| Magnesium alloys are considered as the 21st century green engineering materials owing to their superior performances, such as low density, high strength/weight ratio, and good vibration damping character. They are used widely in many fields such as aerospace, automobile, mechanical electronic components, etc. However, magnesium is a very active metal, the standard electrode potential of magnesium is -2.37 V, which induces the low corrosion resistance of magnesium alloys in many cases and limits their wide applications. Electroplating, electroless plating and chemical conversion coating are effective surface protective measures to improve the corrosion resistance of magnesium alloys. Nevertheless, magnesium alloys are usually oxidized in air to oxide films on the surfaces, which decreases the adhesion of the coating. Chemical polishing can remove the oxide film and level the surface, producing coatings with good adhesion and corrosion resistance. The environment-friendly chemical polishing solution containing H3PO4 was usually used for to improve the corrosion resistance of the conversion coating by leveling the surface.Chemical conversion coating can not only protect the Mg substrates effectively, but also has some advantages over electroplating and electroless plating, such as simple technics and low cost. Chemical conversion coating is traditionally obtained by treating magnesium alloy with hexavalent chromium (Cr6+) compound, which is toxic and pollutes the environment severely. Chromium-free conversion coating has become an important developing direction for protection of magnesium alloys. Phosphate conversion coating which is one kind of the chromium-free conversion coating don't pollute the environment. The phosphate conversion coating can provide enough protection for the magnesium alloy substrate.The chemical polishing process treated on AM60 magnesium alloy and the polishing mechanism for magnesium alloy were studied. The effect of chemical polishing on phosphate-permanganate conversion coating was analyed. The optimum formulation was established. The phosphate-permanganate conversion process as a environment-friendly process and the phosphating mechanism was researched. The coating quality was greatly affected by pH value and pH adjusting agent. The sealing process was performed by immersing the phosphate-permanganate conversion coating in sodium silicate solution and a phosphate-silicate composite coating was yielded. The performance of the phosphate-silicate composite coating was detected. The results show as follows:1. The optimum polishing process is: H3PO4(85%) 60～65% (V/V), C3H8O2 320 cm3·dm-3 (V/V), others are water, time 3～5 min, temperature 45℃. The chemical polishing on AM60 magnesium alloy can be divided into three stages, which are pickling, polishing and over-etching. The process should be controlled in the polishing stage. The leveling surface and highly qualitive phosphate conversion coating can be obtained.2. The optimum chemical conversion process is: KMnO4 40 g·dm-3, K2HPO4 0.5 g·dm-3, NaAc 4.1 g·dm-3, HAc 3.1 cm3·dm-3, pH 3.6～4.6, time 10 min, temperature 45℃. The phosphate-permanganate conversion coating presents net-work features. The main element compositions of the phosphate-permanganate conversion coating are Mg, O, K, P and Mn. The phosphate-permanganate conversion coating with good corrosion resistance is obtained while the pH value was controlled to 3.6～4.6. More Mn element on the coating is produced by using acetic acid to adjust the pH value of the phosphating bath and the corrosion resistance of the coating can be improved. The phosphate-permanganate conversion coating can provide more effective protection than that of traditional chromate conversion coating. The phosphating can be speeded by adding nickel sulfate accelerator. However, the corrosion resistance of the coating can't be improved by adding accelerator.3. The corrosion resistance of the composite coating obtained by sealing phosphate-permanganate conversion coating in 5 g·dm-3 sodium silicate solution is greatly improved. There is silica gel produced in 5 g·dm-3 sodium silicate solution. Silica gel fills up the cracks on the phosphate-permanganate conversion coating. The corrosion resistance of the phosphate-silicate composite conversion coating is higher than that of the coating without sealing.
|
PDF Full Text Request