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.

The origins of persistent interactions among localized corrosion sites were investigated using multielectrode arrays ͑MEA͒. MEAs consisted of one hundred 250 m diam AISI 316 stainless steel wires configured into 5 ϫ 20 close-packed rectangle and 5 ϫ 5 far-spaced configurations. Experiments were conducted in dilute chloride solutions and elevated temperatures to accentuate interactions. Interactions between early dominating pits and the adjacent electrode surface were found to develop as regions of enhanced or suppressed pitting susceptibility triggered by concentration and potential fields developed during growth of large pits, respectively. Transient aggressive species accumulation around active pits was also found to cause ''surface damage'' to electrodes that persisted for long periods after primary pits were repassivated and solutions extensively stirred. Oxide film alteration, mixed metal sulfide inclusion damage, and surface contamination were all considered to be possible origins of persistent interactions. Mn͑Fe,Cr͒S inclusions were implicated to be the origin of such persistent interactions. Persistent interactions were completely suppressed by nitric acid treatment, which is believed to remove or modify susceptible Mn͑Fe,Cr͒S inclusions.

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*,

The use of multi-coupled electrode arrays in various corrosion applications is discussed with the main goal of advancing the understanding of various corrosion phenomena. Both close packed and far spaced electrode configurations are discussed. Far spaced electrode arrays are optimized for high throughput experiments capable of elucidating the effects of various variables on corrosion properties. For instance the effects of a statistical distribution of flaws on corrosion properties can be examined. Close packed arrays enable unprecedented spatial and temporal information on the behavior of local anodes and cathodes. Interactions between corrosion sites can trigger or inhibit corrosion phenomena and affect corrosion damage evolution.

͑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.