The extent of migration of fluoride ions, in anodic oxides on Ta, was determined. By using a pre-formed film, prepared in an electrolyte in which the incorporated Si species were immobile, a marker was produced to which the mobilities could be referred. Upon re-anodizing the pre-formed films in a fluoride electrolyte, secondary ion mass spectroscopic depth-profiling permitted a precise determination to be made of the extent of migration of inwardly mobile fluoride ions. The latter migrated inward at a rate which was 1.85 times faster than that of O2- ions. This led to the development of a fluoride-enriched region of TaF5 between the Ta substrate and the outer anodic oxide.

K.Shimizu, K.Kobayashi, G.E.Thompson, P.Skeldon, G.C.Wood: Journal of the Electrochemical Society, 1997, 144[2], 418-22