Paper AP+AS+SS-TuA9
Atom Probe Tomography Investigations of Surface and Grain Boundary Oxidation in Ni-Cr Alloys Exposed to High-Temperature Water

Tuesday, October 29, 2013, 4:40 pm, Room 203 A

Ni-base, Cr-containing alloys have been selected for use as structural components in many aggressive environments because of their well-known corrosion resistance. In most oxidizing environments, the high Cr content of these alloys results in the formation of a protective Cr-rich oxide film. However, Ni-Cr alloys have been shown to be susceptible to localized corrosion in high-temperature hydrogenated water environments as found in the primary system of pressurized water reactors. Mechanisms controlling this degradation are being investigated using high-resolution analytical electron microscopy and atom probe tomography. Examples will be presented from Ni-Cr model binary alloys (5-30Cr), commercial alloy 600 (Ni-17Cr-9Fe) and alloy 690 (Ni-30Cr-9Fe) samples. In all cases, grain boundaries are found to play a significant but varied role in the observed corrosion behavior. In lower Cr alloys (5-20%), the grain boundaries are preferentially attacked and exhibit extensive oxidation to a much greater depth than the surrounding matrix. In sharp contrast, alloys with 30% Cr form a continuous layer of Cr₂O₃ directly above grain boundaries that protects the grain boundary from oxidation. Localized filamentary oxidation is observed into the metal matrix away from the intersection of the grain boundary with the surface where a continuous layer of Cr₂O₃ does not form.