Phosphate Surface Modification Technique For ZM5

Magnesium alloys have a number of desirable features, including low density, high strength-to-weight ratio, excellent casting capability, good machinability and damping characteristics, and they are now increasingly used in aerospace industry, automobile industry, mechanical engineering, commodities and portable communication equipment. However, poor corrosion resistance has often prevented them from further exploitation. Phosphate technique, one of the surface treatments of magnesium alloys, has attracted much attention due to the possession of a combination of properties, such as economical, non-contaminative, film uniformity, and good corrosion resistance. But, this process is now far from completeness. Therefore, this paper is aimed at providing a systematic study of all aspects concerning phosphate surface technique on magnesium alloys, including the pretreatment process, phosphate chemical film forming process, and coating process. Moreover, the capability of chemical conversion film and coating, and the formation mechanism of chemical conversion film have been studied.During the study of phosphate surface modification process, orthogonal test is used to determine the type of phosphate solutions, the concentration of each solution, treating temperatures and treating times. The thickness and the phase composition of conversion film have been studied by SEM (Scanning Electron Microscopy) and XRD (X-ray Diffraction). The results show that the thickness is about 80-110um and the phase composition is mainly Mg and MgO, and some amorphous phases.The adhesive force of coating is tested by lever-type multi-functional strength testing machine. Results showed that chemical conversion film could greatly improve the adhesive force of coating. The results from the study of corrosion resistance of chemical conversion film and coating by using the constant temperature and humidity box, salt spray test box and SEM (Scanning Electron Microscopy) show that films could strengthen the capability of corrosion resistance of magnesium alloys. An analytical and reasonable explanation to the formation and anti-corrosion mechanismsof phosphate chemical conversion film for ZM5 is put forward accordingly.This paper not only has studied the formation mechanisms of phosphate chemical conversion film for one kind of magnesium alloy (ZM5) theoretically but also will widen the application of phosphate surface modification technique for magnesium alloys practically.