Adhesion, Ductility, and Fracture of Anodic Oxide Films on Tantalum

Abstract

Spectrophotometric thickness measurements have confirmed that anodic oxide films on tantalum can show considerable plastic flow when the oxidecoated metal is stretched or rolled, for example up to 15% reduction in the average thickness of oxide. SEM pictures showed that after a certain amount of deformation the oxide cracks and widening areas of "bare" metal develop. The ductile behavior is seen if chemically polished tantalum is etched in HF or treated with boiling water before anodization. Otherwise the fluoride in the film left by chemical polishing modifies the oxidation process in a way which leads to a poorly adherent anodic oxide film.The mechanical properties of oxide films have been considered to be of importance in corrosion situations where the metal is stressed. In such situations, a complication is that the presence of an oxide film can significantly affect the mechanical properties of the metal. A fairly comprehensive list of literature references is given in a recent paper by Choo and Devereux (1). The mechanical properties of thin oxide films can be unexpected by comparison with those of bulk oxide samples. Some years ago (2), one of us reported that anodic oxide films made on tantalum, which had been chemically polished and then pretreated in a certain way before anodization, showed evidence of a surprising degree of ductility, as indicated by a change in interference color when anodized metal foil was stretched in a tensile machine. The pretreatment was immersion in boiling water for a few minutes. Films formed on chemically polished tantalum without pretreatment are very lightly attached to the metal. The pretreatment was based on a guess that fluoride present in the film left by chemical polishing was somehow responsible for the lack of adherence and that it could be replaced by OH' by the pretreatment. The presence of fluoride has been established by Amsel et aL (3). Pawel et al. (4) haveshown that fluoride ions can be incorporated into the oxide from the solution during anodization and can be drifted by the high applied field to the metal where they do indeed then produce poor adhesion. The ductility was confirmed and quantified by Bubar and Vermilyea (5), who used an electrochemical technique to measure the amount of "bare" metal exposed at cracks in the oxide. A redox current was observed corresponding to the tunneling of electrons through the thin oxide in the crack regions. Substantially no such current was observed in some cases up to 9% elongation of the tantalum (Fig. 4, loc. cir.). These authors used HF to remove the film left after chemical polishing. Dunn (6) obtained larger deformations of the metal by rolling anodized metal. A reanodizing technique, which again gave a measure of the area of metal exposed at cracks, indicated large deformation of the oxide. Eliezer and Brandon (7) applied a dry bulge test to windows of anodic film left by selectively removing the metal from behind. They observed only 9 Electrochemical Society Active Member.

Anodization of Metals Through Thin Solid Electrolyte Films of Calcia Stabilized Zirconia

“…During the anodization of metals covered with a thin CSZ film some stresses can be develoued at the metal oxide-CSZ interface, due to the increase of volume of the underlying metal oxide (Pilling-Bedworth ratio equal to 1.6 or 2.5 for A1 or Ta for example). The ductility of an0dic oxides is known to be very high (27): this should make easier the displacement of the CSZ ]aver located on the top of growing metal oxide. But this last phenomenon is not associated with an ionic migration and so will not be taken into consideration in what follows.…”