Predicting Susceptibility to Intergranular Stress Corrosion Cracking of Alloy 690

Susceptibility to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) of Alloy 690 (UNS N06690) was investigated using chemical etching, double-loop electrochemical potentiokinetic reactivation (DL-EPR), and slow strain rate tests. Alloy 690 was solution-annealed at 1,100°C for 30 min, water-quenched, sensitized at 700°C for 5, 12, 24, 36, 48, and 72 h, and then water-quenched once more. The chemical etching and DL-EPR test in 0.5 M sulfuric acid (H₂SO₄) ± 0.0001 Mpotassium thiocyanate (KCNS) could accurately discriminate the effect of aging on the susceptibility to intergranular corrosion, revealing that the specimen with the highest susceptibility to IGA was the one aged for 48 h. The susceptibility to IGSCC in 0.01 M sodium thiosulfate (Na₂S₂O₃) at 90°C initially increased with increasing the aging time, reaching a maximum value at 48 h; then, it decreased again for longer aging times. The chromium depletion was maximum also for the aging time of 48 h, so it is very likely that chromium carbides are responsible for this type of damage. Also, some particles like titanium oxide (TiO₂) and chromium-nickel-sulfide were found for this specimen, and thus seem to increase the rate of IGA in sulfur-containing environments.