Effects of electrolyte concentration on the microstructure and properties of plasma electrolytic oxidation coatings on Ti-6Al-4V alloy

Abstract

In this investigation, the effect of electrolyte concentration parameters on the plasma electrolytic oxidation (PEO) coatings on Ti-6Al-4V alloy were investigated by an orthogonal experiment involving four factors with three levels by changing the concentration of Ca(CH3COO)2·H2O, NaH2PO4·2H2O, Ethylene-diamine-tetra-acetic acid (EDTA) and NaOH, respectively. The self-corrosion current density, Ca/P, bond strength, phase composition and microstructure of the coatings were studied in detail. The results indicate that the nine PEO coatings had unevenly distributed porous structures comprising different ratios of titanium, rutile, and anatase phase. The effect of electrolyte concentration parameters on the properties and performance of coatings are sequenced hierarchically as follows: EDTA > NaOH > Ca(CH3COO)2·H2O > NaH2PO4·2H2O for corrosion resistance, NaOH > EDTA > Ca(CH3COO)2·H2O > NaH2PO4·2H2O for Ca/P, Ca(CH3COO)2·H2O > NaH2PO4·2H2O > NaOH > EDTA for bond strength. The highest bond strength of the PEO coatings can reach as high as 60 MPa. The optimized electrolyte composition for the Ti-6Al-4V is: CCa(g/L) = 11.44 g/L, CP(g/L) = 5.72 g/L, CEDTA(g/L) = 10 g/L, CNaOH(g/L) = 15 g/L and the corresponding PEO coating had adequate thickness (11.78 μm), satisfactory bond strength (33.69 MPa) and a fitting Ca/P ratio of 1.68.