Statistical investigations of the pitting of passive iron

Revealing the Corrosion Resistance of 316 L Stainless Steel by an In Situ Grown Nano Oxide Film

It is widely accepted that the corrosion resistance of stainless steel originates from a compact Cr₂O₃ layer in the native passive film that serves as a barrier to aggressive ions. However, this suggestion has been questioned by some researchers. They believe that protectiveness might be related to the film recovery. Herein, the pitting development of bare 316 L stainless steel was compared with a corrosion-resistance enhanced steel obtained by tuning the native passive film of the alloy. Statistical software was employed for tracing the size and number of pits on the alloy surface. The statistical results for 12 weeks in 1 M sodium chloride solution (80 °C) revealed that there was a crossover in the growing rates of stable pits (diameter > 9 µm) between the bare alloy and the film-enhanced one. Stable pits on bare 316 L occurred early but showed a comparatively slow increase in the following weeks, demonstrating that self-repairability of metastable pits rather than impermeability of the native passive film plays the key role in the early stage of pitting corrosion.

A Brief Review of Determinism in the Prediction of Localized Corrosion Damage

The great majority of models that have been developed to predict localized corrosion damage are empirical in nature. As such, they lack the ability to effectively predict damage outside of their immediate realm of calibration and generally can only “predict” what is already known. Furthermore, empirical models generally cannot predict new phenomena or relationships. It is shown that it is possible to describe propagation of corrosion damage deterministically,i.e.to describe how the systems evolves from the present state to the future state on the basis of natural laws [conservation of charge, mass-energy, and mass-charge equivalence (Faraday's law),etc.], subject to constraint by the natural laws. It is clear that deterministic models have much more predictive power than do empirical models. However, in the general case, it is possible to describe the development of localized corrosion damage in terms of the propagation of an ensemble of corrosion events, rather than as individual cavities. Accordingly, the prediction can be made in terms of statistical terms, for example, probability that the deepest pit will exceed the critical dimension that defines failure (e.g., thickness of a pipe wall). In doing so, the statistical parameters (mean depth of the deepest pit with its standard deviation,etc.) can be calculated deterministically. In order to perform such calculations, we must possess deterministic models for every stage of cavity development (pit nucleation, propagation and repassivation, transition pit into crack, crack propagation, and so forth). Some of these models are outlined in the current review. The deterministic theory outlined here has been applied for the predation of localized corrosion damage in important, practical systems, including pitting in oil field components, in low pressure steam turbine blades and discs, and in condensing heat exchangers, to name but a few of the current and past applications, some of which are reviewed in this paper.

Sudden Onset of Pitting Corrosion on Stainless Steel as a Critical Phenomenon

Stainless steels undergo a sharp rise in pitting corrosion rate as the potential, solution concentration, or temperature is changed only slightly. We report experiments using real-time microscopic in situ visualizations that resolve the nucleation and evolution of individual pits during the transition. They suggest that the sudden onset of corrosion is explained by an explosive autocatalytic growth in the number of metastable pits and that stabilization of individual pits takes place only later. This finding agrees with a theoretical approach treating the onset of pitting corrosion as a cooperative critical phenomenon resulting from interactions among metastable pits, and it extends perspectives on the control and prevention of corrosion onset.