Effect of post-sealing treatment with different concentrations of NaH2PO4 on corrosion resistance of MAO coating on 6063 aluminum alloy

The objective of this work was to study the influence of post-sealing treatment with different concentrations (0.1 M, 0.5 M and 1 M) of NaH2PO4 on the corrosion behavior of 6063 aluminum alloy coated by micro-arc oxidation (MAO). The coatings before and after sealing were analyzed by XRD, EDS and XPS. The results showed that the post-treatment could effectively seal the pores and cracks, and the main component of sealing layer was AlPO4. During PDP tests, the sealing solution with 0.5 M concentration had better corrosion resistance and the Icorr was only 3.881 × 10−7 A·cm−2. EIS revealed that the|Z|0.01Hz of MAO + 0.5 M coating sample was 107 Ω·cm2, which was two orders of magnitude higher than that of the blank MAO. It is found by calculation that the adsorption of AlPO4 on the surface of MAO followed the Langmuir adsorption isotherm. The adsorption free energy ΔGads of the coatings after sealing was calculated, and the results were all between −20 and −40 kJ·mol−1, indicating that the adsorption of AlPO4 on the coating surface was a mixed adsorption of physical and chemical effects.