Preparation and characterization of anodic films on AZ31B Mg alloy formed in the silicate electrolytes with ethylene glycol oligomers as additives

Oxide coatings are prepared on AZ31B Mg alloy in an environment-friendly electrolyte with additives by plasma electrolytic anodization, and the effect of ethylene glycol oligmers on the performances of the anodized film is investigated. Under a constant current density of 10 mA cm−2, the reaction overpotential of the silicate electrolytes with additives are found higher than that of the original electrolyte as measured by potential–time test. The EIS and DC polarization results reveal that the addition of PEG increases the impedance of the film and reduces its corrosion current density (Icorr) at least by one order of magnitude. The surface morphologies are more and more compact and homogeneous with the increase in EG numbers, while a rougher surface appeared again if the PEG4000 is used as observed by SEM. As detected by XRD, the anodic films are found mainly consist of MgO, MgSiO3 and Mg2SiO4, and their relative amounts are related to the lengths of EGs, resulting in the differences in morphology and anticorrosion variations. Furthermore, the improvement in abrasive resistance of the anodic film formed in the electrolyte with PEG1000 may be attributed to its much more compact surface and the incorporation of ductile PEG chains among those oxides.