Synthesis of corundum rich coating on 7055 aluminum alloy by micro-arc oxidation and its corrosion resistant property in a saline medium

Abstract

The work researched on Micro-arc oxidation (MAO) coatings fabrication method using 7055 aluminum alloy as substrate. The electrolyte was an alkaline solution added by sodium hexametaphosphate with/without sodium tungstate additive. X-ray diffraction (XRD) characterization proved that the α-Al2O3 (corundum)/γ-Al2O3 (gamma alumina) ratio in the coating reached 0.929 with trace tungsten metal, which was produced by thermit reaction. Scanning electron microscopy (SEM) characterization revealed craters up to 100 μm in diameter and micro-cracks on the coating surface. Energy dispersive X-ray spectroscopy (EDS) results indicated that the coating surface contains 0.74 at.% Mg, undetectable Zn and Cu, and 1.39 at.% Zr element, which should be 1.04 at.%, 1.51 at.%, 0.43 at.% and 0.02 % theoretically. Zr element distributed spatially in the same region with P element. The corrosion resistant property of the samples was analyzed by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in 3.5 wt% NaCl. Tafel extrapolation method was utilized to analyze polarization curves and the results revealed that the corrosion potential of the MAO-treated sample was 0.016–0.093 V higher, and the corrosion current density was one order of magnitude lower (1/9.10–1/19.06) than the base 7055 aluminum alloy. The Nyquist diagrams of samples were modeled by equivalent electrical circuits through ZsimpWin software. The iterated data verified a three-layer structure of the MAO coating and the intermediate layer exhibited higher electrical resistance than the barrier layer. The coating film containing the most α-Al2O3 phase displayed the largest open porosity (3.99 % by calculation) and the highest electrical resistance. The friction test revealed that the coating fabricated under a higher current density possessed better wear resistance when the outer layer contacted the counterpair of Si3N4 ball.