Localized Corrosion Mechanisms – a Bayesian Reasoning Perspective

Abstract

The mechanisms for localized corrosion proposed over the last several decades are fall into two two major categories: (1) defect generation in the passive film of the open surface leading to breakdown and enhanced local dissolution and (2) the maintenance of a local electrolyte chemistry that promotes continued localized corrosion. A qualitative argument was proposed by Frankel et al. [1] to bridge the gap between passive film and local chemistry mechanisms. They suggest that passive film breakdown was more important for alloy-environment situations where pit initiation was more difficult, whereas local chemistry was more relevant for situations where pit initiation was more facile. Another difficulty arises when modeling the effect of local chemistry on stabilization of local corrosion. Is it the formation of critical pH, critical metal-anion complexes, or metal-anion salt film that is responsible for the stabilization of localized corrosion? These effects are interrelated because high concentration of metal cations can lead to acidification and salt film formation. Specific experimental techniques may bias conclusions with respect to one mechanism or another. Furthermore, uncertainties exist in the data used to validate models as conditions assumed in the models may be difficult to access. It is convenient under these circumstances to take a probabilistic approach based on Bayesian network (BN) to assess the likelihood of validity of a causal mechanism [2]. In this presentation, the importance of local acidity versus metal-salt formation on localized corrosion is examined through a Bayesian network model. The Bayesian network consists of nodes connected through cause-effect relationships characterized by conditional probabilities. The network computes the marginal probability of an event given causal events. Conversely, given an evidence, the preceding causes are evaluated using Bayes theorem. Thus, BN provides a probabilistic framework for cause-effect evaluations based on experimental evidence. Results from the literature and recent experiments are presented to illustrate the approach. Limitations of the approach are also presented. Frankel, G.S., T. Li, and J.R. Scully, Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability. J. of The Electrochemical Society, 2017. 164(4): p. C180-C181.Pearl, J., Causality. 2009: Cambridge university press.