Assessment of corrosion and scratch resistance of plasma electrolytic oxidation and hard anodized coatings fabricated on AA7075-T6

Abstract

Surface modification of high-strength aluminium alloy 7075-T6 by plasma electrolytic oxidation (PEO) and type II hard anodization (HA) is presented in the current work. PEO-based ceramic oxide coatings were fabricated by employing an alternating current (AC) power source with a current density of 300 mA/cm2. The concentration effect of electrolytes on the alumina coatings was investigated and optimized comprehensively. Three separate aqueous electrolytes with 1:3, 1:1 and 3:1 proportions of sodium silicate (Na2SiO3) and potassium hydroxide (KOH) were utilized to evaluate optimum electrolyte concentration for obtaining desired AC-PEO coatings. X-ray diffraction (XRD) was utilized to investigate the phase composition of the coatings. Field emission scanning electron microscopy (FESEM) was employed to investigate the surface and cross-sectional characteristics of oxide coatings. Scratch testing was used to assess the oxide coatings’ adhesion ability, and potentiodynamic polarization (PDP) was utilized to assess the coatings’ corrosion behaviour in a 3.5 wt% aqueous NaCl solution. Among the AC-PEO and HA coatings, the AC-PEO specimen fabricated with equal ratios of sodium silicate and KOH concentration (Na2- SiO3:KOH 1:1) showed excellent adhesion strength (critical load, Lc = 41.5 N) along with the remarkable corrosion resistance (corrosion current density, icorr = 5.63 9 10–6 mA/cm2).