OXYGEN-TRANSPORT IN GROWING NICKEL-OXIDE SCALES AT 600-800-DEGREES-C

The effect of oxidation conditions on the penetration of oxygen into growing nickel oxide scales has been examined using oxygen isotope tracers and SIMS analysis. Depth profiles and cross-section images of oxide scales show that the proportion of oxygen penetration is increased as the temperature is reduced. At 600°C, the amount of oxygen tracer transported to the scale-metal interface is considerable, approaching that observed for a conventional "duplex" microstructure. The formation of healing oxide along numerous fissures through the oxide is directly observed in SIMS images. These fissures seem to coincide with grain boundaries in the preexisting oxide. As the oxidation temperature is increased, the oxygen transport becomes less uniform, some regions of the scale showing little or no transport. Reduction in the oxygen pressure reduces the amount of oxygen penetrating the scale. © 1990 Plenum Publishing Corporation.