Effect of heat treatment on the stress corrosion cracking of alloy 690

The susceptibility to stress corrosion cracking (SCC) in 0.01 M Na2S2O3 of alloy 690 under different heat treatments has been investigated using slow strain rate tests. Double-loop electrochemical potentiokinetic reactivating tests in 0.5 M sulfuric acid+0.0001 M potassium thiocyanate was used to evaluate the resistance to intergranular attack (IGA). Alloy 690 was solution annealed at 1100 °C for 30 min and water quenched (WQ), followed by sensitization at 700 °C for 5, 12, 24, 36, 48 and 72 h and then WQ. The susceptibility to IGA and intergranular stress corrosion cracking (IGSCC) in 0.01 M Na2S2O3 at 90 °C increased with increasing the aging time, reaching a maximum value at the aging time of 48 h, and then, it decreased again for the specimen sensitized during 72 h. The chromium-depletion profile was deeper also for the specimen aged for 48 h. These results are discussed in terms of consumption of carbon during sensitization, and a continuous diffusion of chromium, ‘back-filling’ the prior depletion profile, recovering the IGSCC resistance. Also, it is suggested that the presence of a continuous network of grain boundary carbides in the specimen sensitized during 72 h, rather than discrete carbides, as in the one aged during 48 h, is another reason for the increased SCC resistance.