Impurity segregation to scale/alloy interfaces and its effect on interfacial properties

This paper summarizes experimental results on the chemical changes at Al(2)O(3)/alloy interfaces as a function of oxidation time, temperature and cooling rates. Surfaces of FeCrAl, FeAl and Fe(3)Al alloys after removal of the scale in ultra high vacuum were analyzed using an Auger microprobe. Sulfur was the only segregant on FeAl and Fe(3)Al, but C and Cr were also present on FeCrAl, where all of the C and some of the Cr segregated during cooling. The S content on FeCrAl built up quickly with oxidation time, reaching a saturation level with rates in accordance with sulfur diffusivity in the alloy. The final concentration on FeAl and Fe(3)Al was only half that of saturation. On Fe(3)Al, S strongly segregated upon initial oxidation; it then desegregated from the interface before slowly building up again to a constant level. The strong initial segregation, however, was not found on FeAl. These results indicate that segregation to oxide/metal interfaces closely resembles that to free surfaces and alloy grain boundaries, where interface microstructure and co-segregation effects dictate the final concentration. A unique feature on growing oxide/meal interfaces is that this concentration can change with time according to changes in interfacial structures as a result of scale growth. Preliminary results on scale spallation behaviors did not indicate a strong relationship between the amount of interfacial sulfur and the interface fracture resistance, whose strength was more heavily related to interfacial pore density.