on the oxidizer. According to this research, compared with oxidizer KMnO4 and KClO3, oxidizer H2O2 is more effective to smooth the pickled surface.(6) The stabilizer, which inhibits the decomposition of hydrogen peroxide, plays a vital role to control the HCl-based pickling process. Moreover, with the mixture of stabilizer D and F added as stabilizer, the pickled surface became smoother.This research holds guiding significance for the resolution of the existing NOx pollution problem and improvement of the pickled surface integrity of ferritic stainless steel sheets.