AEM analysis of stress corrosion cracks

Abstract

Cross-sectional analytical electron microscopy (AEM) was used to study stress corrosion cracking (SCC) cracks in pressurized water in an attempt to understand the mechanism of cracking. AEM is particularly well suited to study the crack tip, the crack sides and the corrosion product inside the crack. These analyses provide clues about the stress (i.e., plasticity), the environment (corrosion product) and material (precipitates/grain boundary composition) which are the key factors controlling SCC. In mis investigation SCC cracks were filled with epoxy, dimpled and ion-milled in argon until electron transparent. A cross-section of an SCC crack in Alloy 600 generated in deaerated water is shown in Figure 1. The crack is intergranular and generally propagates down grain boundaries without carbides as shown in Figure 1. The crack seen in Figure 2 is an exception and propagated down a grain boundary containing Cr7C3 carbides. Although dislocations are clearly observed in bom figures, no deformation is observed uniquely associated with the cracks and no voids have been observed ahead of the cracks.