Electrostrictive stresses and breakdown of thin passive films on stainless steel

A chemomechanical model of passive film breakdown (pitting corrosion) of stainless steel undergoing anodic oxidation is presented; it also likely has validity for many other Cr-containing corrosion-resistant alloys. The model is based on several recent observations. Firstly, remarkable improvements in corrosion resistance of 316L austenitic stainless steel have been realized by low temperature interstitial hardening [1] and [2]. Secondly, as reported by Natishan et al. [3], XPS studies of 316L stainless steel samples polarized in 0.6 M NaCl solutions at various potentials did not reveal any chloride incorporated into or chemisorbed on the passive oxide film. Also, the removal of MnS inclusions by electropolishing does not produce the same improvements in corrosion resistance as interstitial hardening, and the Cr:Fe ratio in the passive film is not enhanced by interstitial hardening [4]. Lastly, we have observed [4] that for 316L stainless steel in 0.6 M NaCl solutions, film breakdown occurs at a critical average film thickness of ≈3 nm. These observations preclude the current models of passive film breakdown, which invariably involve chloride incorporation, while the final finding suggests a chemomechanical effect. To explain these observations, we appeal to the large electric fields present during anodic oxidation which cause large electrostrictive stresses. These add to the “intrinsic” or growth stresses that inevitably accompany thin film growth. The combined stresses in turn can induce morphological instabilities—thickness fluctuations in the passive film—to reduce the strain energy density within the films. Dielectric breakdown leading to pitting corrosion can then occur in the thinnest regions of the films.