Plasma electrolytic oxidation of a zirconium alloy under AC conditions

The microstructures of zirconia coatings formed by plasma electrolytic oxidation of a zirconium alloy, Zirlo, under AC conditions in an alkaline silicate electrolyte have been examined by scanning and transmission electron microscopies. The coatings are shown to consist of three layers. The innermost barrier layer is relatively thin, up to 500 nm in a 100 µm-thick coating formed for 3600 s. The intermediate layer, constituting the main part of the coatings, consists primarily of monoclinic and tetragonal zirconia. The outer, loose layer is rich in silicon species and constitutes a reduced proportion of the coating thickness at increased time of treatment. The porous intermediate and outer layers are readily permeated by the electrolyte. The formation of the coating accounts for < 27% of the anodic charge passed during the treatment.