Oxidation‐Sulphidation of iron aluminides at high temperature

An investigation has been carried out into the oxidation‐sulphidation resistance of a series of novel iron aluminides, containing 5 wt% Cr, 0.2 wt% Zr, from 0 to 0.3 wt% Y and from 8 to 16 wt% Al, at 500°C to 700°C. The test environments were H 2 /H 2 O/H 2 S gas mixtures, giving oxygen and sulphur partial pressures in the ranges of 10 −24 to 10 −31 atm and 10 −9 to 10 −11 atm respectively. The effects of preoxidation on the degradation resistance have also been studied. For comparison, several standard Cr 2 O 3 ‐forming and Al 2 O 3 ‐forming alloys were included in the programme. In general, the Al 2 O 3 ‐forming intermetallics and alloys were much more resistant to degradation than the Cr 2 O 3 ‐forming alloys while, in all cases, preoxidation increased the time to the onset of breakaway‐type corrosion resulting from the initiation and growth of sulphides. In environments in which the Cr 2 O 3 ‐forming alloys developed thick sulphide scales, it was found that the iron aluminides developed and retained protective Al 2 O 3 ‐rich scales more effectively than the conventional Al 2 O 3 ‐forming alloys. At the relatively low temperatures of these tests, it was more difficult to establish the protective scale than at high temperatures, particularly on the alloys of relatively low aluminium concentration. Thus, an iron aluminide containing 16 wt% Al showed good degradation resistance under all conditions while an aluminide containing 8 wt% Al developed more extensive sulphides at 500°C that at 700°C. Overall, the resistance of the intermetallics to degradation increased with increasing aluminium concentration, although none of them showed excessive sulphidation in the 50 h test programmes.