Study On The Technology Of Stannate Conversion Coating For Magnesium Alloy

Magnesium and its alloys have excellent physical and mechanical properties for many applications. In particular its high specific strength makes it an ideal metal for automotive and aerospace applications, where weight reduction is of significant concern.Unfortunately, magnesium and its alloys are highly susceptible to corrosion, particularly in salt-spray conditions. This has limited its use in the automotive and aerospace industries for unavoidable harsh service conditions. The simplest 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 organic/polymer coatings have been tried. For its less cost, chemical conversion coating has been extensively investigated. However, the use of CrO₃ or chromate solutions for the conversion process is harmful to the environment. So 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, rare earth salt and phosphate/permanganate solutions.The properties of conversion coatings are closely related to their microstructure and compositions, which strongly depend on the composition of the substrate magnesium alloys, the pretreatment, the composition of the bath, and the corresponding operating parameters such as solution temperature and treat time.In the present study, a new conversion coating on AZ91D magnesium alloy in solution with sodium stannate as main salt was developed. The effect of different pretreatment processes on the corrosion resistance of stannate conversion coating was analyzed. The optimal bath was obtained. The surface morphologies and corrosion resistance of the stannate conversion coating on AZ91D magnesium alloy at different treat time were investigated. The effect of different alkaline pickling, acid pickling and activation on the corrosion resistance of AZ91D magnesium alloy was analyzed. A new environmentally acceptable chromium-free pretreatment process for the chemical stannate conversion of AZ91D magnesium alloy was developed. The results show that the pretreatment process which uses acid pickling prior is best.The sodium stannate is the main composition which determines the formation, structure and continuity of the coating. The sodium hydroxide adjusts the pH and increases the alkalescency of the bath for the magnesium alloy which is more stabile in the alkaline solution. The accelerant is an indispensability part of the bath which activates the surface of the magnesium alloy. The optimal bath contains sodium stannate 50 g/L, accelerant 30 g/L, additive 10 g/L, sodium hydroxide 30 g/L, The results show that the stannate conversion coating was composed ofαphase,βphase and MgSnO₃·3H₂O. The stannate conversion coating is consisted of a hemispherical particle layer as major overlay formed uniform right on the under layer. This coating provides effective protection for the AZ91D magnesium alloy.The growth mechanism of stannate conversion coating on magnesium alloy AZ91D has been investigated. The coating grows in the following steps. In the first stage of chemical treatment, the coating deposited on the a phase very fast. About 10 minutes later, the coating began deposit on theβphase and then the dissolution of conversion coating accelerated rapidly on the a phase because of its loose structure. Meanwhile coating deposited a phase as well. Finally, the dissolution rate faster than the form rate with the weight of coating reduced. The conversion coating at the treat time of 30 minutes has the best conversion resistance.The environment-friendly chromium-free process was more effective to improve the corrosion resistance of magnesium alloy than the traditional Dow7 process which could instead of the Dow7 process. The advantage of the method is low production cost, simple conversion coating epuipment and technology. It has a better prospect in industrial applications.