Surface characteristics of AC PEO coatings fabricated on commercial Al alloys

Abstract

The effect of alloying elements on coating characteristics was explored in the current work, which involved the fabrication of ceramic oxide coatings on commercial aluminium alloys (AA2024, AA6061, and AA7075) using the plasma electrolytic oxidation (PEO) process. The PEO coatings were fabricated by an AC power source with an exchange current density of about 300 mA/cm2. Field emission scanning microscopy (FESEM) was used to examine the surface, cross-sectional morphology, and alloying elements distribution. X-ray diffraction was used to investigate the phase composition. All PEO coatings were primarily composed of γ-Al2O3 and α-Al2O3 phases. As compared to PEO treated AA6061, PEO treated AA7075, and PEO treated AA2024 suppress the formation of α-Al2O3, resulting in a relatively higher fraction of γ-Al2O3, attributed to the alloying elements residing in the coatings. Because of its dense inner layer and high thickness (51 ± 1 μm), as well as its higher contact angle (114 ± 2°). PEO coated AA6061 sample exhibited superior corrosion resistance with lower icorr = 8.91(±0.24) E-6 mA/cm2 and impedance (Ω-cm2) to the order of 107 in 3.5 wt% aqueous NaCl corrosive medium. The PEO-coated AA6061 sample evinced greater hardness, scratch and wear resistance than the other samples, ascribed to its relatively higher α-Al2O3 phase, lower residual alloying elements, and thick and dense inner layer.