Mg-doped ZrO2 thin film protection of AZ31 magnesium alloy against wet corrosion: A detailed study

Magnesium-based alloys possess attractive properties that minimize the weight of the device and improve its performance efficiency. However, their poor corrosion resistance capabilities compromise their use in many applications. One way to protect against corrosion is to apply a thin film coating on the surface. In this work, pure, 3%, 5%, 8%, and 10% Mg ion-doped zirconia (ZrO₂) thin film coatings are fabricated on AZ31 Mg alloy using the electron-beam physical vapor deposition method to protect its surface from corrosion. The Mg ion doping has stabilized a tetragonal phase of ZrO₂ films, compared to the monoclinic phase in the undoped film. The stabilized tetragonal phase has improved the physical properties of the films compared to the undoped film with monoclinic phase. Corrosion studies conducted using potentiodynamic polarization and impedance spectroscopy showed improved corrosion inhibition characteristics of the Mg:ZrO₂ film in 3.5 wt.% NaCl electrolyte solution. The corrosion rate is reduced from 0.54 mmpy to 0.05 mmpy when coated with 5% Mg doped ZrO₂ film. The corrosion current and potential are obtained to be 2.38 µA/cm² and -1.46 V, respectively. The charge transfer resistance is high and stable in the coated samples compared to the bare substrates, where the resistance dropped heavily after the corrosion. Post-corrosion analysis using scanning electron microscopy, impedance spectroscopy, and X-ray photoelectron spectroscopy, including a quantitative analysis to discern the corrosion mechanism in the Mg:ZrO₂/AZ31 heterostructure, including the presence of Na and Cl from the electrolyte. Surface features and doping quantities have greatly influenced the corrosion inhibition capabilities of the AZ31 Mg alloys.