TRANSPORT PROCESSES IN THE OXIDATION OF NI STUDIED USING TRACERS IN GROWING NIO SCALES

Experimental techniques have been developed for determining Ni63 and O18 tracer distributions in NiO scales ranging in thickness from ∼0.1 to 100 μ. These have been used to investigate Ni and O transport in scales on {100} Ni crystals and polycrystalline Ni in the temperature range 500-1300° C. NiO grown on {100} Ni crystals at 1000°C was uniform and compact and grew by the bulk diffusion of Ni in NiO by a vacancy mechanism. At temperatures below 800°C the principal transport mechanism was short-circuit diffusion of Ni in NiO. At all temperatures short-circuit diffusion of oxygen contributed to scale growth on polycrystalline Ni and was responsible for growth of the inner layer of duplex scales. The oxygen diffusion paths are believed to be micro-cracks induced by growth stresses. © 1979 Plenum Publishing Corporation.