Effect of Electrolyte Concentration and Voltage on the Morphology and Corrosion Resistance of PEO Coated AA6061

Abstract

Aluminum and its alloys are among the most extensively used materials in aerospace and industrial applications owing to its high specific strength, low density, and high abundance on the earth’s crust. However, low hardness, high tendency to adhesion, high corrosion, and poor wear resistance limits its further application. Surface treatments like electroplating, PVD/CVD, and anodizing are proposed to form a protective coating on the surface of Al and its alloys. Plasma Electrolytic Oxidation (PEO), known as micro-arc oxidation (MAO), emerged as a promising surface coating technique producing high-quality ceramic coating for light metal alloys like Al, Mg, Ti, and their alloys. PEO is similar to conventional anodizing but at a higher voltage exceeding its breakdown voltage, forming plasma discharge. In this study, the PEO of AA6061 was carried out in an AC power source at constant voltage 325-400 V, 1000 Hz, and 80 % duty cycle for 30 min. Silicate-based electrolytes of different concentrations (5-10 g/L) Na2SiO3 and (5-10 g/L) KOH were utilized, maintaining 25-40°C electrolyte temperature. The effect of electrolyte concentration and constant voltage on the coating morphology and its relation to hardness were investigated. Surface morphology, cross-section, elemental distribution, and phase composition were characterized using SEM, EDX, and XRD. Different breakdown voltages were determined for each electrolyte concentration. A lower breakdown voltage was achieved at a higher concentration. Porous with pancake-like structure, crater, and nodular structures were observed with coating thickness ranges from 8.37-87.33µm. α-alumina and Ƴ-alumina peaks were detected in all surface coatings.