Structure and corrosion behavior of oxide layer with Zr compounds on AZ31 Mg alloy processed by two-step plasma electrolytic oxidation

• The oxide layer with Zr compounds on Mg alloy was fabricated by two-step PEO process.
• Newly-formed compounds were found in the oxide layer via electrochemical reactions.
• The corrosion resistance of AZ31 Mg alloy was improved via two-step PEO process.

The formation of the oxide layer with Zr compounds on AZ31 Mg alloy processed by two-step plasma electrolytic oxidation (PEO) has been investigated. After the PEO process first in an alkaline phosphate electrolyte and second in an acid electrolyte containing K2ZrF6, the microstructure, chemical composition, and phase composition of the oxide layers were analyzed via SEM, EDS, and XRD, respectively. The electrochemical reaction and the high temperature caused by the plasma discharges in the electrolyte were the main factors leading to the fabrication of an oxide layer containing Zr compounds on AZ31 Mg alloy. The micro-pores were filled with ZrO2 formed during the PEO process. The results of a potentiodynamic polarization test clearly indicated that the corrosion properties of AZ31 Mg alloy were significantly improved by the two-step PEO process.