Study On New Pickling Process Of Ferritic Stainless Steel

The pickling process,which is used to remove the oxide layers produced in rolling and heat treatment,can directly affect the surface integrity of steel products.The traditional pickling process utilized the HNO₃+HF mixed acids to remove the oxide layer completely.However,the use of nitric acid involves extremely serious environmental concerns owing to the emission of NOx and nitrites.In this paper,a new environmental pickling system utilized the H₂SO₄+HF mixed acids was proposed to replace the traditional HNO₃+HF mixed acids pickling solution and a new-type pickling process for ferritic stainless steel was designed,which holds significance to the energy conservation and emission reduction for steel factory.In the present study,the roles of HNO₃ and HF during pickling process were investigated.On the basis of that,an environmental friendly H₂SO₄+HF pickling system was proposed.The corrosion behavior of ferritic stainless steel in the H₂SO₄+HF solutions was discussed and a multi-step pickling process was designed for stainless steel.The conclusions of the study are summarized as following:?1?In the traditional HNO₃+HF pickling process,the increasing of the HF content will accelerate the corrosion rate and improve the pickling efficiency;the increasing of HNO₃ concentration can boost the corrosion potential of stainless steel and the corrosion current density thus increase,which is beneficial for the removal of the oxide layer.?2?With the introduction of oxidant?H₂O₂?to the H₂SO₄+HF pickling system,the effect of HNO₃ in HNO₃+HF solution can be replaced due to the synergy of H₂SO₄ and H₂O₂,which eliminates the emission of NOx.In the H₂SO₄+HF pickling system,the increasing of acid can improve the pickling efficiency,and the H₂O₂ can boost the corrosion potential of stainless steel.?3?The multiple pickling process utilized the H₂SO₄+HF mixed acids can remove the oxide layers deeply and passivate the substrates,so that a smooth surface with high corrosion resistance can be achieved.The reasonable composition of the pre-pickling system is as following:2.74mol/L H₂SO₄+1.37mol/L HF+0.3mol/L H₂O₂.and the reasonable composition of the pre-pickling system is as following:0.69mol/L H₂SO₄+0.137mol/L HF+0.88mol/L H₂O₂.