51312-01501-SCC of sensitized 304SS in high temperature water with anionic impurities contamination

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Product Number: 51312-01501-SG

ISBN: 01501 2012 CP

Author: Fabio Scenini

Publication Date: 2012

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This paper describes stress corrosion cracking on sensitized and cold worked 304 stainless steel in high temperature hydrogenated or deoxygenated water (260ºC/360ºC) occurring as a consequence of ionic impurities and/or superficial defects. The crack morphology was predominantly intergranular with some transgranular regions. Transmission electron microscope (TEM) samples containing crack tips were prepared by focused ion beam (FIB). 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 also found. It is postulated that cracking initiation and propagation might be associated with the formation of oxide on crystallographic planes inside the material. The stress corrosion cracking (SCC) here presented indicated a synergism between sensitization and impurities as well as an aggravating factor of cold work. In fact non sensitized materials did not initiate cracking (within the time scale of the tests) while sensitized materials underwent extensive SCC. Furthermore crack growth rate (CGR) tests of cold worked and/or sensitized material carried out in high purity hydrogenated or deoxygenated water at 300ºC showed sustained crack growth for cold worked while prior sensitization inhibited stress corrosion crack propagation.