Microstructure and Corrosion Performance of Oxide Coatings on Aluminium by Plasma Electrolytic Oxidation in Silicate and Phosphate Electrolyte

Ceramic coatings are fabricated on pure aluminium by plasma electrolytic oxidation (PEO) in three kinds of electrolyte systems E1: 0.05M NaOH+0.033M Na₂SiO₃, E2: 0.025M NaOH+0.008M (NaPO₃)₆; and E3: 0.025M NaOH+0.066M Na₂SiO₃+0.008M (NaPO₃)₆. The voltage--time responses show that the PEO process of E2 has the highest discharging voltage, which results in the biggest pores and heaviest cracks on the surfaces. X-ray diffraction results show that coatings produced in E1 and E3 are mainly composed of γ-Al₂O₃ and mullite, while coatings produced in E2 are mainly composed of α-Al₂O₃. After PEO treatment the corrosion resistance of aluminium is improved significantly and the coatings produced in E3 perform the best corrosion resistance.