Top coating of low-molecular weight polymer MALPB used for enhanced protection on anodized AZ31B Mg alloys

A protective composite coating on an AZ31B magnesium alloy was prepared by anodic oxidation to form an oxide layer, followed by single immersion in maleic anhydride-g-liquid polybutadiene (MALPB) solution to cover a polymer coating on top. As a low-molecular weight polymer with low viscosity, MALPB had a tendency to infiltrate into the pores and cracks in the anodic layer to fill the defects among oxides so that a compact layer could be formed after it was cured by its hardeners, as observed by scanning electron microscopy (SEM). This compact layer, which was composed of anodic oxides integrated with solidified MALPB, possessed thickness around 0.7 μm as detected by energy dispersive X-ray (EDX). The anodized, MALPB-coated AZ31B alloy exhibited enhancement in corrosion resistance superior to that separately coated by anodizing oxides or MALPB, as reflected by its much higher corrosion potentials (E _ccor) and lower corrosion current density (i _corr) in DC polarization tests. Based on electrochemical impedance spectroscopy (EIS) data, we conclude that the anticorrosive performance of the composite coating can be attributed to the barrier effects provided by different layers when the electrolyte passed through the MALPB layer, compact layer, and then the inner anodic layer before it reached the surface of Mg alloy, and that among them, the compact layer acted as a much more effective barrier to the electrolyte. The appearance of damaged areas on the composite coating surface after a much longer duration in a salt spray environment revealed that the life-span of the AZ31B Mg alloy could be greatly prolonged if the pores and cracks on the anodizing films were properly sealed by suitable polymers.