Research And Application Of A New HCL-based Pickling Method For Ferritic Stainless Steel

The mixture of HNO3 and HF is widely utilized in domestic and overseas industry as the pickling solution for various types of stainless steel due to its stable and effective pickling effect.However,this pickling method involves severe environmental concerns due to the emmision of NOx and nitrites.A new environmental-friendly pickling method without the use of HNO3 or HF is therefore urgely demonded.In this paper,the hydrochloric acid is employed as a relatively ecological pickling media and a new HCl-based pickling method is studied and developed comprehensively for ferritic stainless steel(FSS)with improved surface quality.In this work,dynamic pickling behavior of FSS was investigated in both the initial and the intermediate hydrochloric-based electrolytes in consideration of the influences of the consumption of acids and the accumulation of metal cations.Additionally,three inhibitors were employed in the pickling solution and their inhibiting mechanism and inhibiting effects were evaluated respectively.Based on the above research,a new hydrochloric-based pickling technique for hot-rolled FSS plates was developed,and the commercial tests have been proceeded in a domestic stainless steel plant.The principle conclusions are summarized as follows:(1)The HCl-based pickling process was successfully applied in a domestic pickling production line with 15 rolls(450t)of qualified stainless steel sheets.Compared with the HN03-HF picking process,the HCl-based pickling process achieved superior surface quality with higher finish,lower surface roughness as less local corrosion.(2)The introduction of hydrogen peroxide into the HCl-based solution was beneficial for elevating pickling efficiency and improving the surface quality due to its influence on the corrosion potential of FSS.The electrochemical tests indicated that the mutation of corrosion potential occurred with 0.52 mol·L-1 H2O2 in 1.37 mol·L-1 HCl solution.The hydrogen peroxide balanced the redox potential of pickling solution inderectly via oxidizing Fe2+ to Fe3+ when the content was below 0.52 mol·L-1.But it participated in the electrode reaction directly as cathodic when the content was beyond 0.52 mol·L-1 which led to the significant acceleration of corrosion rate of FSS.(3)18C was a suitable and effective inhibitor in hydrochloric-based pickling technique for FSS,with the inhibition efficiency of 93.94%.The results indicated that 18C was an anodic type inhibitor,and the inhibition behavior fit the Langmuir isothermal adsorption,with a negative catalytic effect.(4)During the pickling process of FSS,the redox potential of the electrolyte would be reduced by the accumulation of Fe2+ and enhanced by that of Fe3+.The control of cFe3+/cFe2+thus had a significant influence on the pickling efficiency and the final surface quality.According to the experimental results,the value cFe3+/cFe2+ of over 2 was recommended.(5)An appropriate supplement method is important for the stabilized pickling effect.The weight loss rate of FSS increased gradually with accumulating of Fe2+ and Cl-in a continuous supplement mode;whilst the weight loss rate of FSS decreased gradually with accumulating of Fe2+ and consuming of Cl-in an intermittent supplement mode.