The oxidation lifetime of commercial FeCrAl(RE) alloys

Reliable, lifetime data are a prerequisite for the development of any model aiming at predicting the onset of the ultimate high temperature corrosion degradation mode of FeCrAlRE alloys, namely chemical failure. Thus, the main data generation task of the LEAFA project has been to obtain lifetime data on the oxidation behaviour, in air, of four commercial alloys, Aluchrom YHf, Kanthal AF, Kanthal APM and PM2000. These alloys contained different reactive elements (RE), yttrium, as metal and in the ODS strengthened PM2000 as yttria, zirconium and hafnium either separately or in combination but their aluminium contents were similar to5 mass%. The alloys were manufactured by the main fabrication routes, conventional melting, powder metallurgy and mechanical alloying, and as a consequence had a range of mechanical properties, with the ODS alloy being the most creep resistant. The alloys, in the form either of foils (thickness 30-125 mum) or sheets sections (0.2-2.0 mm thick), were oxidised either isothermally or discontinuously, with thermal cycles of 20, 100 or 300 h duration, at 1100, 1200 and 1300 C. Detailed measurements were made of the scale growth rates, and also of the mechanical failure of scales, through determination of the critical thicknesses for the onset of spallation and of the continuing spall rates. Chemical failure occurred when the aluminium content of the alloy fell below a value [Al-crit], such that the protective alumina scale could no longer be formed/sustained. Breakaway oxidation then ensued leading to the formation of non-protective iron-chromium oxides. Values of both the time to breakaway (t(B/O)) and of [Al-crit] were determined. Critical. parameters affecting the respective oxidation behaviours of the alloys included the substrate alloy mechanical properties and thickness.