Characterization of plasma electrolytic oxidation coatings on Zircaloy-4 formed in different electrolytes with AC current regime

Plasma electrolytic oxidation was undertaken on Zircaloy-4 in alkaline silicate and pyrophosphate electrolytes, with a square waveform AC current regime. The resultant coatings were examined using scanning electron spectroscopy, X-ray diffraction and nanoindentation. The coatings formed in silicate electrolyte comprised mainly a porous inner layer and a more compact outer layer, with characteristic solidification structures being evident following prolonged treatment. The coatings contained monoclinic and tetragonal ZrO2, the latter being mainly present in the outer layer, which was of hardness up to ∼8 GPa. In contrast, extensively cracked coatings resulted from use of pyrophosphate electrolyte; the coating integrity was improved by the addition of silicate to the pyrophosphate electrolyte. The different morphologies of the coatings appeared to be related to the differing nature of the microdischarges and to the incorporation of silicon species that enhanced the formation of t-ZrO2.

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► ZrO2 coatings are grown on Zircaloy-4 by AC plasma electrolytic oxidation.
► Tetragonal and monoclinic ZrO2 are formed using silicate electrolyte.
► Pyrophosphate electrolyte results in flawed coatings of monoclinic ZrO2.
► Silicate favours formation of tetragonal ZrO2, with coating hardness ∼8 GPa.
► Microstructures are related to temperature gradients and solidification rates.