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

钝化膜由金属表面的薄氧化膜组成，起到防止环境侵蚀的防腐屏障的作用。许多作者已经对该薄膜进行了研究，但由于其厚度为纳米量级，其结构、成分等尚不清楚。在本研究中，我们采用电位调制反射(PMR)和光诱导发光技术来研究铁、镍、不锈钢和钛表面的钝化膜，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表现出任何整流作用，而铁、钛和不锈钢在正偏置条件下相对于单个平带电位表现出对阳极电流的强烈抑制。通过阻抗和PMR测量，估算了偏置电势与空间电荷层厚度的关系。对于铁、不锈钢和钛，空间电荷层在薄膜中形成，并起到有效的绝缘层的作用。对于钛表面的钝化膜，进一步测量了光致发光随阳极电位的变化关系。观察到峰值波长约为410 nm的发光光，推测其对应于氧化膜中电子的带间失激。在约720 nm的波长范围内也观察到了极弱的光。光可能反映了阴离子吸附引起的表面态。

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.