High temperature oxidation of Fe13Cr xAl alloys in [formula omitted] vapour mixtures

Abstract

High-temperature oxidation in air of Fe13Cr xAl alloys containing up to 4.5 Al has been studied in the temperature range 680–980°C. A primary aim was to study the oxidation as a function of the Al concentration in the alloys and the water vapour content (from 0.03 to 2.3 vol. % H 2O) in the oxidizing gas. The Fe13Cr alloy exhibits an initial protective behaviour due to formation of protective Cr 2O 3 and Cr 2O 3 Fe 2O 3 films. This protective stage is succeeded by breakaway oxidation due to depletion of chromium in the alloy beneath the oxide scale; double-layered, porous scales develop and chromium is internally oxidized. Under these conditions the oxidation rate increases significantly with increased water vapour content. Additions of aluminium modify the oxidation behaviour. At sufficiently high Al concentrations protective scales of AL 2O 3 (α-AL 2O 3) are found. The critical Al concentration necessary for selective Al oxidation increases as the temperature increases. Thus at 980°C 4Al is necessary while at 680°C 1Al provides excellent oxidation resistance. When continuous Al 2O 3 scales are formed the oxidation is not significantly dependent on the water vapour content in the air. At Al contents below the critical Al concentration, porous, multilayered scales are formed enriched with Al 2O 3 in the innermost layer. Under these conditions the oxidation rate increases significantly with water vapour content. The results strongly suggest that the increased rate of oxidation with water vapour content is due to gaseous transport across voids and pores with H 2O, H 2, and O 2 as carrier gases.