Study Of The Preparation And Performance Of Conversion Coatings On Magnesium Alloys

The optimum formula of coatings prepared in molybdate, stannate and phosphatesystems have been acquired by single factor experiment and orthogonal experiment.Firstly, the process of molybdate is that: Na2MoO430g·L-1; Ca(NO3)24g·L-1;(CH3COO)2Mn5g·L-1; NH4Cl2.5g·L-1; pH5; temperature70℃; time15min. Secondly,the optimum process of stannate is as following: Na2SnO330g·L-1; Na4P2O725g·L-1;CH3COONa8g·L-1; Na2B4O710g·L-1; NaOH6g·L-1; CH3(CH2)11SO3Na2g·L-1;Na2SiO35g·L-1; temperature70℃; time50min. And thirdly, the formula of phosphate isthat Mn(H2PO4)2·2H2O50g·L-1; Mn(NO3)240g·L-1; Ni(NO3)24g·L-1，C6H8O73.5g·L-1;EDTA2g·L-1; pH2; temperature50℃; time40min.The corrosion resistances of the coatings prepared on magnesium alloys inmolybdate, stannate and phosphate system are better than magnesium substrate provedby electrochemical tests including Tafel, EIS and so on. The morphology of molybdatecoatings showed by SEM and EDS presents like dry river bed and the mainly elementsare Mg, Mo, O. In addition, the stannate coatings that presents closely accumulationuniform granular mainly consists of Mg, Sn, O. The coatings without crack prepared inphosphate packed closely like scales are mainly contained Mg, Mn, P, O. The specificmaterials analyzed by XRD of the molybdate coatings are Mg2Mo3O11, MgO2; thestannate coatings are MgSn(OH)6, Mg(OH)2and the phosphate coatings are Mn3O4,Mg6MnO8, Mn3P6O18.The coatings prepared in molybdate, stannate, phosphate and chromate systems aretested by CuSO4drip experiment, Tafel, EIS and total immersion test of NaCl solution.The results show that the corrosion resistances of all the above coatings are better thansubstrate. And among those, the resistance of the coatings prepared in phosphate systemis even better than traditional chromate coatings and furthermore, the resistance ofcoastings prepared in molybdate and stannate are neck and neck.