| It has often been assumed that pitting events occur randomly in time and space and that there is no effect of one event upon another. However, when a pit begins to form and current flows, the local environment is altered. Changes can occur in the local concentration of aggressive species, the potential field and damage can occur to the passivating oxide film. These three competing effects work on different time scales and can affect electrode areas through different distances. Therefore, pitting events can have some influence on the probability of future nearby events.; Current time series data collected on a single working electrode experiencing metastable pitting corrosion was first analyzed. The metastable pitting events were found to have correlation and were not randomly distributed in time.; Further investigation was performed on an array of 25 working electrodes in order to obtain information on the spatial interactions among metastable and stable pitting sites. The 5-by-5 array consisted of closely spaced, 0.025 cm diameter 316 stainless steel wires. The flush-mounted wire tips were exposed to 0.05 M NaCl solution at 47°C. To examine the environmental changes created by a stable pitting site, one or more electrodes in the array were held at a 1 V vs. SCE causing the entire electrode to corrode at a high rate simulating a single large pit. Two types of interactions were observed when a pit was created in the center of the array. First, inhibition of pitting on nearby electrodes occurred due to ohmic potential drop near the actively corroding pit site. Second, enhancement of pitting was observed due to alternations in the local solution composition and oxide film created by a deactivated pitting site. The ohmic shielding effect was dominant near the active site, however, it dissipated almost immediately after the pit site was deactivated. The other two interactive effects increased pitting probabilities as they endured for a longer time after the current ceased to flow, with oxide film changing lasting more than 10 minutes.; The information obtained about the interactions among corrosion sites were then used to develop a spatial metastable pitting model that simulated the behavior found experimentally. |
