The effect of ceria coatings on the high-temperature oxidation of iron-chromium alloys

The effect of sputtered ceria coatings (0.2–30 nm thick) on the oxidation of Fe25Cr alloys at 1100 °C has been studied using the 18O/secondary ion mass spectrometry technique. Samples were oxidized either in 16O 2 only or sequentially first in 16O 2 and then in 18O 2 for periods up to 20 h. Oxidation rates were reduced with increasing ceria thickness up to 4 nm; there was no significant further reduction in rate for thicknesses of 4 nm or more. The secondary ion mass spectrometry sputter profiles indicated that scales were mainly Cr 2O 3; iron was present in the outer part of the oxide layer at levels of 1% or less. With increasing ceria coating thickness the maximum in the cerium sputter profile moved from the vicinity of the alloy-oxide interface towards the gas-oxide interface. When Fe25Cr was sequentially oxidized, 18O was found to have diffused towards the alloy-oxide interface. The secondary ion mass spectrometry data suggest that when ceria is located within the scale there has been a change in mechanism from predominantly cation to predominantly anion diffusion.