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

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 alpha-Al2O3 (corundum)/gamma-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 mu m 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 alpha-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.