Preparation And Corrosion Resistance Of An Environment-friendly Calcium Phosphate Conversion Coating On AZ91D Magnesium Alloy

Magnesium and its alloys have many excellent properties such as highstrength-weight ratio, low density and excellent cast capability. They have been used inmany different fields, particularly in aerospace, military industry, automobile, etc.Unfortunately, magnesium and its alloys are highly susceptible to corrosion, especiallyin salt-spray conditions due to theloose and porousoxidation coating. This has limitedits practical applications. The simplest way to avoid corrosion is to coat the substratewith conversion coatings to prevent contact with the environment. In this study,calcium phosphate coatings were prepared on AZ91D magnesium alloys and thephophating bath composition as well as the parameters was also discussed. Thisphosphate treatment not only can improve the corrosion properties of magnesiumalloys but also has the advantages of green environment protection, simplicity ofoperation and cost-effectiveness. The surface morphology, phase composition andanticorrosive properties of the as-prepared phosphatecoating were analyzed byscanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-raydiffraction (XRD), polarization curves test, immersion corrosion test andelectrochemical impedance spectroscopy (EIS).(1) The preparation process of calcium phosphate coating on AZ91D magnesiumalloys was systematically studied in this study. The base composition of thephosphating bath includes25g/l Ca(NO3)2and25g/l NH4H2PO4. The effects of processing parameters such as temperature, phosphating time and pH value onmorphology and corrosion resistance of coatings on magnesium alloys were discussed.The optimized processing parameters were eventually confirmed (pH value3,temperature40°C, time20min).(2) The additives in phosphating bath were proved to have a great influence onthe composition and structure of calciumphosphate coatings. In this study,triethanolamine (TEA) was selected as an additive to prepare calcium phosphatecoating. The optimum concentration of TEA (1.4g/l) was confirmed by discussing theinfluence of different amount of TEA on the anticorrosive property of the substrate.(3) The calcium phosphate coating was composed of a large number of leaf-likeparticles. The substrate of magnesium alloys was covered by the phosphate coatingcompletely. The addition of TEA in phosphating bath can make the leaf-like particlesrefinement and improve the density of the phosphate coatings. The cross sectionanalysis showed that the surface of magnesium alloys was covered by phosphatecoating uniformly. The phosphate coating showed a certain thickness about10μm. TheEDS results showed that the calcium phosphate coating was composed of elementsofcalcium, phosphorus and oxygen. The results of XRD confirmed that the chemicalcomposition of the phosphate coating was CaHPO42H2O crystals with a highcrystallinity without other impurity elements.(4) The analysis results of polarization curves, immersion corrosion test and EISshowed that the anticorrosive property of magnesium alloys could be improved byphosphate conversion treatment effectively. Moreover, the calcium phosphate coatingson magnesium alloys prepared with TEA had much better corrosion resistance thanthat of coatings prepared without TEA.