Abstract
Electrophoretic (EP) coating is one of the most useful methods for corrosion protection of Mg alloys. The phosphate conversion film (P film) is required before electrophoresis to improve the adhesion strength and corrosion resistance. In this work, the P films with different thickness prepared on the AZ31 Mg alloy by controlling the phosphating time. The adhesion strength and corrosion resistance of the P films and the corresponding electrophoretic composite (P-EP) coatings were investigated by pull-off tests and electrochemical impedance spectroscopy (EIS). The results show that the thickness of the P film is an important factor affecting the comprehensive performance of P-EP coatings, that is, with the increase of P film thickness, the corrosion resistance of P films increases, but the adhesion of P-EP decreases. The P-EP coating exhibits good adhesion but poor corrosion resistance when the P film is thin, and the P-EP coating with the P film thickness of about 3.5 μm shows the best comprehensive performance at phosphating time of 10 min. Compared with the EP coating without phosphating pretreatment, the adhesion strength of the optimum P-EP coating increases by 50 % and the resistance value increases by two orders of magnitude.
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