Electrolytic etching of AZ91D Mg alloy in AlCl3-EMIC ionic liquid for the electrodeposition of adhesive Al coating

In this paper, electrolytic etching pretreatment of AZ91D Mg alloy in AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid was employed for the electrodeposition of well adhesive Al coatings. The quantitative measurement of coating adhesion was carried out and the mechanism of electrolytic etching on the improvement of coating adhesion was studied through Scanning Electron Microscopy (SEM), energy dispersive X-ray (EDS), X-ray Photoelectron Spectroscopy (XPS) and Grazing Incidence X-ray Diffraction (GIXRD). With electrolytic etching pretreatment at 25 mA/cm2, the adhesion of subsequently electrodeposited Al coatings on AZ91D was higher than 4 MPa. The improvement of adhesion strength was attributed to both the complete removal of oxide layers on the surface and the uniform dissolution of Mg solid solution phase (α phase) and intermetallic phase Mg17Al12 (β phase). Meanwhile, further electroless replacement of metallic Al was observed at the position of α phase. In the case of 5 mA/cm2 electrolytic etching pretreatment, only partial surface oxide layers were removed and etching grooves formed on the surface, which resulted in a low adhesion strength of 0.5-1 MPa.