Oxide dispersion strengthened FeCrAl alloys are of great interest for fossil energy applications because of their superior oxidation and creep resistance at very high temperature (1200ºC). The durability of these alloys in air is known to depend on two critical parameters: the total Al reservoir and the rate at which Al is consumed because of oxidation. Two ODS FeCrAl alloys have been thermally cycled at T > 1100ºC in industrially relevant atmospheres rich in H2O and CO2. The effect of the environment on the onset of breakaway oxidation was assessed, and lifetime models were adjusted according to each specific atmosphere.
The chemical compositions of MA956 (UNS S67956) and PM2000 (Werstoffe Number WN 1.4768) alloys, provided by Special Metals Corp. (Huntington, West Virginia) and Metalwerk Plansee (Reutte, Austria) respectively are given in Table 1. Rectangular 18X10 mm2 specimens with thicknesses ranging from 0.45 to 2 mm were machined, and the specimen surfaces were polished down to a 0.3 µm surface finish, and ultrasonically cleaned in acetone and methanol. Each 1h oxidation cycle consisted of 1h at 1200°C in a specific atmosphere, followed by 10 min cooling at room temperature in laboratory air. The specimens were cycled until failure in three different flowing environments: dry O2, air+10vol.%H2O and 50vol.%CO2 (containing 1500ppm O2 as a buffer) + 50vol.%H2O. The mass gains of the specimens were measured using a Mettler model XP205 balance. After exposure, the surfaces of the specimens were observed using optical and scanning electron microscopy (SEM), as well as x-ray diffraction (XRD). Electron probe microanalysis (EPMA) was used to determine the remaining Al content in the specimens after failure. EPMA measurements were carried out on a JEOL 8200, and at least 3 Al line profiles per specimen were generated. Details of the line profile measurements by EPMA can be found elsewhere.6
Cyclic oxidation of UNS S67956 Specimen mass gain versus number of cycles for UNS S67956 coupons oxidized in O2, air+10%H2O and the mixture of 50%H2O-50%CO2 are presented in Figure 1. As expected, the onset of breakaway oxidation, characterized by a fast increase in the mass due to the formation of Fe-rich oxides, is highly dependent on the specimen thickness. Except for the thinner specimens (0.5mm), the lifetime, for a given thickness, is lower in air+10%H2O compared to O2. However, the amount of mass gain was very similar in all three atmospheres before the onset of breakaway oxidation. Specimens exposed to the 50%H2O-50%CO2 mixture have undergone only 500 cycles to date, but the mass gain curves are thus far identical to the ones obtained in O2 and air+10%H2O. The thinnest specimen exposed in 50%H2O- 50%CO2 exhibited a similar lifetime as specimens of the same thickness exposed to O2 and air+10%H2O, but longer exposure of thicker specimens is required to more accurately assess the effect of CO2 on the UNS S67956 oxidation behavior.