Investigation Of Stability And Deterioration Process Of AZ91D With MAO Coating Prepared By Alternative Square-wave Power Source

Micro-arc oxidation (MAO) coating was prepared on magnesium alloy AZ91D using an alternative square-wave power source. The effects of electric parameters, such as the applied voltage, the frequency and the duty cycle, on the thickness, surface morphology and structure of the MAO coating were investigated by orthogonal experiment. The results indicate that the thickness and porosity of MAO coating increase with the applied voltage and the duty cycle, but decrease with the frequency. The thickness and porosity of MAO coating show opposite effects on its corrosion resistance. The corrosion resistance increases with the thickness, but decreases with the porosity, resulting in an optimal value for the applied voltage and the duty cycle for obtaining higher corrosion resistance of AZ91D.FTIR microscopic mapping was used to investigate the formation and transformation of Mg(OH)2in MAO coating. In the MAO coating, Mg(OH)2is formed accompanying with micro-arc sparking and the amount and distribution of Mg(OH)2is related to the intensity and position of sparking, which depends on the applied voltage and duty cycle. In NaCl solution, Cl ions are easy to be absorbed on the surface of a MAO coating where Mg(OH)2is present and promote Mg(OH)2transform into the soluble magnesium salt MgCl2Meanwhile, MgO in a MAO coating can be transformed into Mg(OH)2because Mg(OH)2is more stable than MgO in aqueous solution. Therefore, micro pores and cracks of the MAO coating are exposed into the corrosive media which is allowed to contact the magnesium alloy substrate directly and cause its corrosion.The polarization curves of magnesium alloy AZ91D with MAO coating showed several typical patterns caused by differences in the composition and structure of the MAO coating. The pattern of the polarization curve of magnesium alloy AZ91D with MAO coating depends on its primary composition, structure and many experimental factors, such as the concentration of chloride ions, pH value of the electrolyte, degree of cathodic polarization and the exposed area of the specimen. These factors change the pattern of polarization curve of magnesium ally AZ91D with MAO coating by affecting the main composition and structure of the MAO coating because of its instability in aqueous solution. For magnesium alloy AZ91D with a MAO coating immersed in NaCl solution, the rate determining steps of the anodic and the cathodic reactions are the mass diffusion and charge transfer steps, respectively. As a result, the corrosion current density fitted from the polarization curve is not an accurate corrosion rate.Magnesium alloy with MAO coating dissolves with local activity in aqueous solution because there are many pores in a MAO coating, which allow the corrosion media contacts with magnesium alloy substrate. These small active regions or sites can be detected by Scanning Electrochemistry Microscopy (SECM). The results indicate that the size and distribution of the active sites and/or regions are related to the structure of MAO coating. The pregnance, growth and evolution of active sites can be observed on the magnesium alloy with thick and porous MAO coating, while several active sites and active regions coexist in the case of thin MAO coating. The pregnance, growth and evolution of active sites are affected by chloride ion in aqueous solution because of the influence of chloride ion on the stability of MAO coating. Comparing the EIS data with the SECM data, the macroscopic electrochemical behavior of magnesium alloy AZ91D with MAO coating is consistent with its microscopic electrochemical behavior.