Effects of Disodium Phosphate Concentration (Na2HPO4·2H2O) on Microstructure and Corrosion Resistance of Plasma Electrolytic Oxidation (PEO) Coatings on 2024 Al Alloy

Since the electrolyte composition plays a pivotal role in the microstructure and corrosion behavior of plasma electrolytic oxidation (PEO) coatings, the effects of disodium phosphate (Na2HPO4·2H2O) concentration on the microstructure and corrosion resistance of PEO coatings fabricated on 2024 Al alloy were studied in this investigation. Accordingly, electrolyte with four different concentrations of phosphate ion (5, 10, 15 and 20 g/L) was used. All PEO processes were conducted at constant current density of 15 A/dm2 for 15 min. The surface and cross-sectional morphologies of the coatings indicated that the coating formed in the electrolyte with 10 g/L Na2HPO4·2H2O (with 9.14 µm thickness) had the most compact and uniform structure with the lowest and smallest micropores. Furthermore, studying the corrosion behavior of samples in 3.5 wt.% NaCl solutions by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests revealed that the sample coated in the electrolyte with 10 g/L Na2HPO4·2H2O had the highest corrosion potential, the lowest corrosion current density and, accordingly, the best corrosion resistance. The corrosion resistance of this coating was 4.574 × 105 Ω cm2, which could increase the corrosion resistance of uncoated 2024 Al alloy substrate 48 times. The x-ray diffraction pattern of this coating proved that the coating was composed of α-Al2O3 and γ-Al2O3 phases.