Improvement Of Manganese-based Phosphate Conversion Coating On Magnesium Alloy

One of the main challenges in the use of magnesium alloys, particularly for outdoor applications is its poor corrosion resistance. An effective way to protect the substrate and improve its corrosion resistance is chemical conversion treatment. An effective method improving the performance of Magnesium manganese-based phosphate conversion coating was investigated; the consumption of manganese ion, the variation of the coating's corrosion resistance and the morphology during treating process were studied; a novel barium-based phosphate conversion coating on AZ91D die-cast was studied, the concentration of the added ingredients and the optimum process parameters were settled; a novel local micro-arc oxidation(MAO) method for Magnesium TIG welded joint was investigated and the corrosion condition of the welded joint before and after treatment was studied.Nitrate of lime was added into the solution, the surface of coating turned to smooth and the crack of the coating became narrow when concentration of Ca(NO3)2 in conversion solution exceed 2 g/L.The corrosion resistance of coating was best at 5 g/L of Ca(NO3)2. XPS result shown that the coating was composed of Mn, P,O, Mg, Ca and Al, Ca2+ existed in the coating in the form of CaCO3,CaO and a kind of amorphous calcium phosphate salt.With the increase of treated area, the concentration of Mn2+ decreased linearity, the corrosion resistance was not change greatly; when the treated area exceed 100 dm2/L,the crack in the coating became wide.The optimum concentrations of Ba(NO3)2, Mn(NO3)2 and NH4H2PO4 were 25 g/L,15 ml/L and 20 g/L respectively, treating time, solution temperature and the value of pH value were settled and they were 5 to 30 min,50 to 70℃and 2.35 to 3.0. The corrosion resistance of barium conversion coating was better than that of manganate conversion coating. The coating was composed of Ba, P, O, Mg, Zn, Mn and Al.AZ31 sheets were connected together with tungsten inert gas (TIG) welding, corrosion resistance of the welded zone and matrix was equivalent when evaluate respectively; the corrosion resistance of the welded zone was poorer when welded joint was checked by immersion test; the corrosion resistance of local MAO coating was worse than that of MAO coating; after local MAO treatment, the corrosion resistance of welded zone was better than that of matrix, the coating in the region of heat affect zone had more probability to break.