The mechanism for tuning the corrosion resistance and pore density of plasma electrolytic oxidation (PEO) coatings on Mg alloy with fluoride addition

Abstract

Here we prepared PEO coatings on Mg alloys in silicate–NaOH–phosphate electrolyte containing different concentrations of NaF addition. The detailed microstructural characterizations combining with potentiodynamic polarization and electrochemical impedance spectra (EIS) were employed to investigate the roles of fluoride in the growth and corrosion properties of PEO coating on Mg. The result shows the introduction of NaF led to a fluoride-containing nanolayer (FNL) formed at the Mg/coating interface. The FNL consists of MgO nanoparticles and insoluble MgF2 nanoparticles (containing rutile phase and cubic phase). The increase in the NaF concentration of the electrolyte increases the thickness and the MgF2 content in the FNL. When anodized in the electrolyte containing 2 g/L NaF, the formed FNL has the highest thickness of 100–200 nm along with the highest value of x of ∼0.6 in (MgO)1-x(MgF2)x resulted in the highest corrosion performance of PEO coating. In addition, when anodized in the electrolyte containing a low NaF concentration (0.4–0.8 g/L), the formed FNL was thin and discontinuous, which would decrease the pore density and increase the coating's uniformness simultaneously.