Compositions at Al2O3/FeCrAl interfaces after high temperature oxidation

Interfacial Segregation, Pore Formation, and Scale Adhesion on NiAl Alloys

“…Although sulfur was found to be the major segregant at these scale/alloy interfaces, the segregation behavior, in terms of rate and amount, varied significantly with different alloys and differed from surface segregation. For example, at the Ah03/FeCrAI interface, cosegregation of Cr with S took place, which gave rise to greater than one monolayer of S. The amount saturated as early as 12 minutes at 1000°C, where the rate was determined by S diffusion in the alloy [5,7,8]. For the iron aluminides, S was the sole segregant at these scale/alloy interfaces, but its steady state level was slowly reached only after a complete a-…”

“…The segregation of indigenous sulfur impurity from an alloy to the Ah03 scale/alloy interface during high temperature oxidation is often considered the major cause that weakens the interface [1][2][3][4]' Systematic studies of the chemical changes at Ah03/alloy interfaces as a function of oxidation time have in recent years been carried out for FeCrAI [5], Fe3AI and FeAI [6,7], where the alloys normally contain about 20 ppm of sulfur. Although sulfur was found to be the major segregant at these scale/alloy interfaces, the segregation behavior, in terms of rate and amount, varied significantly with different alloys and differed from surface segregation.…”

Impurity Effects on Alumina Scale Growth

“…The segregation of sulfur to growing oxide/metal interfaces has been studied using conventional Auger electron microscopy (AES) with a typical probe size of about 1µm 15,16,61,62 , and with field emission AES of probe sizes ~30nm 63 . The chemical composition of the alloy surface was examined after spalling the oxide scale in ultra high vacuum (UHV) by scratching an oxidized specimen surface 61,64 .…”

“…Other than sulfur, there were also excess carbon and chromium at the interface. The carbon segregated during cooling and the Cr co-segregated with sulfur 15,65 . This co-segregation caused excess sulfur to be present at the interface, giving rise to the high concentration that amounts to ~2 monolayers, where a monolayer is defined as one physical layer on the alloy surface.…”

“…Non-metallic impurities that are present in almost all commercial grade alloys may segregate to the scale/alloy interface during oxidation. This has been demonstrated with sulfur 15,16 whose presence is believed to deteriorate scale adherence 17,18 . However, it is not certain if the segregated sulfur will diffuse or be carried into the scale to exert any effect on scale growth.…”