Study On Phosphate Conversion Technique On AZ31 Magnesium Alloy Profiles

Magnesium alloys have a number of excellent properties for many applications, such as low density, high strength-to-weight ratio, outstanding casting capability, significant mechanism and damping characteristics. They are increasingly used in aerospace industry, automobile industry, mechanical engineering, commodities and portable communication equipment. However, magnesium alloys are highly susceptible to corrosion, and poor corrosion resistance limited its use in the automotive and aerospace industries for unavoidable harsh service conditions and prevented them from further exploitation. The way to avoid corrosion is to coat the magnesium alloys to prevent from contacting with the environment. Various methods for applying protective coatings, such as plating, chemical conversion coating, anodizing, gas phase deposition, and polymer coatings have been tried. With the development of anti-corrosion technique, much research has been carried out to search for more environmental acceptable processes instead of chromate treatment in surface modification process of magnesium alloy. Several non-chromate solutions are thus investigated, such as phosphate, stannate and rare earth salt solutions. In this paper, phosphate was used as chemical conversion treatment solution, the properties and the forming mechanism of the conversion coating would be discussed.During the study of the phosphate conversion processing treatment on AZ31 magnesium alloys profiles, influencing factors of the phosphate conversion processing treatment were considered, concentrating on solution parameter, the processing time, the processing temperature and accelerant. Orthogonal test was used to determine the primary and secondary subsequence of influencing factors. The morphology and the composition of the treated specimens were analyzed by using Quanta-200 scanning electron microscopy(SEM) equipment with the Energy Dispersive Analysis System (EDS) . The results show that the phosphate conversion coating is compact and smooth and the composition of the coating are mainly Ba,O,F,P,Al and Mg. The best quality of the conversion coating on AZ31 magnesium alloys profiles have gotten in the phosphate solution of treatment solution parameter 15 g/L, accelerant1.0g/L at 90℃for 15min.The properties of conversion coatings are closely related to their microstructure,compositions and growth characteristics, which strongly depend on the AZ31 magnesium alloys profiles matrix's dissolution at the beginning of the coating growth.The test results of corrosion resistance of phosphate chemical conversion coating by immersion test and intermission salt spray test show that phosphate conversion coating could strengthen the capability of the corrosion resistance of magnesium alloys. An analytical and reasonable explanation to the formation and anti-corrosion mechanisms of phosphate chemical conversion coating for AZ31 magnesium alloy profiles is put forward accordingly.This paper not only has studied the formation mechanisms of phosphate chemical conversion coating for AZ31 magnesium alloy profiles theoretically but also will widen the application of phosphate surface treatment technique for magnesium alloys practically.