Enhanced corrosion resistance of interstitially hardened stainless steel: Implications of a critical passive layer thickness for breakdown

Immense interstitial hardening of 316L austenitic stainless steel via low-temperature paraequilibrium carburization also leads to greatly improved corrosion resistance. Both the hardening and the improved corrosion resistance owe their origin to a "colossal" supersaturation of interstitial carbon. The corrosion resistance of stainless steel involves a Cr2O3-rich passive film, and the composition and thickness of the passive film developed during anodic polarization at various potentials were determined for both carburized and non-treated steels using grazing incidence X-ray photoelectron spectroscopy. Passive oxide film breakdown is a necessary step in pitting corrosion, and appears to occur in these steels at a critical film thickness of ≈3 nm. We suggest that this breakdown is of chemomechanical origin. Long wavelength thickness perturbations occur during film growth to reduce the strain energy density in the passive film arising from intrinsic and electric field-induced stresses. At the critical thickness, the localized thinning is sufficient to lead to dielectric breakdown and nucleation of pitting corrosion. The improved corrosion resistance for the carburized material results from thinner passive films at a given potential and hence a delay in the detrimental effect of the thickness perturbations.