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

The objective of the present study was to investigate the corrosion behavio r of weldable Fe-Al alloys in environments representative of low NOx gas co mpositions, i.e., high partial pressures of sulfur [p(S-2)] and low partial pressures of oxygen [p(O-2)]. Using thermogravimetric techniques, binary a lloys 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 degrees C for va rious times up to 100 h. Post-exposure characterization consisted of surfac e and cross-sectional microscopy in combination with energy dispersive spec troscopy and/or electron probe microanalysis. It was found that the Fe-Al a lloys exhibited three different stages of corrosion behavior: inhibition, b reakdown, 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 initiall y 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 allo ys with 7.5 wt% Al. The final stage (steady-state) found the diffusional gr owth of thick sulfide scales on alloys with less than 7.5 wt% Al that led t o relatively high weight gains. Overall, the results from this study indica te that weldable Fe-Al compositions, approaching 10 wt% Al, have excellent corrosion resistance in aggressive low NOx gas compositions at service temp eratures below 600 degrees C. With the potential promise for applications r equiring a combination of weldability and corrosion resistance in moderatel y reducing environments, these alloys are viable candidates for further eva luation for use as sulfidation resistant weld overlay coatings.