Effects of fluoride ions in the growth of barrier-type films on aluminium

•The effect of fluoride on growth of barrier-type anodic alumina films is studied.•Fluoride ions incorporated into the film migrate faster than oxygen ions.•A fluoride-rich layer is formed at the base of the film.•Fluoride promotes the field-assisted ejection of aluminium ions.•A sufficient fluoride concentration leads to formation of a porous film.