The reactive element effect on thermally grown chromia scale residual stress

Uncoated and Nd2O3-coated Fe–30wt.% Cr alloys were oxidized at 1000 °C in air under atmospheric pressure. The oxidation rate was decreased by one order of magnitude when a Nd2O3 film was applied to the alloy surface before the oxidation test. X-ray diffraction (XRD) was used to determine residual stresses in chromia scales grown on both uncoated and coated samples. Stresses relaxed by chromia scale spallation from the uncoated alloys. For Nd2O3-coated samples, even if stresses were released, no scale spallation was observed. In that case, stresses were supposed to relax by the creep of the chromia scale, owing to the modification of the chromia grain morphology. Indeed, large columnar chromia grains grown on uncoated alloys induced oxide scale failure during cooling to room temperature, whereas small equiaxed chromia grains developed on Nd2O3-coated alloys, led to stress relaxation by creep of the scale without cracking. It is assumed that this modification of the chromia-scale plasticity is the main factor to improve the spallation resistance of alloys under thermal shocks.