Research On Corrosion And Protection Of Magnesium Alloys

As the lightest metal structural material, magnesium alloys are found using widely in automotive, electronic and aeronautical industries because of a number of desirable features, including high strength/weight, high hardness/weight, damping characteristic and recycled easily. However, their applications are limited severely for their low corrosion resistance. The corrosion properties of Mg-Al-Zn alloys and the measures to improve their corrosion resistance are researched in this paper.The influences of main elements, phases, defects and casting-state to corrosion resistance of Mg-Al-Zn alloys are researched by optical microscope, scan electron microscope, weight-loss test, collecting gas test and polarization curves in this paper. The results show that corrosion resistance reduces with increasing aluminum and improves by adding some zinc and enhances to some extent with increasing manganese. All the secondary phases speed corrosion for their high potentials, defects reduce corrosion resistance because they make localized corrosion happen easily. The corrosion resistance of rapid-cooling alloys is high for their compact structure and little defects.The protective films on Mg-Al-Zn alloys forms by chemical conversion, anodic oxidation and electroless nickel plating. The properties of the films are researched through observing morphology of films, analyzing compositions of films and corrosive experiments . The results show that one-step treatment chemical conversion has many excellences such as simple operation, low treatment temperature and steady-going solution. The chemical conversion film connects well with matrix and has high corrosion resistance. Anodic oxidation film connects well with matrix and has high corrosion resistance for its much less holes and smooth surface. Most of the two protective flims are used directly as protective surface while used as the bottom of other organic coating. During unwash electroless nickel plating, the water film that forms before the alkaline electroless nickel plating makes plating-coating connect well with matrix. The coating has excellent properties and high corrosion resistance.The new opinions in this paper are that the corrosion resistance of Mg-Al-Zn alloys is very poor. The protective ability of the film formed in one-step treatment chemical conversion is very good. Anodic oxidation film has low holes ratio and high corrosion resistance. The water film that forms before the alkaline electroless nickel plating makes electroless nickel plating easier and improves the quality of the coating .