Influence of a CaNa2(EDTA) Additive on Plasma Electrolytic Oxidation of Zirlo Alloy and the Properties of the Resulting Coatings

The plasma electrolytic oxidation (PEO) of Zirlo alloy was carried out in a phosphate electrolyte with CaNa2(EDTA) as an additive (0–15 g/L) to improve its corrosion and wear resistance. The PEO behavior, microstructure, phase composition, and performance of coatings were characterized as a function of the concentration of the additive. The results indicate that the addition of CaNa2(EDTA) promotes coating growth and improves the coating structure and phase composition. When the additive concentration is 5–10 g/L, the coating shows an improved thickness, and denser microstructure. The coatings consist of m-ZrO2 and t-ZrO2 as the main crystalline phases, as well as amorphous materials with Ca and P. The t-ZrO2 phase content rises sharply when CaNa2(EDTA) is added into the electrolyte (81.3% t-ZrO2 is obtained under the condition with 10 g/L CaNa2(EDTA)). Potentiodynamic polarization tests demonstrate that PEO treatment significantly enhances the corrosion resistance of Zirlo alloy. Under the condition of 5 g/L CaNa2(EDTA), the corrosion current density of the coating decreases by two orders of magnitude compared to the substrate, achieving the best corrosion resistance. Friction and wear tests also show that the coating obtained at 5 g/L CaNa2(EDTA) exhibits the shallowest wear scar and the lowest wear rate, demonstrating optimal wear resistance. This study shows the novelty of obtaining high-quality PEO coatings on Zirlo alloy based on Ca and P incorporation.