Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel in High-Temperature Water with Anionic Impurities Contamination

Abstract

This paper describes some results selected from a larger program that was aimed at understanding the stress corrosion cracking (SCC) initiation of Type 304 stainless steel (UNS S30400) in high-temperature deaerated water. Out of a large number of statically loaded samples, only a small minority of the tested samples underwent SCC. The occurrence of SCC indicates a synergism between sensitization, ionic impurities (mainly chloride and sulfate), and/or superficial defects and cold work. In fact, none of the nonsensitized materials initiated cracking (within the time scale of the tests), while only three sensitized samples underwent extensive SCC. The crack morphology of the fractured sample was predominantly inter-granular with some transgranular regions. Transmission electron microscopic samples containing crack tips were, in most respect, in line with the literature: a magnetite/spinel duplex layer on the crack surfaces, a Cr-rich oxide at the crack tip, and Ni enrichment at the metal/oxide interface and oxidized deformation bands intercepting the crack flanks. Also, finger-like features protruding several hundreds of nanometers along the slip planes intersecting the intergranular crack were found on grain boundaries with a high degree of localized deformation. These results support the theory that cracking initiation and propagation might be associated with the formation of oxide on crystallographic planes inside the material.