1986 Volume 50 Issue 5 Pages 500-506
The oxidation behavior and mechanism of an Fe-30 mass%Cr-4 mass%Al alloy and similar alloys containing 0.06, 0.09, 0.19 and 0.38 mass%Hf were studied at temperatures between 1273 and 1573 K for times up to 79.2 ks in stagnant air by thermogravimetric method, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The oxide formed on these alloys was mainly α-Al2O3, though the inclusion of HfO2 particles in scales increased with an increase in hafnium content in the alloy. Oxidation kinetics and scale adherence were discussed in terms of the morphological details of the scale and the structural change in the scale-alloy interface. The addition of a small amount of hafnium lowered the oxidation rate, but further hafnium addition tended to increase the oxidation rate by providing rapid diffusion paths in the form of HfO2 stringers. The scale adherence, on the contrary, showed a reverse trend, i.e., the higher the hafnium content, the stronger the adherence of scale in the experimental addition range. Metallographic evidence suggested that the improved scale adherence was due principally to a pegging mechanism; hafnium promoted the growth of stringers of α-Al2O3 into the alloy.
