Anomalous anodic layer growth on titanium occurring in electrolyte comprising nitrate and water

Abstract

The anodization of a titanium substrate using an aqueous nitrate solution led to an anomalous surface covered with a collapsed-like structure, and a homogenous oxide layer was never obtained, which is different to the case of most aqueous electrolytes. The objective of this study was to elucidate the primary cause inducing this anomalous layer growth that occurs in an aqueous nitrate electrolyte. To this end, the anodization of a titanium substrate was conducted using a non-aqueous electrolyte consisting of ethylene glycol and ammonium nitrate, wherein a trace amount of water was added. Microscopic analysis revealed that the addition of only 2% water into the electrolyte induced the growth of the anomalous layer, while the non-aqueous electrolyte without water addition resulted in a homogenous layer. Regardless of the water addition, the formed surface was comparatively flat at the initial stage. However, cabbage-like precipitates appeared and increased with the treatment time only in the case of water addition, and subsequently gradually converted to a collapsed-like surface. Nitric ions coexisting with water were capable of capturing an electron from the molecule oxygen ions in the anodic layer, and thereby developed the oxygen-filled cavity. The generation of a local spark at the cavity led to the formation of the cabbage-like precipitates, and thus, the layer growth did not proceed normally.