Influence of acid pickling on electrodeposition of aluminum on magnesium alloy in room temperature molten salts

The effects of different acid pickling processes on the adhesions between the AZ91D magnesium alloy substrate and aluminum coatings electrodeposited from the acid AlCl3-EMIC room temperature molten salts were studied. The adhesions were estimated by score-tearing test. The surface morphology, phase compositions and element valence of coatings/etched substrate were analyzed by OM, SEM, EDS, XRD and XPS. The corrosion resistance was tested by salt-spray. The formation mechanism of coating morphologies was also discussed. The results show that the aluminum coating is not dense and the adhesion between the coatings and substrate is not good after mechanical pretreatment on AZ91D substrate. The dense and uniform coatings can be electrodeposited on the AZ91D substrate after pretreated by hydrofluoric acid pickling. The density of coatings and the adhesion can be improved simultaneously by using diluted phosphoric acid as pickling solution, and thus, the coating provides good protection to the substrate.