Characterization of the roles of electrochemistry, convection and crack chemistry in stress corrosion cracking

Abstract

Understanding the role of ionic current flow within a crack and near the crack tip is fundamental to modeling of environmentally assisted crack advance. Critical conceptual issues and models related to ionic current flow within cracks, and the associated ``crevice`` chemistry and metal oxidation that results, are presented and examined in the light of experimental evidence. Various advanced techniques have been developed to evaluate the roles of electrochemistry, transport, and crack chemistry in stress corrosion cracking, with emphasis on high temperature ``pure`` water. These include high resolution crack length measurement by dc potential drop performed simultaneously with microsampling, electrochemical microprobe mapping, microinjection of species, and micropolarization of the crack. Conceptual issues addressed include the importance of the corrosion potential vs. oxidant concentration, the absence of oxidants and associated low corrosion potential within cracks, the location and role of macrocell currents associated with potential gradients from differential aeration cells, the localized nature of the microcell currents associated with dissolution at the crack tip, the importance of pH and adsorbed species on repassivation and crack advance, and the role of convection in crack chemistry and crack advance. Correct concepts are shown to be an essential pre-cursor to quantitative modeling.