Plasma electrolytic oxidation (PEO) treatment of zinc and its alloys: A review

Recently, few papers about plasma electrolytic oxidation (PEO) treatment on pure zinc, ZA27 and Zn-0.1 wt.% Mg alloys have been reported. The PEO procedure of pure zinc in electrolytes of KOH showed that the KOH amount and additives control the growth of a compact and dense oxide. The additives are mostly included in an outer film of the coating, while the inner layer of coating is mainly formed by ZnO. For ZA27 alloy, the PEO technique made successfully dense coating on this alloy by electrolytes of aluminate, silicate and aluminate/borate. The results showed that the coatings that were applied using composite electrolyte of aluminate/borate and aluminate electrolyte have the most and least growth rates of coating, respectively. The coatings are mostly consisted of Al2O3, ZnAl2O4 and CuAlO2. Moreover, ZnSiO4 might be also detected in the coating that was made by electrolyte of silicate. Produced coatings using electrolyte of silicate and aluminate/borate have a normal structure of three-layer PEO coating i.e. porous outer film, dense film, and inner barrier film, while the coating produced using electrolyte of aluminate has only a dense single-layer structure. For Zn-0.1 wt.% Mg, porous coatings containing ZnO and Zn2SiO4 were formed successfully by PEO procedure. The results revealed that the surface morphology and thickness of these coatings can be modified by the time of PEO procedure. Also, the corrosion tests indicated that less corrosion current density, higher impedance and more positive corrosion potential were found on the PEO coated Zn-0.1 wt.% Mg in m-simulated body fluid (SBF). The better corrosion behavior can be ascribed to the protective and steady oxide coating.