Structure and corrosion properties of the two-step PEO coatings formed on AZ91D Mg alloy in K2ZrF6-based electrolyte solution

- Two steps PEO (plasma electrolytic oxidation) coatings were produced on AZ91D Mg alloy.
- Microstructure of the primary coatings significantly influenced the final Morphologies of the coatings.
- Discharge channels in the secondary coatings were found directional in nature.
- The observed major phases in the coating were ZrO2, MgO and Mg2SiO4.
- Corrosion resistance and hardness were significantly enhanced with secondary step process.

In this work, PEO (plasma electrolytic oxidation) coatings were produced on AZ91D magnesium alloy using two-step PEO (plasma electrolytic oxidation) technique. Microstructure and phase composition of the two-step coatings were analyzed using SEM, and XRD, respectively. From the microstructural analysis, it was found that the microstructural features of the primary coatings significantly influenced the properties of the secondary step PEO coatings. Further, it was found from the structural analysis that unlike the primary coatings, discharge channels in the secondary coatings are directional in nature. From the XRD patterns, major phases in the coating were detected as ZrO2, MgO and Mg2SiO4 along with some minor phases. Due to the excellent structural properties of the coatings obtained through secondary step process, hardness and corrosion results of the coatings were significantly improved. The highest corrosion resistance measured through potentiodynamic experiment was found as ~ 505.3 × 103 Ω/cm2, with the hardness value for the same specimen as ~ 1170.24 HV.