Surface Deformation Nanostructures and Stress Corrosion Crack Precursors Originating from Surface Grinding

Abstract

Stress corrosion cracking (SCC) in light water reactor components has long been studied from a post mortem perspective, yielding insights into water chemistries and effects on crack propagation. Analysis of a cracked component does not effectively provide information on the corrosion events or on SCC initiation. It is important that microstructures of these early stages be understood because the original surface of the component formed during fabrication is often not the final surface condition that is exposed to reactor water. Pre-service grinding of reactor components and welds is performed for a variety of reasons, from aesthetics to preparation for non-destructive testing. It is this final surface microstructure that often controls SCC initiation. Surface and near-surface characteristics have been investigated in 304SS metal coupons on which controlled grinding was performed. These examinations indicate the extent of subsurface microstructural damage before high-temperature water exposure. Analytical electron microscopy techniques have been used to gain insights into possible surface precursors to corrosion damage and SCC initiation. Nanocrystalline grains were commonly found at the surface in lightly ground to heavily abraded materials within the first ˜0.5-10 ųm along with high dislocation densities, twinning and lath structures.