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This paper describes the preliminary results of corrosion protection of AZ91D Mg alloy coated with a micro-arc oxidation (MAO) coating. The corrosion behavior of coated substrates was evaluated by means of immersion and electrochemical tests in 5 wt.% NaCl solution. The immersions tests include weight loss measurement after full or partial immersion with the whole coat and full immersion with the damaged coat. The electrochemical tests include polarization curves and electrochemical impedance spectroscopy (EIS) measurements. The results of immersion and electrochemical tests show that micro-arc oxidation (MAO) surface treatment can significantly improve the corrosion resistance of the AZ91D Mg alloy in 5% NaCl solution, and the corrosion protection effect of the MAO coating is not sensitive to pores or cracks in the coating. The charge transfer resistance R_ct, increases from about 1.669Ωcm² for the uncoated alloy to about 1113Ωcm² after being coated with MAO coat.

Immersion and electrochemical tests have been applied to study corrosion protection of AZ91D Mg alloy coating with chrome-free chemical conversion (CCC) coat in 5wt% NaCl solution. The immersions tests include weight-loss measurements after full or partial immersion with whole coating and full immersion with damaged coating. The electrochemical tests include electrochemical impedance spectroscopy (EIS) and polarization curves measurement. The results of immersion and electrochemical tests show that chrome-free chemical conversion (CCC) surface treatment can significantly improve the corrosion resistance of AZ91D Mg alloy in 5% NaCl solution, and that the corrosion protection effect of CCC coating is not sensitive to pores or cracks. The equivalent circuit models are fitted from Nyquist plots for the uncoated specimens and the CCC-coated specimens. The charge transfer resistance, R_ct, increases from about 1.669 Ω cm² for the uncoated alloy to about 210 Ω cm² after the alloy is coated with CCC coating.

Influence of surfactants on the corrosion properties of chromium-free electroless nickel deposit were investigated on AZ91D magnesium alloy. The corrosion tests were carried out by immersion test (1 M HCl) and electrochemical polarization test (3.5 wt% NaCl). The surfactants in the electroless nickel bath increases the corrosion resistance properties of the deposit on the magnesium alloy. In addition, smoothness and amorphous plus nano-crystalline phase were increased and accounted for the significant corrosion resistance. As a consequence, the corrosion potential moved towards the positive direction and the corrosion current density decreased. The immersion tests also provided the same trend as that of electrochemical polarization test. On the whole, the study concluded that corrosion resistance was enhanced by including a surfactant in the electroless deposits on magnesium alloy.