Characterization of internal and intergranular oxidation in Alloy 690 exposed to simulated PWR primary water and its implications with regard to stress corrosion cracking

Oxidation testing of Ni-base Alloy 690 was conducted in simulated PWR primary water, and the internal and intergranular (IG) oxidation layers were systematically characterized by analytical transmission electron microscopy to obtain insight into its IG stress corrosion cracking (IGSCC) resistance and behavior. The internal oxidation layer consisted of upper NixFe1-xCr2O4 and innermost Cr2O3. Each internal oxidation layer had a crystallographic orientation relationship with the Alloy 690 matrix, indicating that the layers formed through solid-state reactions between inward diffusing oxygen and the alloying elements. Strong evidence that the Cr2O3 layer formed through the outward grain boundary diffusion of Cr was found, leaving severe Cr depletion along the grain boundary with a steep composition gradient toward the surface. The innermost Cr2O3 layer had a continuous and compact band-shaped morphology around the surface grain boundary. This Cr2O3 layer was revealed to be protective of the inward grain boundary diffusion of oxygen from the surface and therefore responsible for the prevention of grain boundaries from IG oxidation. IG Cr carbides suppressed the internal and IG oxidation further. The key findings obtained in this work may present the main reason for the high resistance of Alloy 690 to IGSCC under PWR normal operating conditions.