07487 Selective Dissolution and Oxidation Zones in NiCrFe Space and Their Relationship to Stress Corrosion Cracking

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Product Number: 51300-07487-SG

ISBN: 07487 2007 CP

Author: Roger C. Newman and Zoe Coull

Publication Date: 2007

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A framework is presented that may explain a wide range of stress corrosion behaviour in aqueous environments at high temperatures, using only two hypotheses for the applicable crack growth mechanisms. The framework is mainly relevant to reducing (hydrogenated) environments, but may account for some aspects of SCC in oxidizing ones. Binary Fe-Ni alloys have been used to explore some of these issues, as well as engineering alloys. In the Ni-rich region, SCC propagates by intergranular oxidation; high Cr content, as in Alloy 690, provides protection by forming an external Cr-rich oxide. In Fe-rich alloys, classical de-alloying occurs in hot caustic solutions; on austenitic stainless steel or binary Fe- 10Ni this generates a nanoporous metallic layer enriched in Ni. In high-temperature nearneutral or mildly-acid solutions, there is also Ni enrichment, most clearly seen in binary Fe- 10Ni. This Ni accumulates under a magnetite film. It is not yet known whether this Ni is also nanoporous, but the behaviour of noble-metal alloys in analogous conditions suggests that it may be. Higher Ni contents, as in Alloy 800 or model binary alloys, protect against SCC by forming a very thin protective Ni layer rather than a porous one, in accordance with ordinary de-alloying theory.