TEM investigation of intergranular stress corrosion cracking for 316 stainless steel in PWR environment

Type 316 stainless steel foils containing stress corrosion cracks grown in high temperature aqueous environments have been examined by transmission electron microscopy. It was found that the crack tips are oxidized and have a three-layered morphology where all the layers taper towards the crack tip. The inner layer is a microcrystalline spinel sandwiched between the outer layers of a nano-crystalline oxide. The outer layers are enriched in Cr, and the inner with Fe, relative to the matrix. Cu was observed to segregate at the interface between oxide and matrix at one crack in type 316 steel. The inner oxide growth is dominated by different mechanisms before and after the grain boundary cracks.