Salt film development during pitting of nickel

Abstract

<p>Pitting corrosion is one of the major causes of industrial failure of metallic parts. Unfortunately, such attack cannot always be detected before perforation occurs. Understanding the mechanism of pit development will provide us with a clue of how to prevent pitting corrosion in advance. From an engineering point of view, it is important to keep the pit, once it forms, repassivated rather than allow continuous growth, leading to the perforation. The key factors controlling pit development must therefore be determined. A comprehensive model of pitting processes is proposed which considers interfacial dissolution, salt film formation, multiple species in solution, reaction equilibria, migration and diffusion. The model is capable of determining local chemistry within pits and characterizing the conditions necessary for stabilization of pitting corrosion. The model is experimentally verified using microelectrodes developed to measure the local properties of pitting sites. The electrodes were characterized through studies on their potential stability, spatial resolution, and the interference of other ions involved in the corrosion of nickel. The study was performed on a nickel model pit in a saline environment. The in situ measurements obtained by microelectrodes provide the direct evidence of existence of a salt layer, comprised of corrosion products, at the bottom of a developing pit. The condition for formation of the salt film in active pits appears to require that the pit depth be sufficient to ensure the maintenance of a critical local solution chemistry. The experimental results are consistent with the theoretically predicted critical depth at which a metal chloride salt layer starts to form at the pit bottom. The measurements of the potential distribution within the salt film have provided data for the potential drop across the salt film, its thickness and therefore its conductivity. These help us to achieve a better understanding of the nature of the salt film. The anodic behaviour of nickel was studied in solutions which simulate the pit electrolyte, based on the in situ measurements of the local pH and chloride ion concentration within a pit at different stages of development. The factors controlling the pit growth were deduced.</p>