A study on metastability phenomena of passive films for corrosion resistant alloys

In chloride-containing environments, passive films on corrosion-resistant alloys (CRA) are characterised by instability phenomena. These in turn depend on environmental conditions, type of alloy, sample geometry and heterogeneities on the metal surface. The localised corrosion process for these systems can be described by a three-state model: (1) passivity, (2) metastability, and (3) localised corrosion. Corrosion resistance is based on the permanence of states I or 2, while transition to state 3 indicates the beginning of a localised attack. In this paper, the metastability state of passive films on CRA is studied in order to find a suitable model for predicting the transition from metastability (state 2) to localised corrosion (state 3). Samples are tested for properties such as frequency and charge quantity of metastability phenomena, and time versus survival probability distribution is also investigated. Our working hypothesis is that localised corrosion can be described by a Markov chain stochastic process. Using this model, two indexes are calculated for localised corrosion resistance, and their use in establishing a ranking of specific systems is considered. (c) 2007 Elsevier Ltd. All rights reserved.