Tantalum oxide films on smooth, pure substrates can be deformed as much as 50% before fracture. On surfaces containing impurity particles the films contain many flaws which nucleate cracks after relatively little deformation. Zirconium oxide films showed some evidence of deformation, but much less than tantalum oxide. Aluminum oxide films showed no evidence of deformation.Protection against corrosion is usually afforded by a thin film of reaction product. The behavior of such a film during deformation of the underlying metal would be of great importance in determining corrosion under such conditions and may play a decisive role in stress corrosion cracking and corrosion fatigue. It is usually assumed that reaction films are brittle, and one currently popular theory of stress corrosion cracking asserts that repeated cracking of the passive film allows continued corrosion. Bradhurst and Leach (1) have shown that barrier-type anodic oxide films on aluminum are brittle, while Edeleanu and Law (2) have demonstrated that both porous anodic aluminum oxide films and the natural corrosion film on aluminum are brittle. Young (3), on the other hand, reported that anodic films on tantalum changed color during deformation of the metal indicating some deformation of the oxide, and Dunn (4) has told us that anodic films on tantalum deform along with the substrate when tantalum sheet is rolled. In this paper we report the use of a new technique to obtain quantitative information about the deformation of anodic films on aluminum, zirconium, and tantalum.

An estimate of the ductility of a passive film can be made by straining the metal covered by the film and observing the current required to keep the film at its normal thickness. For the metals tested, the relative ductilities of passive films as measured by this technique decrease in the order tantalum; 16 Cr, 14 Ni, bal. Fe; type 304 stainless steel; iron; aluminum; 60 Ni, 16 Cr, bal. Fe; zirconium; and 80 Ni, 20 Cr.

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