Microstructural evolution of Alloy 690 during sensitization at 700 °C

A structural characterization of sensitized Alloy 690 has been carried out. Alloy 690 was solution annealed (SA; 1100 °C for 30 min, water quenched, WQ) and sensitized at 700 °C for 5, 12, 24, 36, 48 and 72 h, followed by water quenched. Techniques employed included scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction and corrosion weight-loss tests. It was found that the cubic Cr0.19Fe0.7Ni0.11 phase was the main component in all the conditions. In addition, a grain refinement was observed when the aging time was increased; but after 48 h of aging, a discrete, semicontinuous network of Cr23C6 precipitates was detected by X-ray diffraction, in addition to the NiCrO4, Ni9S8 and (Fe,Ni)9S8 phases found by TEM at the grain boundaries, making this alloy more susceptible to intergranular attack (IGA). After 72 h of aging, chromium continues diffusing, “back-filling” the prior depletion profile, recovering the IGA resistance.