Insights into structure and high-temperature oxidation behavior of plasma electrolytic oxidation ceramic coatings formed in NaAlO2–Na2CrO4 electrolyte

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti 2 AlNb alloy in various NaAlO 2 electrolytes containing 2 g L −1 , 4 g L −1 , and 6 g −1 Na 2 CrO 4 additive, respectively. The influence of Na 2 CrO 4 additive in NaAlO 2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na 2 CrO 4 additive in the NaAlO 2 electrolyte not only promotes the formation of γ-Al 2 O 3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr 3 O and (Al 0.948 Cr 0.052 ) 2 O 3 phases. The PEO ceramic coatings formed in NaAlO 2 electrolytes with 2 g L −1 and 4 g L −1 Na 2 CrO 4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO 2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO 2 electrolyte with 4 g L −1 Na 2 CrO 4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO 2 basic electrolyte without any Na 2 CrO 4 additive.