SEGREGATION BENEATH OXIDE SCALES

Studies are reported and discussed on Auger analyses of the region beneath Cr2O3, Al2O3, or NiO layers on their metal substrate. Small concentrations of S, C, and P were detected in areas which had been connected to the oxide layer, most probably due to segregation in defects, such as misfit dislocations, microvoids, grain boundaries, etc. For high oxygen pressures at the interface (Ni-NiO) P also can be enriched in the inner layer as phosphate. Sulfur starts to segregate to the free-metal surface as soon as the scale and metal separate, stabilizing voids and accelerating their growth to cavities or favoring the detachment of scale in the case of growth stresses. In this surface segregation S displaces C and P from the metal surface.