Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta2O5, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a "pancake" type and then a "coral reef" type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of "pancake" structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich "coral reef" structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.