Improvement of corrosion properties in an aluminum-sprayed AZ31 magnesium alloy by a post-hot pressing and anodizing treatment

The corrosion protection afforded by an aluminum arc-spray coating and a post-treatment consisted of hot pressing and anodizing on an AZ31 magnesium alloy has been studied by using polarization and electrochemical impedance techniques. The corrosion current densities of the Al arc-sprayed AZ31 specimens in their spray-coated condition, and after hot pressed under 500 and 1600 MPa conditions, were found to be 2.4×10−4, 2.1×10−5 and 8.1×10−7 A cm−2, respectively. Apparently, the corrosion resistance of the Al arc-sprayed AZ31 specimens was significantly improved by the densification of the Al-sprayed layer. By adopting an anodizing treatment the corrosion current density of the AZ31 Mg alloy with dense Al coating was further reduced to 3.7×10−8 A cm−2, showing an improvement of the corrosion resistance of the Al-coated AZ31 alloy by the anodizing treatment. The overall electrochemical impedances ranging from 0.01 Hz to 10 kHz of the Al arc-sprayed+hot pressed+anodized (AHA) specimen were approximately 2 orders of magnitude higher than those of the AZ 31 Mg plate, indicating that through the Al arc-spraying and a post-hot pressing and anodizing treatment, the corrosion resistance in AZ31 Mg alloy can be significantly improved. This was achieved, according to the X-ray photoelectron spectroscopy (XPS) analysis, due to the formation of a protective Al2O3 layer on top of the Al-coated AZ31 Mg alloy.