Comparative behavior of Alloys 600, 690 and 800 in caustic environments

Since 1991, secondary side cracking has become a major concern in EDF steam generators. At 350 C, IGSCC of Alloy 600 was found in pure NaOH solutions from 4 to 500 g/l, with a maximum at 100 g/l. From the present results and available data, IGSCC of Alloy 690 occurred in pure NaOH solutions from 40 to 500 g/l, but the greatest sensitivity was observed at 100 g/l. TG/IGSCC of Alloy 800 became more severe with increasing NaOH solutions, from 10 to 500 g/l. On the whole, the ranking for SCC-resistance was Alloy 600 < Alloy 800 < Alloy 690. A significant improvement was still obtained with 690TT with intergranular carbides (industrial tubes) and perhaps with Alloy 800 NG (nuclear grade), but both alloys cracked at the highest stresses. IGSCC susceptibility of Alloy 600 MA was higher for the tubes with the lowest values of the yield stress. The depth of IGA in the case of Alloy 600 was higher than for Alloy 690; for this alloy, it appeared to be general oxidation. Lead was found to significantly increase both IGA and SCC of these alloys in caustic solutions in the range 300-350 C. For tests performed at high stresses,more » and so for short durations, IGSCC (Alloy 600 MA) or TGSCC (Alloys 690 and 800) were the main cause of failure. At low stresses and for long durations, failure occurred predominantly by IGA (and also IG/TGSCC for Alloy 600TT). The worst SCC-resistance was observed on Alloy 800. Resistance was better for Alloys 600 MA land 690 TT. Alloy 600 TT exhibited the best resistance.« less