超细晶304L不锈钢在含Cl^-溶液中点蚀行为的研究

INVESTIGATION ON PITTING CORROSION BEHAVIOR OF ULTRAFINE-GRAINED 304L STAINLESS STEEL IN Cl^- CONTAINING SOLUTION

采用动电位极化、循环极化、电化学阻抗谱、Mott-Schottky曲线结合表面形貌观察,研究了利用等通道转角挤压方法制备的晶粒尺寸为(130±30)nm的超细晶304L不锈钢在含Cl-溶液(0.05 mol/L H2SO4+0.05 mol/L Na Cl)中的点蚀行为.研究表明,超细晶材料比粗晶材料具有更高的腐蚀电流密度和钝化电流密度,更低的腐蚀电位、破钝电位和保护电位,且钝化区更窄.严重塑性变形引起304L不锈钢材料晶粒显著细化,一方面增加了表面钝化膜的施主密度和扩散系数,降低了钝化膜的致密性,使Cl-在材料表面的吸附能力增强;另一方面增加了晶界含量,使Cl-沿晶界向内扩散能力增强,促进了点蚀形核和长大.

The electrochemical behavior and pitting corrosion in a Cl^- containing solution （0.05 mol/L H2504＋ 0.05 mol/L NaC1） of the ultrafine-grained 304L stainless steel （304L SS） with average grain size of （130±30） nm prepared by equal channel angular pressing （ECAP） technique were examined using potentiodynamic polarization curves, cycle polarization curves, electrochemical impedance spectroscopy （EIS）, Mott-Schottky （M-S） curve measurements together with SEM observation of surface morphology. As compared to the coarse-grained counterpart,the ultrafine-grained sample exhibited a higher corrosion current density icorr of 81.74 A· cm^2 and a lower corrosion potential Eoo, （vs SCE） of-466 mV, and having a higher passivation current density ip of 32.38 μA· cm^2 and a narrower passive region （-315-450 mV） together with a breakdown potential Eb decrease of 100 mV and a protection potential Ebp decrease of 190 mV. On one hand, the grain refinement induced by severe plastic deformation deteriorates the compactness of the passive films and is helpful for the Cl^- absorption, resulting in a 1.6 times increase of the carrier density and one order of magnitude increase of the diffusion coefficient in the passive films. On the other hand, the significant increase of grain boundaries provides more possibility for Cl^- diffusion along grain boundaries, and thus promotes the pitting nucleation and growth.