Non-Equilibrium Dissolution of Aluminum under Cathodic Polarization

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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