Oxide Film Characterization Along Crack Paths in Stainless Steel in Aerated and Deaerated Water Environments

The oxide films that formed along stress corrosion crack (SCC) paths in both high temperature aerated and deaerated water (AW and DW) were characterized through extensive analytical transmission electron microscopy (ATEM), energy dispersive spectroscopy (EDS), Auger, X-ray diffraction (XRD), and focused ion beam/scanning electron microscopy (FIB/SEM) analyses. SCC growth rate experiments were conducted in AW and DW environments at temperatures as high as 360°C, with anion additions during portions of testing. Rapid growth rates occurred in anion faulted AW and were as much as two orders of magnitude faster than those measured in the DW environments. The oxides generally consisted of a dual-layered structure with an inner layer of chromium enriched oxide (relative to the metal substrate) and an outer layer of iron rich oxide. Nickel enrichment was often observed at and ahead of crack tips and along crack flanks in the DW environment and was infrequently observed in the AW environment. Localized corrosion, here referred to as “oxide bulbs” were solely detected in the AW environment and occurred periodically along crack paths and at some crack tips. The development of a nickel enriched zone ahead of the crack tip in the DW environment is speculated to be a signature of a slow growing crack rather than an essential signature of the crack propagation mechanism.