Corrosion Resistance of Plasma Electrolytic Oxidation Coatings on LD7 Alloy in Oxalate and Dihydrogen Phosphate Electrolytes

Two plasma electrolytic oxidation (PEO) coatings were fabricated on LD7 aluminum alloy in oxalate and dihydrogen phosphate electrolytes. The phase composition, morphology and corrosion resistant of the two PEO coatings were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical test. Both PEO coatings presented three-layer structure, i.e., a porous outer layer, a relative dense intermediate layer with nano-size micro-pores and an inner nanoscale barrier layer at the coating/substrate interface. It was found that the porosity of the coating in oxalate electrolyte was lower than that in dihydrogen phosphate electrolyte. The small quantity of the nanosize micro-pores might be attributed to the appearance of "soft spark" in oxalate electrolyte, whereas the large number of micro-pores and cracks should be related to intensive micro-discharges in dihydrogen phosphate electrolyte during PEO process. The compact coating with nano-size micropores in oxalate electrolyte provided better protection than that in dihydrogen phosphate electrolyte from corrosion.