| Abstract:In order to improve the corrosion resistance of2519aluminum alloy with micro-arc oxidation(MAO) coating further, polypropylene (PP) and polystyrene (PS) was used to cover the surface of sample. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray (EDS) analysis were used for the structure and surface morphology of organic film and MAO coating. The electrochemical polarization test, electrochemical impedance spectroscopy (EIS) and immersion test were used for studying the corrosion behavior of MAO coating and organic film.The corrosion protection properties of PP film on the MAO coating were studied. The results show that due to the fact that the micro pores and cracks on the surface of MAO coating were sealed by PP film, the corrosion resistance of covered sample were enhanced. From the results of electrochemical polarization test, the corrosion current density of the sample with MAO coating is about two orders of magnitude higher than that of the sample with PP film on MAO coating. From the results of immersion test, the sample with MAO coating was stable in NaCl solution during the initial stage of immersion. After168h immersion, there are several corrosion pits on its surface. When the immersion time is prolonged to720h, many white corrosion products can be observed on its surface, while for the sample sealed by PP almost no corrosion pitting and corrosion products can be observed on its surface. As shown of EIS results, during the period of immersion, for the unsealed sample, the appearance time of Warburg element is1h, but for the sealed sample, the appearance time of Warburg element is360h. At the end of immersion, corrosion resistance of unsealed sample is about103Ω·cm2, but the sealed sample is above105Ω·cm2.The corrosion protection properties and surface morphology of the sample with PS film on MAO coating through different concentrations were investigated. The results show that the anti-corrosion performance of the sample with PS film on MAO coating was enhanced. Compared to the sample with MAO coating, the corrosion current density of the sample treated by5%PS solution decreases about one order of magnitude and that of the sample treated by10%and15%PS solution about four and three orders respectively. The results of EIS indicate that during the period of immersion, for the sample sealed by5%PS film, the appearance time of Warburg element is96h, but the sample sealed by10%and15%PS film is1104h and696h respectively. At the end of immersion, the resistance of all samples is stable. The resistance of sample sealed by5%PS is about103Ω·cm2, but that of sample sealed by10%PS and15%PS is105Ω·cm2and104Ω·cm2.10%is the optimum.The failure of the MAO coating and organic film was investigated. The results show that in the initial stage of immersion, the corrosion media penetrate into the organic film and outer porous layer through the defects of organic film and porous layer. With the prolong of immersion time, the corrosion media diffuse into the barrier layer and react with the substrate. Due to the barrier effect of MAO coating and organic film, the corrosion products accumulate in the interface between the MAO coating and substrate. Since the molar volume of corrosion products is larger than that of substrate, the structure of organic film and MAO coating is broken. |
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