Intergranular Attack of Alloy 600 in PWR Steam Generator Tubes Simulation Tests

Abstract In some steam generators, intergranular attack (IGA) has been detected on alloy 600 tubes within or near the tubesheet crevice region. A work was undertaken to reproduce IGA in an experimental device simulating actual steam generators conditions. A special tubesheet assembly was designed with representative materials and geometry. Thermal-hydraulic conditions of the hot leg side of the generators and sludge piles were simulated. During three runs of four tests each, chemical parameters and test procedures were adjusted to finally obtain a high concentration of alkalies in the tubesheet crevices and a representative corrosion attack of tubing materials. In the two first runs, stress corrosion cracking (SCO) in the tubing cut short the tests, and IGA was minor. In the third run chemicals were initially forced into the crevices so that SCC was delayed and IGA favoured. Mill-annealed (MA) alloy 600 was the most susceptible to IGA. A thermal treatment (TT) at 700°C given to that material decreased the amount of IGA. The least vulnerable material was an experimental MA alloy 690.