This research illustrates a spatially distributed cellular automaton aimed at simulating the pitting corrosion process in stainless steels. The cellular automaton model is based on the metal surface discretization into square cells; in any instant, each cell is found to be in one of the following states: (i) passivity (the passive fi lm on the cell surface is stable and protective), (ii) metastability (the passive fi lm is unstable and can either break, causing pitting initiation, or repassivate), and (iii) pitting (the cell has turned into a stable attack area, with no repassivation chance). At the very beginning, all cells are in passivity state. Subsequently, a stochastic process drives the changes in the state of cells, following rules drawn from the kinetics of pitting localized corrosion. In particular, the transition probabilities of each single cell from one state to another are infl uenced by the adjacent cells. The comparison between simulations run with the proposed model and laboratory experimental results demonstrates that the model is suitable for describing pitting corrosion processes.

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Electrochemical tests lasting 600 h to 6,000 h, at different potentials, were carried out on Type 316L (UNS S31603), Type 904L (UNS N08904), and nickel alloy 825 (UNS N08895) in sodium chloride (NaCl) solutions. A statistical approach was adopted using samples of 10 to 60 specimens. The pitting or crevice corrosion behavior was reported using cumulative frequency of passive film breakdown vs. time. In this work, we considered the formation of stable occluded cells as the breakdown of passive film. Each corrosion system “passive alloy-environment-geometry-time” was characterized by straight lines (i.e., breakdown rate of passive film, ρb), with a higher slope in the initial stage and a lower slope after some tens/hundreds of hours. The ρb can be used in the design for material selection and industrial maintenance schedules.

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