Non-Equilibrium Dissolution of Aluminum under Cathodic Polarization

利用扫描电子显微镜、透射电子显微镜、电感耦合等离子体原子发射光谱、原子力显微镜、共聚焦激光扫描显微镜和相位检测干涉显微镜，在恒电位和电位扫描条件下，研究了铝表面不同类型的阳极氧化膜的阴极极化。结果总结如下：a)膜溶解效应在E_a=15、20和25V(vs.Ag/AfC_1)的电解铝样品上形成多孔型和阻挡型阳极氧化膜，然后将E_a=25V的样品浸入草酸溶液中，通过化学溶解使膜(或阻挡层)变薄。样品在中性硼酸盐溶液中通过阴极扫描进行极化，以测量样品上形成的凹坑的数量N_p和尺寸L_p以及溶解的Al^lt；3+和…的量的变化发现两种类型的阳极氧化膜的N_p值约为10^7m^~(-2)，与E_a无关，N_p最高可达10^~(-2)。在100V下，在电抛光样品(样品-BS)、表面粗糙样品(样品-BR)、水热处理样品(样品-HA)和热处理样品(样品-TA)上形成势垒型氧化膜，然后在中性硼酸盐溶液中对样品进行阴极扫描，发现凹坑的数量按样品-TA&GT；样品-BR&GT；样品-HA&GT；C)溶液成分的影响在硫酸溶液、硫酸钠溶液和硫酸/硫酸铜溶液中，在恒电位下对覆盖有25V阻挡层的阳极氧化膜进行阴极极化。在硫酸钠溶液中有孔洞形成，而在硫酸溶液中没有。在硫酸铜溶液中发现了铜的局部沉积。结论a)在阳极氧化膜覆盖的铝的阴极极化过程中，由于氧化物/金属界面上的氢气泡形成的应力作用，在阳极氧化膜的缺陷处局部开始了膜的破裂和凹坑的形成。由于缺陷处pH值的增加，通过薄膜破裂将金属衬底暴露在溶液中会导致金属衬底的腐蚀。凹坑中气泡的积累和释放使凹坑在相对较低的电位下生长。b)阳极氧化膜中的缺陷是从金属表面的粗糙表面形成的，并在薄膜生长过程中通过无定形氧化膜的结晶形成。阳极氧化膜中缺陷的数量与阴极极化曲线有关，阴极极化技术可以用来估计缺陷的分布。较少

Cathodic polarizaiton of aluminum covered with different types of anodic oxide films has been examined under potentiostatic and potential scanning condition by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), atomic force microscopy (AFM), confocal laser scanning microscopy (CLSM), and phase detection interferometric microscopy (PDIM). Results are summarized in the following.a) Effect of film dissolutionPorous type and barrier type anodic oxide films were formed on elec-tropolshied aluminum specimens at E_a = 15,20, and 25 V (vs. Ag/AfC1), and E_a = 25 V specimen was then immersed in an oxalic acid solution to thin the film (or barrier layr) by chemical dissolution. The specimens were cathodically polarized by potential scanning in a neutral borate solution to measure the change in the number, N_p, and size, L_p, of pits formed on the specimens, and in the amount of dissolved Al^<3+> … More ions, W_d, during catho-dic polarization.It was found that the N_p value for specimens covered with both types anodic oxide films is ca. 10^7 m^<-2>, independent of E_a, and that it increases up to 10^<10> m^<-2> by decreasing the film thickness by chemical dissolution.b) Effect of preparation of anodic oxide filmsBarrier type oxide films were formed at 100 V on electropolished specimens (Specimen-BS) , surface roughened specimens (Specimen-BR), hydrothermally treated specimens (Specimen-HA), and thermally treated specimens (Specimen-TA) , and then the specimens were cathodiclly polarized by potential scanning in a neutral borate solution.It was found that the number of pits decreases in the order of Specimen-TA>Specimen-BR>Specimen-HA>Specimen-BS,and that it can be related with the cathodic polarization curve.c) Effect of solution compositionSpecimens covered with 25 V barrier type anodic oxide films were cathodically polarized under potentiostatically in a sulfuric acid solution, soduium sulfate solution, and sulfuric acid/copper sulfate solution. Pit formation was found in the sodium sulfate solution, but not in the sulfuric acid solution. In copper sulfate solution, the local copper deposition was found.Conclusiona) Film breakdown and pit formation duriring cathodic polarization of aluminum covered with anodic oxide films start locally at the imperfections in the anodic oxide film by the stress formed by H_2 gas bubble at the oxide/metal interphase. Exposure of the metal substrate to the solution by film breakdown leads to corrosion of the metal substrate, because of the increase in pH at the imperfection. Accumulation and release of gas bubble from the pit allow the pit to grow under a relatively low potential.b) Imperfections in anodic oxide films are formed from the rough surface of the metal surface and through the crystallization of amorphous oxide films during the film growth. The number of imperfections in the anodic oxide film can be related with the cathodic polarization curve, and the cathodic polarization technique can be used for the estimation of distribution of imperfections. Less