Investigation of mutual effects among additives in electrolyte for plasma electrolytic oxidation on magnesium alloys

Plasma electrolytic oxidation (PEO) coatings were prepared on AZ91D magnesium alloys in alkaline silicate-based electrolyte with and without additives. The mutual effects among additives including TiC particles, dispersant polyethylene glycol 6000 (PEG6000) and anionic surfactant sodium dodecyl sulfate (SDS) were studied based on orthogonal experiment. The content and distribution of TiC deposited in the coatings were measured by EPMA and EDS. The thicknesses, phase compositions, microstructures and corrosion resistances of the coatings were examined by using TT260 eddy current thickness gage, XRD, SEM and electrochemical test, respectively. The results show that the experiment design of this study is the key to study the mutual effects among these additives. Each additive and their interactions all remarkably influence TiC content and corrosion resistance of the coatings. Smaller size TiC is much easier to migrate towards the anode, and the interaction between PEG6000 and SDS both effectively prevents its agglomeration and increases the number of its negative surface charges, which further increase the migration rate and the deposited uniformity of TiC and make TiC have more opportunity to deposit in the discharge channel. Thus, when smaller size TiC, PEG6000 and SDS are all added into the electrolyte, they could improve the anti-corrosion property of the coating to the largest extent attributed to higher TiC content and the densest microstructure of the coating.