The influence of the high Fe and Cr contents of Alloy 800 on its inter- and intragranular oxidation tendency in 480°C hydrogenated steam

Alloy 800 was exposed to 480°C hydrogenated steam with conditions maintained at an oxygen partial pressure well below the dissociation pressure of NiO. Oxidation phenomena in Alloy 800 were investigated using high resolution electron microscopy techniques. Limited oxygen penetration was detected at grain boundaries while extensive internal oxidation occurred intragranularly. Internal oxidation was possible in an alloy containing a reactive solvent element, Fe, with the solute element, Ni, displaced to the surface. Several stress relief mechanisms are observed to operate simultaneously. The relevance of the observed oxidation phenomena on the primary water SCC resistance of Alloy 800 is discussed.