Evaluation of environmental effects on intergranular attack of Alloy 600. Final report

Parametric studies of laboratory initiated intergranular attack (IGA) in mill annealed Alloy 600 were conducted in advance of remedial action testing. In isothermal capsule tests an increase in test temperature from 600 to 6500F and, to a lesser extent, an increase in caustic concentration from near 10 to 50 weight percent enhanced IGA. The possibility that mildly alkaline silicates could concentrate in a superheated crevice and cause IGA was investigated. Vapor pressure measurements did not identify a composition of silicates that could concentrate. Exposures of capsules to organic solutions (sodium-acetate, sodium-phenoxide, and sodium formate) at 6100F for 180 days did not produce noticeable IGA or stress corrosion cracking (SCC). However, stressed C-rings exhibited IGA and SCC in the last two, less stable environments but not in the sodium-acetate solution. In superheated crevice experiments near 6100F, chemical additions were made of 10 ppM sodium carbonate or of sodium hydroxide plus potassium hydroxide. At the top of the tubesheet crevice, denting was observed for both environments as a consequence of caustic corrosion of the tubesheet. The denting rate was much higher for carbon steel than for A508 low alloy steel tubesheets. IGA rates were the highest near the bottom of the tubesheet crevice. Increasing the tubesheet temperature to 6500F and packing the crevices with chromium oxide, effected similar amounts of denting, but enhanced SCC and IGA. IGA rates up to 47.4 mpy were obtained, which could be suitable for remedial action testing. Such IGA rates are comparable to those obtained in Alloy 600 capsules at 6500F using the most aggressive environment of caustic plus chromium oxide. Neither sulfur contamination nor hydrogen markedly influenced the IGA rates. An attempt to remove crevice deposits with the EPRI crevice cleaning solvent was partially effective. 29 refs., 41 figs., 36 tabs