Plasma electrolytic oxidation of Zircaloy-2 has been investigated in dilute and concentrated aluminate electrolytes, under a pulsed-bipolar current regime, in order to develop coatings of high wear resistance. Coating growth kinetics, cell potential-time responses and discharging behaviours depend significantly on the electrolyte concentration. The coatings formed in dilute aluminate electrolyte reveal a three-layered structure, with pancake structures at the coating surfaces. "Soft sparks" occur during PEO in dilute aluminate electrolyte, causing a relatively fast growth of the inner layer and resulting in a large amount of alumina-enriched material beneath the pancake structures, and hence an increased wear resistance of the coating. In contrast, more homogenous coatings, free of pancakes, result with the concentrated electrolyte. The main phase in the coatings is t-ZrO2, with gamma-Al2O3 also present in coatings formed in the latter electrolyte. The coatings formed in the concentrated electrolyte display a high wear resistance, even for thin coatings formed for short times, which is attributed to the relatively high alumina content of the coatings. (C) 2013 Elsevier Ltd. All rights reserved.