Studies on niobium anodization in the mixture of 1 M H3PO4 and 1 wt % HF at galvanostatic anodization are described here in detail. Interestingly, duplex niobium oxide consisting of thick barrier oxide and correspondingly thick porous oxide was prepared at a constant current density of higher than 0.3 mAcm −2, whereas simple porous type oxide was formed at a current density of lower than 0.3 mAcm −2. In addition, simple barrier or porous type oxide was obtained by galvanostatic anodization at a single electrolyte of either 1 M H3PO4 or 1 wt % HF, respectively. The formation mechanism of duplex type structures was ascribed to different forming voltages required for moving anions.
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