A rapid inward migration of fluoride ions in a growing anodic oxide, in a high electric field, was studied by anodizing Ti–12at%Si; in which film growth proceeded with almost 100% efficiency in selected electrolytes. The Si species which were incorporated into the anodic film were immobile, and acted as marker species. The migration rate of fluoride ions was determined precisely by using a 3-stage anodization which consisted of initial anodic film formation at a constant current density of up to 50V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by re-anodizing at 55V in ammonium fluoride electrolyte and final anodizing in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by using glow-discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of Si marker species. The fluoride ions migrated inwards at twice the rate of O2− ions. Thus, anodizing of Ti in fluoride-containing electrolytes produced a fluoride-rich layer that separated the alloy substrate from the anodic oxide; with eventual detachment of the film from the substrate.

Fast Migration of Fluoride Ions in Growing Anodic Titanium Oxide. Habazaki, H., Fushimi, K., Shimizu, K., Skeldon, P., Thompson, G.E.: Electrochemistry Communications, 2007, 9[5], 1222-7