Clarification of stress corrosion cracking mechanism on nickel base alloys in steam generators for their long lifetime assurance
- Sumitomo Metal Industries Ltd., Amagasaki (Japan). Research and Development Center
Thermally treated (TT) Alloys 600 (16%Cr-8%Fe-bal.Ni) and 690 (30%Cr-10%Fe-bal.Ni) have been successfully used in the steam generators of operating pressurized water reactors (PWRs). This paper deals with intergranular stress corrosion cracking (IGSCC) mechanisms in Ni-base alloys in various corrosive environments such as deaerated water, air-saturated chloride medium, and caustic solutions at high temperatures with focus on Cr content and Cr carbide precipitation at grain boundaries in the alloys. Nickel base alloys of high purity, or with different Cr, C, and B contents with different heat treatments were put to various corrosion tests. SCC resistance of Alloy 600 is affected differently by water chemistry of environments, while Alloy 690 is almost immune to the environments investigated: (1) Cr depletion at grain boundaries is clearly detrimental to IGSCC resistance of Alloy 600 in air-saturated water containing Cl{sup {minus}} ions at 300 C. (2) High purity Alloy 600 has weaker SCC resistance in deaerated water at 360 C than commercially available Alloy 600. Cr depletion along grain boundaries is detrimental to the IGSCC resistance, however its detrimental effect disappears when Cr carbides precipitate at grain boundaries in semi-continuous or continuous way. The NiCr{sub 2}O{sub 4} film formed on the metal surfaces enhances the IGSCC resistance. Similar relationship between Cr depletion and Cr carbide precipitation is also observed in Alloy 600 in deaerated caustic solutions at high temperatures. (3) Concerning intergranular attack (IGA), which occurs in oxidizing caustic solutions at high temperature, existence of semi-continuous or continuous Cr carbides improves the IGA resistance regardless of Cr depletion. A dual layer corrosion protective film composed of an upper layer of NiO and lower layer of Cr{sub 2}O{sub 3} formed on metal surfaces, of which formation is accelerated by selective Cr carbide dissolution, may be responsible for the IGA resistance.
- OSTI ID:
- 203812
- Report Number(s):
- CONF-950816-; ISBN 1-877914-95-9; TRN: 96:009778
- Resource Relation:
- Conference: 7. international symposium on environmental degradation of materials in nuclear power plants: water reactors, Breckenridge, CO (United States), 6-10 Aug 1995; Other Information: PBD: 1995; Related Information: Is Part Of Seventh international symposium on environmental degradation of materials in nuclear power systems -- Water reactors: Proceedings and symposium discussions. Volume 1; Airey, G.; Andresen, P.; Brown, J. [eds.] [and others]; PB: 664 p.
- Country of Publication:
- United States
- Language:
- English
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