High-temperature oxidation of Fe-Cr-Al alloys: influence of Al content

ABSTRACT Four different alloys containing chromium and aluminum were oxidized at high temperature (800-1100°C). Three of them were cast alloys containing 20wo%Cr and x%Al (x = 2, 4 and 6). The last material was prepared from a Fe-17Cr alloy which was pack - aluminized. The oxidation kinetics was studied by thermogravimmetry. Analyses were performed by using X ray diffraction, SEM + EDS and glow discharge optical spectroscopy. Classical results were exhibited by the alloys containing respectively 4 and 6%A1. The parabolic law was obeyed during entire oxidation runs. A circonvoluted α-Al 2 O 3 scale is formed on the alloys. When the Al content is as low as 2%, the results are more complicated. Two different parts are observed on the kinetic curves. During the initial part (high oxidation rate) an external scale containing Cr and Fe oxides grows on the alloy. Alumina forms by precipitation at the metal- scale interface. When a continuous Al 2 O 3 barrier is obtained, the kinetics is modified; the oxidation rate is decreased and the parabolic law is obeyed again. Concerning the pack aluminized alloy, the analyses show that the surface Al content is very high (>50%). A unique parabolic law is never observed during the oxidation, whatever is the temperature. But linear short parts can be plotted one after another on the (δ/S) 2 vs t curves. The parabolic rate constants which can be then calculated are decreasing when the oxidation time is increasing. A limit value is attained after about 50 hours (depending on the temperature). The analyses show that only pure α-alumina is formed during the tests. These results can be explained if one consider that the scale, which contains initially numerous deffects like cracks, becomes more and more compact as the oxidation proceeds.