316L奥氏体不锈钢在不同电位下的点蚀和再钝化行为研究

Pitting and Repassivation Behavior of 316L Austenitic Stainless Steel under Different Potentials

目的研究外加电位对316L奥氏体不锈钢点蚀和再钝化行为的影响。方法采用循环极化、恒电位极化,电化学阻抗谱(EIS)等多种电化学测试方法,研究了系列电位与混合电位对316L奥氏体不锈钢点蚀敏感性的影响,并采用扫描电子显微镜(SEM)、X射线光电子能谱(XPS)观察分析钝化膜点蚀形貌和元素含量。结果在60℃的饱和CO_2的10 g/L NaCl溶液中,316L奥氏体不锈钢的钝化区间为-0.394~0.168 V,但电位在-0.100~0.168V之间,即亚稳态点蚀区时,电流出现一定的波动。在钝化区极化时,316L奥氏体不锈钢的稳态电流密度非常低,随外加电位的升高而略有增加,极化后试样表面无点蚀;在亚稳态点蚀区,极化的电流密度较高,极化后,试样表面出现明显的点蚀坑;混合区极化时,电位从0.1 V转换到-0.1 V时,电流密度急剧下降,并稳定在一个较低的电流值。XPS结果表明,在钝化区电位极化后,Cr、Mo元素含量有所升高,而Fe元素发生了选择性溶解。结论 Cr、Mo元素是耐蚀性元素,其氧化物或氢氧化物的存在可促进钝化膜的局部修复,因此电位转换到低电位后,试样表面发生再钝化现象,钝化膜的稳定性增强,材料的耐蚀能力提高。

The work aims to investigate the effect of applied potentials on the pitting and repassivation behavior of 316 L austenitic stainless steel. Cyclic polarization, potentiostatic polarization and electrochemical impedance spectroscopy tests were selected to study the effect of series potential and mixed potential on pitting sensitivity of 316 L austenitic stainless steel. In addition, SEM and XPS were used to observe the pitting morphology and analyze the elemental concentration of passivation film. At 60 ℃, the passive region of 316 L austenitic stainless steel in saturated CO2 and 10 g/L NaCl test solution was-0.394~0.168 V, but when the potential was between-0.100~0.168 V, there was a certain current fluctuation, namely metastable pitting zone. The current density of 316 L stainless steel in passive region was very small, slightly increasing with the increase of potential and the pit was not observed on the sample surface after polarization. However, the current density of 316 L austenitic stainless steel in metastable pitting region was high and obvious pitting appeared on the samples. In the mixed polarization with potential from 0.1 V to-0.1 V, the current density droped sharply and eventually stabilized at a lower current value. Meanwhile, the results of XPS showed that the Cr and Mo concentration increased with the potential transition, but the Fe element was selectively dissolved in this condition. Cr and Mo are corrosion resistance elements, and the existence of the oxides or hydroxides can promote the local repair of passivation film. Therefore, the surface of 316 L stainless steel presents repassivation behavior, the stability of the passivation film is enhanced and the corrosion resistance is improved with the transition to low potential.