Study On Processing And Corrosive Resistance Of Phoshate Conversion Film On The Surface Of Magnesium Alloys

In order to improve the corrosion resistance of AZ91D Mg alloy, a uniform and well corrosion resistance of phosphate film was fabricated on surface of Mg alloy by chemical and electro-chemical phosphating treatment. The content of phosphate bath, technological parameters and phosphating additives were studied.Orthogonal design experiment was used in order to acquire the best formula. The anticorrosive performance, morphology and chemical composition of phosphate coatings on AZ91D Mg alloy of different technological conditions were analysis by OM micrograph, X-Ray diffraction (XRD), E-t curves, electrochemical impedance spectroscopy (EIS), Tafel polarization cuvres, Corrosion weight-loss and Drop corrosion.The formation mechanism of phosphate conversion film was also sutdied. The results showed that: the chemical phosphate conversion film with well corrosion resistance was acquired (phosphoric acid 27.0g/L, Zinc oxide 7.0g/L, tartaric acid 1.0g/L, D-glucose acid sodium 0.5g/L, four boric acid sodium 1.2g/L, ammonium molybdate 1.0g/L), and the best processing parameters (temperature 30℃, time 15min, pH 2.18). The optimized current density of electro-chemical phosphating is 4.0A/dm².To chemical phosphating process, the properties of phosphating film formed in compound additives are better than those in single additives. The best content of compound additives respectively is as followed: urea 1.0g/L, sulfourea 1.0g/L, sodium wolframate 0.5g/L and diethanol amine 1.2g/L. Contrasted with bare substrate of magnesium alloy and the phosphate films formed at optimum formula, adding composite additives have well corrosion resistance, In addition, The OM micrograph results showed that adding composite additives would effectively reduce the number of cracks and holes, the phosphating film forming more completely. The best corrosion resistance of chemical phosphate film is adding the composite additives of urea, and wolframate can refine the grain of the phosphate film. To the electro-chemical phosphating process, the three conditions of adding additive 0.5g/L tartaric acid, 5.0g/L sodium dihydrogen phosphate, and 0.5g/L tartaric acid and 5.0g/L sodium dihydrogen mixed can promote the growth of phosphating film, improve the corrosion resistance. Among them, the corrosion resistance of mixed additives is the best.The results of XRD showed that the main composition of chemical phosphate film is Zn₃(PO₄)₂. The E-t curve of chemical phosphate film on Mg alloy surface results indicated that the process of film formation was composed of three steps: the corrosion dissolve of bare substrate of magnesium alloy, the deposite of the phosphate conversion film and the stabilize formation of phosphate film.