Effect of NaOH concentration on microstructure and corrosion resistance of MAO coating on cast Al−Li alloy

Ceramic coating was prepared on cast Al−Cu−Li alloy by micro-arc oxidation (MAO) in a silicate electrolyte solution with additives including Na2WO4 and NaOH. The effect of NaOH addition varying from 1 to 9 g/L on the microstructure and corrosion behavior of the coatings was investigated. The coating was characterized by XRD, XPS and SEM with respect to the composition and microstructures. The results show that NaOH promoted the formation of oxides in the reaction of MAO and contributed to the increase of layer thickness. Electrochemical tests (polarization curve and EIS) and mass loss test indicate that the corrosion resistance of the coated Al−Cu−Li alloy was improved with the increased addition of NaOH. Moreover, the results prove that the alumina coating formed in the electrolyte with 7 g/L NaOH showed the best corrosion resistance. The examination of the full immersion corrosion test presents the same result of corrosion resistance.