Structure And Corrosion Resistance Of Micro-arc Oxidation/polymer Coating On Surface Of AZ31Magnesium Alloy

In order to improve the corrosion resistance of AZ31magnesium alloy with micro-arc oxidation(MAO) coating further, polypropylene (PP), epoxy resin and polystyrene (PS) were used. Fourier transform infrared spectroscopy (FTIR) was used for detecting the structure of organic film and the chemical bonds between organic film and MAO coating. Scanning electron microscope (SEM) and energy dispersive X-ray (EDS) analysis were used for observing the surface morphology and distribution of all kinds of elements. 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 micro pores and cracks on the surface of the MAO coating can be filled by the PP. The corrosion current density of the sample with PP film on MAO coating is about three orders of magnitude lower than that of the sample with MAO coating, and the impedance at low frequency is increased two orders of magnitude higher in the initial immersion stage in3.5wt%NaCl solution.The influence of silane pre-treatment on corrosion performance of epoxy film on MAO coating was investigated. The results show that the silane pre-treatment could improve the adhesion between the MAO coating and the epoxy film. Compared to the sample without silane pre-treatment, the corrosion current density is reduced by about one order of magnitude, the impedance at low frequency is increased three orders of magnitude in the initial immersion stage in3.5wt%NaCl solution.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 thickness and the uniformity of the PS film on MAO coating are enhanced with the increase of concentrations. 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 two and three orders respectively. The results of EIS indicate that the corrosion resistance of the sample treated by15%PS solution is the highest, the corrosion resistance of it is above105Ω·cm2, about two orders of magnitude higher than other samples at the end of immersion.The failure of the MAO coating and organic film was investigated. The results show that the corrosion media penetrate into the organic film and the MAO coating, then react with the substrate. Due to the blocking effect of MAO coating and organic film, the corrosion products are difficult to transfer into solution in time and accumulate in the interface between the MAO coating and substrate. Since the molar volume of corrosion products is larger than that of substrate, it will give a swelling effect to the coating, then the structure of organic film and MAO coating is broken.