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The stress-corrosion cracking (SCC) resistance of commercially fabricated mill-annealed and thermally treated Inconel Alloy 600 steam generator tubing was evaluated over a range of sodium hydroxide concentrations (10 to 50 percent), test temperatures [316 to 343 °C (600 to 650 °F)] and applied stresses (50 to 150 percent yield stress). The grain boundary microstructure was compared with that previously observed in laboratory-treated material. The SCC resistance of thermally treated tubing was superior to that of the mill-annealed product for all test conditions. It was confirmed that the improvement in caustic SCC resistance correlated with the presence of a semicontinuous grain-boundary chromium carbide precipitate. Grain boundary segregation (phosphorus, boron) and chromium depletion do not adversely affect SCC performance. The presence of copper oxide reduced SCC resistance, whereas the presence of silica inhibited SCC. These observations were rationalized by reference to the anodic polarization curve.
microstructure, alloys, Inconel 600, stress-corrosion cracking, caustic, thermal treatment, grain boundary, carbide precipitation, element segregation, chromium depletion
Fellow engineer, Westinghouse Research and Development Center, Pittsburgh, Pa.
Senior engineer, Nuclear Technology Division, Westinghouse Electric Corp., Pittsburgh, Pa.