Influence of a TiO2 surface treatment on the growth and adhesion of alumina scales on FeCrAl alloys

Deposits of TiO2 on FeCrAl alloys were obtained by surface TiO2 slurry application or by immersion of samples in tetraisopropylorthotitanate (TIPT) solution followed by air dry which gave the thinnest coatings. Isothermal oxidation of treated samples showed strong modification compared to non-treated ones, particularly in the temperature range of 850-925 degrees C where parabolic rate constants rapidly decreased when alloys were TiO2 treated. SEM surface observation, X-ray diffraction and ruby fluorescence showed that the presence of TiO2 promoted the formation of alpha-Al2O3 whereas non-treated samples exhibited large amounts of transition aluminas. An interesting effect of the rapid change from metastable to stable a-alumina was a strong increase of scale adhesion, determined by tensile testing, from 300-400 to 2000 J/m(2) for scales grown at 850 degrees C on Aluchrom YHfAl. This was explained not only by the change from outward to mainly inward growth but also by the volume reduction at the transition to alpha transformation.