The influence of the oxide/metal interface composition on the adherence of oxide layers on metal substrates

Well adherent and corrosion-protecting oxide layers are of great importance for high-temperature alloys. The adherence of the oxide layers is affected by the morphology and the chemical composition of the oxide/metal interface. In this study scanning Auger microscopy (SAM) is used to investigate the oxide/alloy interface of oxidized FeCrAl alloys (undoped or doped with Ti, Ce and Y) after partly removing the oxide layer by in situ bending. The metal surface shows individual voids and rugged parts, indicating imprints of the removed oxide. For the undoped alloy poor adherence of the oxide layer is observed. Sulphur is strongly enriched at the surface of the voids. In several areas of the rough interface no sulphur was detected. On the Ti-containing alloys the oxide layer is again poorly adherent. Sulphur is strongly enriched at the surface of the voids and at a lower but constant level at the rough area. On Y- and Ce-containing alloys the oxide layer is well adherent and no sulphur enrichment is detectable beneath the scale. The poor adherence of the oxide layers on undoped and Ti-doped FeCrAl alloys is correlated to the presence of sulphur at the alloy surface. Sulphur enrichment is explained by sulphur segregation to the free alloy surface for the undoped alloy and additional sulphide formation for Ti-containing alloys. The positive effect of Y and Ce on the adherence of the oxide layer is explained by sulphide precipitation, preventing sulphur segregation to the free alloy surface.