Corrosion behavior of weldable Fe-Al alloys in oxidizing-sulfidizing environments

The objective of the present study was to investigate the corrosion behavior of weldable Fe-Al alloys in environments representative of low NO x gas compositions, i.e., high partial pressures of sulfur [p(S 2 )] and low partial pressures of oxygen [p(O 2 )]. Using thermogravimetric techniques, binary alloys with 0-12.5 wt% Al were exposed in oxidizing-sulfidizing environments [p(S 2 ) = 10 -4 atm and p(O 2 ) = 10 -25 atm] at 500-700°C for various times up to 100 h. Post-exposure characterization consisted of surface and cross-sectional microscopy in combination with energy dispersive spectroscopy and/or electron probe microanalysis. It was found that the Fe-Al alloys exhibited three different stages of corrosion behavior: inhibition, breakdown, and steady-state. Observance and/or duration of these stages was directly related to the aluminum content of the alloy. The inhibition stage was characterized by growth of a thin, gamma alumina scale that suppressed rapid degradation of the underlying substrate for alloys with greater than 7.5 wt% Al. During the breakdown stage, mechanical failure of the initially formed alumina scale, and the inability to re-establish itself, resulted in the growth of nodular sulfide products due to short circuit diffusion of sulfur and iron through the passive layer. This typically occurred on alloys with 7.5 wt% Al. The final stage (steady-state) found the diffusional growth of thick sulfide scales on alloys with less than 7.5 wt% Al that led to relatively high weight gains. Overall, the results from this study indicate that weldable Fe-Al compositions, approaching 10 wt% Al, have excellent corrosion resistance in aggressive low NO x gas compositions at service temperatures below 600°C. With the potential promise for applications requiring a combination of weldability and corrosion resistance in moderately reducing environments, these alloys are viable candidates for further evaluation for use as sulfidation resistant weld overlay coatings.