Characterization and properties of PEO coatings on 7075 Al alloy grown in alkaline silicate electrolyte containing KMnO4a dditive

Abstract

Plasma electrolytic oxidation (PEO) was used to modify the surface of 7075 Al alloy by applying a bipolar pulsed-current in a silicate based electrolyte containing 0 to 3 g L− 1 KMnO4. For the coating produced in the base electrolyte, crystalline γ-Al2O3 was the main phase, while the coatings formed at the presence of KMnO4 consisted of α-Mn2O3 containing Si and Al oxides. A pancake structure was observed on surface of the coating produced in the base electrolyte, while a foam feature along with some volcano-like areas was detected on the surface of the coatings produced in the presence of KMnO4. The KMnO4 changed the discharge type leading to diminish the pores at the metal/coating interface and also increased the coating thickness. The Nyquist plots of the coatings in 3.5 wt% NaCl solution at pH 4 showed two capacitive loops related to the outer porous and inner compact layers, where the resistance of inner layer was very high indicating that it could determine the overall corrosion performance. The inner layer resistance increased with increasing KMnO4 concentration. At prolonged immersion times, the barrier property of the coatings was lost to some extent, but the coating produced in 3 g L− 1 KMnO4 solution showed superior corrosion performance due to its lower permeability. The corrosion current density of the specimen coated in the presence of 3 g L− 1 KMnO4 was ~ 3 times lower than that observed for the specimen coated in the base electrolyte.