Formation of Aluminum Composite Passive Film on Magnesium Alloy by Integrating Sputtering and Anodic Aluminum Oxidation Processes

Anodic aluminum oxidation (AAO) process on sputtered-Al film to form an oxide coating as an anticorrosion layer on AZ91D magnesium alloy was performed. The cross-sectional scanning electron microscope images showed an increase on the thickness of anodized oxide layer with increasing oxidation duration. By X-ray photoelectron spectroscopy and X-ray diffraction analyses, the anodized alumina layer had an amorphous structure, where the chemical composition is mainly composed of aluminum oxide. From the results of anticorrosion test, the higher charge transfer resistance of oxide layer was obtained for all AAO specimens by electrochemical impedance spectroscopy, as compared to the raw AZ91D substrate. In addition, the potentiodynamic polarization measurement showed that the specimen with 15 min anodizing treatment had the lower corrosion current density (0.29 μA/cm2), and the higher protection efficiency (99.64 %). These results proved that Al oxide/Al bi-layer composite passive film on AZ91D alloys is significantly improved the corrosion resistance behaviors and the optimal parameter for AAO process is 15 min.