SCC of sensitized 304SS in high temperature water with anionic impurities contamination

This paper describes some results selected from a larger program which was aimed at understanding the stress corrosion cracking initiation of 304 stainless steel in high temperature deaerated/hydrogenated water. With a large number of statically loaded samples it became apparent that it was a challenge to initiate SCC on statically loaded samples tested in high temperature deaerated/hydrogenated water. However, only a small minority of the tested samples, which were thermally sensitized underwent SCC. The SCC presented here shows the findings from the investigation of these samples and indicates a synergism between sensitization, ionic impurities and/or superficial defects as well as the aggravating factor of cold work. In fact, none of the non sensitized materials initiate cracking (within the time scale of the tests), while only a three sensitized sample underwent extensive SCC. The crack morphology of the fractured sample was predominantly intergranular with some transgranular regions. Transmission electron microscope (TEM) samples containing crack tips were prepared by focused ion beam (FIB). The analysis revealed the crack to contain a dual layer oxide (outer magnetite and inner Cr rich oxide) and metallic Ni enrichment ahead of the crack tip. Furthermore, finger-like features protruding several hundreds of nanometres along the slip planes intersecting the intergranular crack were found. It was also noted that the oxidation of deformation bands developed asymmetrically with the oxide advancing further into the grain with the most favourable crystallographic orientation. This is likely to be caused by the dominating deformation of one grain and consequent generation of a strain gradient across the grain boundary. It is therefore postulated that cracking initiation and propagation might be associated with the formation of oxide on crystallographic planes inside the material. ©2012 by NACE International.