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

Passivity films on corrosion resistance alloys (CRA) in chloride-containing environments are characterised by instability phenomena, which depend on type of material, environmental conditions, sample geometry and presence of heterogeneities on the metal surface. The localised corrosion process for these systems could be described by a three-stage model: 1-passivity, 2-metastability and 3-localised corrosion; the corrosion resistance is based on the system permanence in states 1 or 2, while the transition to state 3 indicates the beginning of localised attack. This paper studies the metastability state of the passivity film for CRA with the aim to propose a model able to predict the transition to localised corrosion (state 3). Some properties of tested samples, such as the frequency and charge quantity of metastability phenomena, are examined in this paper. Time versus survival probability distribution was also investigated. The hypothesis has been put forward that localised corrosion could be dealt with a Markov chain stochastic process, and an index of localised corrosion resistance “pLCP” was calculated using this model.