不锈钢点腐蚀发生的早期过程Ⅱ.电化学扫描隧道显微研究

应用电化学扫描隧道显微技术,考察外加电位、介质条件及时间等因素对不锈钢发生微点腐蚀早期阶段表面形貌的影响,从纳米级空间分辨度进一步探索不锈钢表面钝化膜破坏及微点腐蚀发生早期过程机理.结果表明,不锈钢在点蚀早期阶段,首先发生不稳定微点腐蚀,其表面形貌随着不稳定微点腐蚀的发生、消长而变化.不稳定微点腐蚀的几何尺寸约为10nm,极化电位愈正,不锈钢钝化膜局部溶解破坏和微点腐蚀发生的倾向性愈大,不稳定的微点腐蚀在一定的条件下可发展为稳定点腐蚀.即使在开路电位下,Cl-也可能在不锈钢表面呈不均匀分布,进而导致钝化膜

ELECTROCHEMICAL SCANNING TUNNELING MICROSCOPE STUDIES OF INITIAL PITTING CORROSION IN 18/8 STAINLESS STEEL

The surface topographies of stainless steel in an early stage of pitting corrosion were imaged by a home made electrochemical scanning tunneling microscope (ECSTM). The influences of potential, chloride and time on the surface topographics were examined in nanometer spatial resolution corresponding to the polarization behaviors. It was further indicated that the local breakdown of passive film and micropitting corrosion always occur in the potential region which was much more negative than that of the ordinary critical pitting corrosion. The initiation, temporary development, and repassivation of the metastable micro pitting corrosion were important reactions during an very early stage of pitting corrosion. The tendency of metastable micro pitting corrosion of approximate 10 nanometer geological size increased with polarized potential. Chloride appeared un uniform distribution on the surface and induced the local breakdown of passive film for the 18/8 stainless steel in NaCl solution.