New Insights into Internal Oxidation of Alloy 690 and Model Alloys in Hydrogenated Steam

The oxidation behavior of Alloy 690 and Ni-xCr-10Fe model alloys exposed to 480°C hydrogenated steam, in the nickel metal stability region, was studied using a variety of techniques, including time-resolved scanning electron microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and electron energy loss spectroscopy. The alloy underwent internal oxidation intragranularly, resulting in the expulsion of metallic Ni nodules to the surface. A compact Cr-rich “healing” layer developed over time at the intragranular internal oxidation front near grain boundaries and significantly retards further internal oxidation. Protective external Cr-rich oxide formed at and very near the grain boundaries, hindering intergranular oxidation. In view of the fact that the protective oxide forms micrometers away from the grain boundaries, one cannot simply say that Cr diffuses up the boundary and forms an oxide that spreads laterally—internal stress relief must play a role, just as it does within the grains, only with different mechanisms at play.