Metallographic preparation and degradation of the tau-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 invest igated both during metallographic preparation and subsequent exposure to th e ambient environment. The primary phases formed were an outer layer of iro n sulfide (Fe1-xS) and an inner layer composed of tau -plates (FeAl2S4) and iron sulfide particles. No difficulties were found concerning the stabilit y of the iron sulfide phases, but it is known that the tau -phase is easily hydrolyzed by water. Therefore, standard metallographic procedures where w ater is used as a lubricant and/or cleansing solution during preparation co uld not be exercised. Using scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis, the effect of the use of v arious lubricants and/or cleansing solutions was examined in order to produ ce good quality, polished cross-sections of the corrosion scales. The best results were obtained using 200-proof dehydrated ethyl alcohol as the lubri cant and cleansing solution. It was also observed that post exposure of pol ished samples to the ambient environment degraded the microstructure with t ime. It is believed that moisture from the air reacted with the tau -phase, resulting in the evolution of hydrogen sulfide gas. (C) 2000 Elsevier Scie nce Inc. All rights reserved.