Fe-Mn-Al-C alloys:: a study of their corrosion behaviour in SO2 environments

The corrosion reaction of four Fe-Mn-Al alloys exposed to a cycling, dry-humid, SO2 (0.001% by volume) polluted atmosphere was studied. ICEMS, XPS, AES-SAM and transmission Mossbauer spectroscopy at different temperatures were employed to characterize the corrosion products. The analytical results indicate that (i) ferrihydrite is the main component of the rust; (ii) there is an abundant presence of Mn882+ and SO32-/SO42- on the top of the corrosion layer, the concentration of SO42- increasing with the number of cycles; and (iii) the magnetic hyperfine pattern exhibited by the series of low-temperature spectra of the rust is quite different from that observed in the rust formed under similar corrosive environments on iron and weathering steel. This latter finding is correlated with a slow rate of transformation of the Fe3+ species formed at the early stages of corrosion into alpha-FeOOH, the usual final product of this type of corrosion processes. The sulphate anions, abundant inside the electrolyte during the wet periods, could be incorporated to the ferrihydrite structure being responsible for the Mossbauer spectral pattern recorded from the corrosion products at low temperatures.