Studies on Preparation and Corrosion Resistance of Molybdate Conversion Coating Formed on AZ91 Magnesium Alloy

An environmental friendly molybdate conversion coating was prepared on AZ91 magnesium alloy by chemical conversion method.The factors such as pH value,temperature and mass concentration of Na2MoO4 were investigated by single factor experiment and orthogonal experiments,and the optimum conditions of chemical conversion were determined as following:pH 3.5,temperature 70 ℃,30~40 g/L Na2MoO4 and chemical conversion time 50 min.The macro surface morphology of as-prepared coating on Mg-based substrate was compact and yellowish,while presenting microscopic small cracks.XRD showed that main component of the molybdate conversion coating were Mg2Mo3O8 and MgMoO4.Electrochemical polarization test analysis showed that the molybdate conversion coating could effectively improving the corrosion resistance of magnesium alloy and increasing the self-corrosion potential,and the self-corrosion current density was decreased by two orders of magnitude.