SEGREGATION BENEATH OXIDE SCALES

Studies are reported and discussed on Auger analyses of the region ben eath Cr2O3, Al2O3, or NiO layers on their metal substrate. Small conce ntrations of S, C, and P were detected in areas which had been connect ed to the oxide layer, most probably due to segregation in defects, su ch 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-m etal surface as soon as the scale and metal separate, stabilizing void s 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.