Figure 2. a) Typical current–time (j–t) characteristics after a voltage step in the absence (a) and presence (c) of fluoride ions in the electrolyte. Either compact oxide (fluoride free) or porous/tubular metal oxide formation (containing fluoride) forms by different morphological stages (I–III). The inset shows typical linear sweep voltammograms (j–U curves) for different fluoride concentrations resulting in either electropolished metal (high fluoride concentration), compact oxide (very low fluoride concentration), or tube formation (intermediate fluoride concentration). b,c) Schematic drawing showing fieldaided transport of mobile ions through the oxide layers in the absence and presence of fluoride ions: rapid fluoride migration leads to accumulation at the metal–oxide interface.