||The combination of a high strength-to-weight ratio, excellent dimensional stability and good damping capacity of magnesium and its alloys is leading to more comprehensive applications for automobile and aircraft components. However, the high susceptibility to corrosion of magnesium and its alloys has greatly hindered their wide spread use in many applications, especially in acidic environments and in salt-water conditions.
In order to improve the corrosion resistance of magnesium alloys, proper treatments have been used. Some surface modification techniques have been developed for the protection of Mg alloys. Micro-arc oxidation (MAO), is a promising novel process to form ceramic-like coatings of thickness ranging from tens to hundreds of microns on metals(Al, Mg and Ti etc.) and their alloys for corrosion protection, wear resistance.
In a single electrolyte system, the thickness of the prepared ceramic layer is thin and the compactness is poor, and it is difficult to form effective protection to the substrate. Thus, some additives, such as phosphates, hydrides, and borates, have been used for some applications. Among them, phosphate is the most widely used and soluble in water, and it can form phosphate anions with strong absorption in water. Therefore, it is often used as an additive component to micro-arc oxidation electrolyte.
The optimum coating conditions were deduced according to the coating quality, and electrochemical and immersion corrosion performance. The results show that the pH can significantly affect the coating process, microstructure, and coating porosity percentage, and thus influence the corrosion protection efficiency of the ceramics coated AZ61.