SEGREGATION OF SULFUR DURING GROWTH OF OXIDE SCALES

There is no equilibrium segregation of sulfur to the intact metal/oxide interface, as was confirmed by taking AES sputter profiles of the chromia layer on an Fe-Cr alloy and the alumina layer on the intermetallic phase beta-NiAl after short term oxidation (10 min) and segregation anneal at high temperatures. After prolonged oxidation (> 3 h) a monolayer of sulfur was detected in regions where the scale had cracked and spalled, however, in this state large cavities had formed beneath the scale and the sulfur obviously had segregated to the free metal surface of these cavities. The adverse effect of sulfur on scale adherence in the oxidation of high-temperature materials containing some ppm sulfur can be explained by the reduction of surface energy caused by sulfur segregation which favors the formation of voids and accelerates their growth to cavities.