The Kinetics of Nucleation of Metastable Pits on Metal Surfaces: The Point Defect Model and Its Optimization on Data Obtained on Stainless Steel, Carbon Steel, Iron, Aluminum and Alloy-22

Abstract

The theory of the kinetics of metastable pit nucleation in terms of the Point Defect Model (PDM) has been applied the first time to describing the evolution of the nucleation rate of metastable pits on a variety of metallic substrates. The PDM successfully accounts for the experimental data that have been reported in the literature on stainless steel, carbon steel, iron, aluminum, and Alloy-22, and which are judged to be reliable and reproducible. Important fundamental parameters related to metastable pitting such as total number density of pitting nucleation sites, dissolution time of the cap over the pit, energy related to absorption of the aggressive ions into oxygen vacancies in the surface of the barrier layer, vacancy condensation rate, and the time at which the nucleation rate of metastable pits is maximum were obtained from the optimization of the PDM on the experimental data, as reported in the present paper. The values obtained for those parameters are in good agreement with values and observations reported elsewhere. The present work successfully demonstrates the capacity of the PDM in accounting for experimental observations of metastable pitting and that the PDM can be applied as an underlying theory for studying and understanding metastable pitting on metal surfaces. The passive state is not perfectly protective and passivity breakdown occurs for various reasons, giving rise to enhanced general corrosion rate or to localized corrosion. Localized passivity breakdown is especially of great concern, because it results in the nucleation and propagation of pits and the subsequent nucleation and growth of cracks provided that there is sufficient tensile stress in the system to initiate crack nucleation at the stress raiser represented by the pit. Metastable pitting, in which passivity breakdown occurs followed immediately by repassivation, is now well-accepted as being an integral part of the process of pitting corrosion on a metal or alloy, culminating in the accumulation of damage via the development of stable pits. It is also well-recognized that repassivation reflects the failure on the part of the pit nucleus to establish a spatial separation between the local cathode and the local anode with the former occurring on the external surfaces and the latter taking place in the nucleus, which is necessary for the buildup and maintenance of the aggressive conditions of high [Cl − ] and high [H + ] (low pH) that cause the pits to grow as stable pits, until they, too, die via delayed repassivation (due to the inability of the pit to maintain separation of the local cathode and the local anode, because of the limited availability of resources of the cathodic depolarizer on the external surfaces).Numerous authors have reported metastable pit nucleation rate data measured under more-or-less well-defined conditions. Williams et al.1-3 reported a good proportional correspondence for Type 304L SS between the nucleation frequency of metastable pits and the frequency of occurrence of propagating (stable) pits, that ...

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