Shalva Gedevanishvili

Isothermal oxidation behavior of a sheet alloy of Fe–40at.%Al at temperatures between 1073 and 1473 K

A sheet Fe–40at.%Al alloy with 200mm thickness was oxidized isothermally in air at temperatures between 1073 and 1473 Kfor up to 608.4 ks. The oxidation behavior was classified into three temperature ranges. At temperatures below 1223 Ka mixture of a- and y-Al2O3 grew fast, followed by slow oxidation with the formation of only a-Al2O3. The initial rapid oxidation is due to the formation of y-Al2O3. Between 1273 and 1323 Kthe oxidation obeyed a parabolic rate law over the whole oxidation time with the formation of a-Al2O3 scale, except for the very initial stage of oxidation up to 3.6 ks. At temperatures above 1423 Kthe parabolic rate constants tended to decrease with oxidation time, while the oxide grain size of the columnar structure increased by 3–4 times. The change in the parabolic rate constants with time at 1473 Kwas reasonably well explained using the oxygen inward diffusion model along the grain boundary with a hypothetical width of 2 nm. The Fe–40Al showed the lowest oxidation rate and the activation energies for a parabolic Al2O3 growth are 422 kJ/mol above 1273 Kand 241 kJ/mol below 1223 K . Oxide scale exfoliation was not observed for the Fe–40Al oxidized isothermally in air at the experimental temperatures here. # 2003 Elsevier Science Ltd. All rights reserved.

Corrosion behavior of Fe-40Al sheet in N2–11.2O2–7.5CO2 atmospheres with various SO2 contents at 1273 K

A 200 mum thick Fe-40 at.%Al sheet, produced by powder metallurgy and hot extruding, was corroded at 1273 K for up to 360 ks in N-2-11.2 vol.%O-2-7.4 voI-%CO2 atmospheres containing 100, 500, or 2000 ppmSO(2). The alloy oxidized rapidly at the very initial stage of corrosion, forming sulfides in addition to both theta- and alpha-Al2O3 scales, it was suggested that sutfides may be formed by direct reaction Of SO2 gas molecules with the alloy. Sulfide or dissociated sulfur at the alloy-scale interface may promote exfoliation of the external scale. With further oxidation, the oxide scale consisted solely of alpha-Al2O3, and grows obeying a parabolic rate law. The parabolic rate constants are between 5.5 x 10(-12) and 7.2 x 10(-12) kg(2) m(-4) s(-1), independent Of SO2 concentration. After 360 ks of corrosion the Al content in the alloy sheet remained at about 38 at. %Al, with an almost flat profile across the entire sheet

Effect of pre-oxidation on the corrosion behavior of Fe–40Al sheet in a N2–11.2O2–7.5CO2–500 ppm SO2 atmosphere at 1273 K

A 200 mum thick Fe-40Al sheet, produced by powder metallurgy and hot extruding, was pie-oxidized at 1373 K in air for different times and then the pre-oxidized specimens were corroded at 1273 K for up to 360 ks in a N-2-11.2 vol.% O-2-7.4 vol.%CO2-500 ppm SO2 atmosphere. The pre-oxidation followed a parabolic rate law, forming continuous, adherent alpha-Al2O3 oxide scales, accompanied by the formation of valley-like flaws. The valley-like flaws originate from the inclusion of additives in the alloy substrate. The inclusion of additives as alpha-Al2O3 in the Fe-40Al sheet leads to some local exfoliation of the oxide scale in the SO2 containing atmosphere. The Fe-40Al pre-oxidized for longer than 14.4 ks showed very good corrosion resistance in the N-2-11.2 vol.% O-2-7.4 vol.% CO2-500 ppm SO2 atmosphere, and there is little exfoliation of the oxide scale after 360 ks oxidation at 1273 K