The Effect of Anion Adsorption on the Stability of Resistive Passivation Film on Steels

钝化膜是金属表面的一层氧化膜，起着保护金属免受环境侵蚀的屏障作用。该膜已被许多作者研究，然而，由于其厚度为nm量级，因此其结构、组成等，并不详细。采用电位调制反射（PMR）和光致发光技术研究了铁、镍、不锈钢和钛的钝化膜，发现钝化膜覆盖的镍电极对溶液中氧化还原物种的反应电流没有整流作用，而铁、钛、不锈钢和钛的钝化膜对溶液中氧化还原物种的反应电流没有整流作用，和不锈钢在相对于各自平带电位的正偏压条件下表现出对阳极电流的强烈抑制。通过阻抗和PMR测量，估计了偏置电位与空间电荷层厚度的关系。对于铁、不锈钢和钛，空间电荷层形成在膜中，并且用作有效的绝缘体屏障。对于钛上的钝化膜，进一步测量了作为阳极电位的函数的光激发发光。观察到一个峰值波长约为410nm的发光，认为这是氧化膜中电子的带间退激。在约720nm的波长区域中也观察到极弱的光。光可以反射由阴离子吸附引起的表面状态。

The passivation film consists of thin oxide film on metals and functions as a barrier of anticorrosion against the environmental attack. The film has been studied by many authors, however, since its thickness is in nm order, the structure, composition, etc., are not known in detail. In this research, we adopt potential-modulation reflectance (PMR) and light-induced luminescence for the study of the passivation film on iron, nickel, stainless steel, and titanium.Nickel electrode covered by the passivation film does not reveal any rectification for the reaction current with redox species in solution, while iron, titanium, and stainless exhibit a strong suppression of the anodic current under the positive bias condition relative to the individual flatband potentials. From impedance and PMR measurement, the relation between the bias-potential and the space charge layer thickness was estimated. For the iron, stainless, and titanium, the space charge layer is formed in the film, and functions as an effective insulator barrier. For the passivation film on titanium, photo-excited luminescence was further measured as a function of anodic potential. A luminescence light with a peak about 410nm wavelength is observed, which is assumed to correspond to an inter-band deexcitation of electrons in the oxide films. An extremely weak light is also observed in a wavelength region about 720 nm. The light may reflect a surface states induced by anion adsorption.