| During the production of stainless steel,oxide scales with complex composition and compact structure are formed on the surface after rolling and annealing process.An uneven Cr-depleted layer(CDL)is produced due to the preferential oxidation of Cr.Both the oxide and Cr-delpleted layer needs to be removed for the preparation of subsequent process,such as cold rolling,electroplate and coloring.A typical pickling process for stainless steel is pre-pickling with sulfuric acid followed by HNO3-HF mixed-acids pickling.The mixed-acids pickling process undertakes multiple function,including removing the residual oxide scales and the CDL,and passivating the base metal,so that the pickled stainless steel sheets have defects such as CDL residual and local attacks.Moreover,the use of HNO3 inevitably produces NOx and nitrites pollution,which increases pressure to the steelmakers.A new environmental friendly pickling process without nitric acid for stainless steel is therefore urgently required.Two typical stainless steel sheets,304 austenitic stainless steel and 430 ferritic stainless steel were employed as raw material in this study.The high-temperature oxidation behavior of the two stainless steel sheets were investigated to illuminate the structural characteristics of oxide scales and CDL.A multi-steps environmental-friendly pickling process was then designed for the two types of stainless steel.Several theoretical issues including accelerated corrosion,transition mechanism of corrosion product film,local attack control and passivation involved during pickling process were in-depth studied.The role of H+,halide ions and oxidant during pickling was discussed and the corrosion mechanism of stainless steel in different pickling system was proposed.The principle conclusions of this study are as follows:(1)For the austenitic and ferritic stainless steel,the oxide scales consist of a Fe-riched(Fe2O3)outer layer,a Cr-riched(Cr2O3)inner layer and a mid-layer composed of spinel and corundum(FeFe2-xCrxO3)phases.The Cr-depleted layer grows along the oxidation frontier ranging from 1.5 to 3 μm.Due to the selective oxidation of Si at the oxidation frontier,the Cr-depleted phenomena can be alleviated.(2)During the oxidation process,the oxide scales on 430 ferritic stainless steel grows inward evenly,resulting in relatively even oxidation frontier and CDL.However,oxide nodules with alternated Cr-riched dense oxide scales and Ni-riched scales,are commonly formed on 304 austenitic stainless steel during oxidation.Since the oxide nodules have varying depth and radiuses,the CDL is not even for 304 stainless steel.Therefore,pickling 304 austenitic stainless steel in more difficult than 430 ferritic stainless steel.(3)For the austenitic and ferritic stainless steel,the oxide scales consist of a Fe-riched(Fe2O3)outer layer,a Cr-riched(Cr2O3)inner layer and a mid-layer composed of spinel and corundum(FeFe2-xCrxO3)phases.The Cr-depleted layer grows along the oxidation frontier ranging from 1.5 to 3 μm.Due to the selective oxidation of Si at the oxidation frontier,the Cr-depleted phenomena can be alleviated.(4)During the oxidation process,the oxide scales on 430 ferritic stainless steel grows inward evenly,resulting in relatively even oxidation frontier and CDL.However,oxide nodules with alternated Cr-riched dense oxide scales and Ni-riched scales,are commonly formed on 304 austenitic stainless steel during oxidation.Since the oxide nodules have varying depth and radiuses,the CDL is not even for 304 stainless steel.Therefore,pickling 304 austenitic stainless steel in more difficult than 430 ferritic stainless steel.(5)The corrosive manner of stainless steel in pickling solutions is determined by the concentrations of oxidant and halide.For the austenitic and ferritic stainless steel,the oxide scales consist of a Fe-riched(Fe2O3)outer layer,a Cr-riched(Cr2O3)inner layer and a mid-layer composed of spinel and corundum(FeFe2-xCrxO3)phases.The Cr-depleted layer grows along the oxidation frontier ranging from 1.5 to 3 μm.Due to the selective oxidation of Si at the oxidation frontier,the Cr-depleted phenomena can be alleviated.(6)For the austenitic and ferritic stainless steel,the oxide scales consist of a Fe-riched(Fe2O3)outer layer,a Cr-riched(Cr2O3)inner layer and a mid-layer composed of spinel and corundum(FeFe2-xCrx03)phases.The Cr-depleted layer grows along the oxidation frontier ranging from 1.5 to 3 μm.Due to the selective oxidation of Si at the oxidation frontier,the Cr-depleted phenomena can be alleviated.(7)The passive film formed in H2SO4-based solution is composed of Cr2O3,Fe2O3,Cr(OH)3,FeOOH and sulfates and its stable thickness is 3.4±0.2 nm.According to the PDM and Mott-Schottky analysis,the passive film is an n-type semiconductor with the donor density ranging of 1.6～24×10-21 cm-3.The calculated electrical field strength and defect diffusivity in the film are 75367 V·cm-1 and 8.034×10-16 cm2/s,respectively. |
