1996 W.R. Whitney Award Lecture: Statistical and Stochastic Approaches to Localized Corrosion

Corrosion science has been based mainly upon deterministic approaches, particularly the electrochemical theory of corrosion. Localized corrosion, however, cannot be explained without statistical and stochastic points of view because of the large scatter in data common in the laboratory and the field. Toshio Shibata was the 1996 recipient of the W.R. Whitney Award sponsored by NACE International. In his award lecture at CORROSION/96, Shibata reviewed successful applications of statistical approaches to localized corrosion in engineering data and presented a stochastic theory of pitting corrosion based upon sensitivity analysis of parameters in the stochastic model that rationally could explain statistical distributions of pitting potential (Epit) and induction time for pit formation. The most successful application in the statistical analysis was found in the extreme-value analysis using the Gumbel distribution to estimate the maximum pit depth that will be found in a large-area installation by using a small number of samples with a small area. A birth and death stochastic process model was developed to explain the statistical distribution of Epit and induction time for pit generation. Effects of potential sweep velocity (v), surface area (S), alloying elements, and solution flow velocity on Epit were explained. A Monte Carlo simulation using the birth and death model simulated the observed distribution. Image analysis of stainless steels (SS) exposed to an atmospheric environment was discussed as a means to characterize rust spot distribution.