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.

Detailed, time-resolved, in situ visualizations of the onset of pitting corrosion of stainless steel directly in the electrolyte (0.05 M NaCl) by ellipsomicroscopy for surface imaging and specially adapted high-resolution contrast-enhanced optical microscopy indicate that the sudden onset of corrosion can be explained by an explosive autocatalytic growth of metastable pits. 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. The given analysis brings investigations of localized pitting corrosion into a conceptual framework of autocatalytic processes and nonequilibrium pattern formation phenomena in reaction-diffusion systems. -(PUNCKT, C.; BOELSCHER, M.; ROTERMUND, H. H.; MIKHAILOV, A. S.; ORGAN, L.; BUDIANSKY, N.; SCULLY, J. R.; HUDSON*,

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͑1981͔͒ were used to characterize the spatial patterns of pitting sites on AISI 316 stainless steel. Populations of artificial pitting sites were generated ͑e.g., clustered, anti-clustered, random, or periodic͒ to test the ability of the selected spatial statistics methods to characterize these patterns. Experimental pitting patterns on AISI 316 stainless steel analyzed by the same methods indicated that interactions occur between micrometer scale pits over multiple micrometer distances. Spatial statistics results indicated that positive ͑e.g., clustering of pit sites͒ occur between pitting sites when they are grown potentiodynamically. Processes associated with the acid/halide pitting mechanism have been shown to promote interactions between pit sites that give rise to such spatial patterns ͓J.

Experiments were carried out on bursting oscillations in an electrochemical system, the dissolution of iron in sulfuric acid. Bursting characteristics were investigated as functions of the parameters applied potential and external resistance. We develop a model that incorporates the effects of oxide and salt films. This model reproduces all the important features of the dynamics seen in experiments including both the slow activepassive oscillations and the faster oscillations on the mass-transfer-limited plateau as well as the bursting phenomena. During this bursting, two time scales are important: the slow variation of the salt-film thickness and the faster dynamics of a three-variable (potential drop and proton and iron(II) concentrations) subsystem.