An ATEM study of oxidation behavior of SCC crack tips in 304L stainless steel in high temperature oxygenated water

Oxidation behavior at the tips of several stress corrosion cracking (SCC) crack, including intergranular SCC (IGSCC) and transgranular SCC (TGSCC) cracks, in heat-treated 304L stainless steel in high temperature oxygenated water was studied using a field emission gun-analytical transmission electron microscopy (FEG-ATEM). These SCC cracks were produced during SCC tests in two different heat-treated 304L stainless steels in high temperature pure water with two different levels of dissolved oxygen (DO). The study showed that oxygen played a key role in nucleation and propagation of SCC cracks. For the IGSCC crack, oxidation took place followed by nucleation and growth of microcracks in the oxide ahead of the crack tip on the grain boundary plane. The IGSCC crack then propagated by linkage with these microcracks. Oxides formed in the high temperature water with different DO had different configurations and microstructures. In the case of low DO, separate oxides of Cr2O3 and Fe3O4 types were formed, while the oxide formed in high DO consisted of mixed FeCr2O4 and Fe3O4. For the TGSCC crack, oxidation along deformation bands and nucleation and growth of microcracks within oxide were important mechanisms. It was found that Si was preferentially oxidized ahead of the crack tip, promoting SCC crack propagation. Ni enrichment at some SCC crack tips was also revealed.