The formation of niobium oxide microcones on niobium substrates was investigated in HF electrolytes. The microstructure of the layers depends strongly on the HF concentration, the formation potential, and the anodization time. Microcones can be formed under a range of experimental conditions. Electrochemical determination and scanning electron microscopy indicate the microcones were formed due to the fast expansion of the oxide volume and the electric break under electric field. X-ray diffraction results verify the formation of Nb2O5. A possible growth mechanism has also been presented on the basis of experiments. (c) 2007 The Electrochemical Society.