Surface Treatment Of Magnesium Alloy With High Strength And Toughness

Magnesium alloys have attracted great attention for applications in automotive, electrical and aerospace industries because of their low density, high specific strength and high specific stiffness, etc. However, magnesium has a very low normal electrode potential, which makes it become one of the most active structural metals. This paper use three surface treatment methods(micro-arc oxidation,direct chemical plating, chemical conversion) to produce protective coatings magnesium alloys. The coatings can increase the corrosion resistance of magnesium alloy and extend the service life.Micro-oxidation coating was prepared on the surface of ZK61 magnesium alloy in silicate and aluminate systems. With the increasing of time and current density, the amount of micropores in the micro-arc oxidation coating decreases, but the size of micropore and the roughness of coating become greater, and thickness shows a linear growth, adhesion force decrease significantly. The corrosion increases first and then declines with the processing time and current density. Current density shows a greater impact on composition of micro-oxidation film. The coating is mainly composed of MgO (periclase, syn) and MgAl₂O₄ (spinel) in aluminate system. The coating contains MgO (periclase, syn), Mg₂SiO₄ (forsterite, syn) and SiO₂ in silicate system. The optimum processing parameters for magnesium alloy are in a processing time of 40 min and a current density of 0.20 A/cm².Direct chemical nickel plating was processed on ZK61 magnesium alloy based on the main salt of basic nickel carbonate and reductant of sodium hypophosphite. Coating has no obvious defects, and surface morphology shows cell bar shape, pH has greater impact on surface morphology. With the increase of pH value, cell bar shape reduces in size first and then increases. With increasing of thiourea concentration, depositing rate decreases straightly. The corrosion resistance of coating increases with the increasing of temperature and time, and increases first and then declines with increasing of pH value. The optimum processing parameters for direct electroless nickel plating are in a processing thiourea density of 1mg/L, temperature of 85℃, time of 60min and pH value of 6.4.Surface morphology of conversion film shows broken tile flake, and has cracks and obvious flaws. With increasing of temperature,time and pH value, the cracks increases in size. Thickness rises with increasing of time and temperature, increases first and then decreases with increase of pH value. The corrosion increases first and then reduces with increasing of time,temperature and pH value. The film is mainly composed of Mg₃(PO₄)₂. The optimum processing parameters for phosphate chemical conversion are in a processing time of 15min, temperature of 40℃, and pH value of 3.5.