Reactive sputter deposition of alumina films on magnesium alloy by double cathode glow-discharge plasma technique

In order to overcome the problem of the corrosion resistance of AZ31 magnesium alloy, the nanocrystalline Al2O3 film was deposited on AZ31 magnesium alloy by double cathode glow-discharge plasma technique. The microstructure, chemical composition and elemental chemical state of the sputter-deposited nanocrystalline Al2O3 film were analyzed by means of scanning electron microscopy equipped with an energy dispersive spectroscope, X-ray diffraction), transmission electron microscope and X-ray photoelectron spectroscopy. The results indicated that the sputter-deposited nanocrystalline Al2O3 film consisted of single θ-Al2O3 phase with average grain size about 60 nm. The hardness and the elastic modulus of the as-deposited nanocrystalline Al2O3 film were about 17.21 GPa and 217 GPa measured by nanoindentation instrument, respectively. The corrosion behavior of the sputter-deposited nanocrystalline Al2O3 film in 3.5%NaCl solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The amount of porosity for the sputter-deposited nanocrystalline Al2O3 film calculated by two electrochemical methods was equal to 0.0086% and 0.168%, respectively. The sputter-deposited nanocrystalline Al2O3 film exhibited excellent corrosion resistance, which was attributed to its dense enough structure to prevent magnesium alloy from corrosion in aggressive solutions.