Metallographic preparation and degradation of the τ-phase (FeAl2S4) formed after high-temperature oxidation–sulfidation of Fe–Al alloys

The stability of corrosion products formed after high-temperature exposure of an Fe–5 wt.% Al alloy in an oxidizing–sulfidizing environment was investigated both during metallographic preparation and subsequent exposure to the ambient environment. The primary phases formed were an outer layer of iron sulfide (Fe1−xS) and an inner layer composed of τ-plates (FeAl2S4) and iron sulfide particles. No difficulties were found concerning the stability of the iron sulfide phases, but it is known that the τ-phase is easily hydrolyzed by water. Therefore, standard metallographic procedures where water is used as a lubricant and/or cleansing solution during preparation could not be exercised. Using scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis, the effect of the use of various lubricants and/or cleansing solutions was examined in order to produce good quality, polished cross-sections of the corrosion scales. The best results were obtained using 200-proof dehydrated ethyl alcohol as the lubricant and cleansing solution. It was also observed that post-exposure of polished samples to the ambient environment degraded the microstructure with time. It is believed that moisture from the air reacted with the τ-phase, resulting in the evolution of hydrogen sulfide gas.