SECONDARY ION MASS-SPECTROMETRY AND SCANNING-TRANSMISSION AND TRANSMISSION ELECTRON-MICROSCOPY STUDIES OF THE EFFECTS OF REACTIVE ELEMENTS ON NICKEL OXIDATION

Implantation of reactive elements (REs), such as cerium and yttrium, at doses of 0.5-5 x 10(16) ions cm-2 has been shown to reduce the NiO scale growth rate substantially during the oxidation of nickel in oxygen at 900-degrees-C, at least during 5 h exposures. Understanding the mechanism involved necessitates establishing the location and form of the RE within the scale. These questions have now been addressed, primarily by secondary ion mass spectrometry (SIMS) and by scanning transmission and transmission electron microscopy (TEM/STEM) examination, on NiO scales formed on nickel implanted with either cerium or yttrium ions to 5 x 10(16) cm-2 during oxidation in oxygen at 900-degrees-C for periods between 0.25 and 150 h. SIMS demonstrated that initially the peak RE concentration within the scale was located near the gas interface but with increasing time it became positioned at increasing depth. NiO growth on unimplanted nickel is controlled primarily by outward cation transport. Assuming the position of the RE peak concentration is indicative of the original nickel surface then, at least initially, RE ion implantation changed the scale growth such that inward oxidant movement was dominant. However, with time, growth reverted increasingly to outward cation transport control. The resurgence or cation transport occurred earlier during the oxidation of cerium than yttrium implanted nickel. TEM/STEM has established that cerium was present as small oxide particles within both the NiO grains and grain boundaries and also that cerium and yttrium segregated along the scale grain boundaries. The segregant concentrations diminished with oxidation time, indicating diffusion along the boundaries, which was consistent with the SIMS observations. There would seem, therefore, to be a plausible correlation between the disposition of the REs within the NiO scale and their effect on the oxidation behaviour of nickel.