Hot corrosion of carbon-alloyed Fe3Al-based iron aluminides

The oxidation and hot corrosion behavior of two Fe3Al-based iron aluminides, Fe-25Al and Fe-27.5Al-3.7C (at.%) have been studied at I 100, 1225 and 1330 K. Hot corrosion studies were conducted by coating the specimen surfaces with 2.5 +/- 0.2 mg cm(-2) of Na2SO4 prior to exposure in pure oxygen. The oxidation kinetics of the carbon-alloyed iron aluminide were generally slower than that of the binary alloy. Alumina was identified in the scale after oxidation of both the alloys. The rates of hot corrosion were generally higher than the rates of oxidation for both the alloys. The presence of alpha-Fe2O3 in addition to alumina was indicated by Xray diffraction analysis of the scales present on the surface of the samples after hot corrosion. Fourier transform infrared spectra from the spalled scales in hot corrosion divulged the presence of alpha-Al2O3, alpha-Fe2O3 and sulfate. Cross-sectional microscopy revealed that the scale-metal interfaces were pitted under hot corrosion conditions and the pits contained aluminum sulfide. Aluminum sulfide was also identified along the grain boundaries in the binary aluminide matrix below the scale-metal interface. The hot corrosion process has been explained based on sulfide formation and its subsequent oxidation. The lower rate of hot corrosion in the carbon-alloyed iron aluminide has been related to the blocking effect of carbides, present along the grain boundaries, for the penetrating sulfur. (C) 2002 Elsevier Science B.V. All rights reserved.