Behavior of uncoated and coated pure Fe in metal dusting conditions

The behavior of uncoated and coated pure Fe exposed to an atmosphere of CH4 + H2 + residual O2 at 1073 K for various times was studied by thermogravimetric analysis. For the uncoated Fe, extensive coking and carburizing was preceded by the formation and reduction of Fe oxides that occurred in the first 5 h of exposure and produced a highly un-protective porous Fe-rich structure on the surface. The metastable Fe-O-C diagram for the present temperature indicates that Fe oxides may be stable until carbon accumulation due to CH4 decomposition builds up to a level high enough to start reduction of the oxides. Regardless of the Fe oxides that form, eventually wustite will be present during reduction, and its conversion to Fe involves a change in volume that results in the porous Fe observed, which is evidently non-protective. Thus, Fe-based materials exposed to carburizing-oxidizing atmospheres will be always potentially susceptible to this adverse effect. The presence of a Cr/Al oxide thin film coating applied by d.c. magnetron sputtering helped to decrease coking and carburization serving as a diffusion barrier for outward Fe and to avoid formation of the oxides, and for inward C in the coking and carburizing stage.