THE EFFECT OF CHROMIUM, CARBON, AND YTTRIUM ON THE OXIDATION OF NICKEL-BASE ALLOYS IN HIGH-TEMPERATURE WATER

Since the surface film has been implicated in several models of intergranular stress corrosion cracking (IGSCC) of nickel-base alloys. this study was initiated to provide a foundation for the future study of a possible link between the nature of the surface film and IG crack susceptibility. The influence of chromium, carbon, and yttrium on the nature of the surface film formed on nickel-base alloys was investigated after 100 h of exposure in high purity, deaerated, hydrogenated water at 360-degrees-C. The chemical composition of an ultrahigh purity (UHP) alloy of Ni-17Cr-9Fe-0.0030C was altered by varying the Cr content, doping with C, or ion implanting Y. X-ray photoelectron spectroscopy and scanning transmission electron microscopy provided information on the chemical composition, structure, morphology, and thickness of the surface film. Increasing the chromium content from 5 to 17 weight percent (w/o) of a Ni-xCr-9Fe-low C alloy dramatically changes the surface film from predominantly Ni(OH)2 to Cr2O3. An additional increase in Cr from 17 to 30 w/o does not significantly alter the type, distribution, or thickness of the oxide phases formed. This suggests that there is a critical chromium concentration that controls the formation of Cr2O3 after 100 h of exposure to high purity, deaerated hydrogenated, water at 360-degrees-C. The presence of Cr2O3 in these alloys is attributed to the high affinity of Cr for oxygen, and the increased stability of Cr2O3 over many other oxides in reducing environments. The addition of 300 wppm carbon to a Ni-17Cr-9Fe-0.0030C alloy increases the film thickness while promoting the formation of Ni(OH)2, possibly by an increase in the diffusion of Ni or by an enhancement of the surface activity of the Ni atoms. The addition of 300 wppm carbon to a Ni-30Cr-9Fe-0.0020C alloy increases the film thickness without changing Cr2O3 as the dominant surface species. Yttrium implantation to 2.4 atom percent at a depth of 70 nm in Ni-17Cr-9Fe-0.0030 and 0.030 C alloys produces a similar film thickness and similar composition profiles containing slightly more Ni(OH)2 than Cr2O3. Yttrium is known for forming Y2O3 which most likely promotes the formation of Cr2O3 by acting as nucleation sites for the similarly structured chromia.