Corrosion Behavior of Fe–25Cr in a N2–0.1% SO2 Atmosphere at 973 K Under Mechanical Loading

The corrosion behavior of an Fe-25Cr alloy was investigated in a N₂-0.1 vol.% SO₂ atmosphere at 973 K with and without stress. Without stress, the surface scale formed at the very initial stage consisting of Cr oxides and sulfides, which later changed to a Cr₂O₃-rich scale with the initially formed sulfides remaining. Under relatively low constant stress, +15 and +20 MPa, as well as cyclic stress of ±30 or ±40 MPa, the total strain for 36 ks was less than 0.1% and there was little cracking of the external scale, which consisted of Cr₂O₃ and (Fe, Cr)₃O₄ with small amounts of sulfide, growing faster. The external scale here was poorly adhesive. Under high stress, +25 and +35 MPa, cracks formed in the external scale and both oxides and Fe sulfide grew rapidly through the cracks to form nodules. The nodules consisted of an FeS-rich core surrounded by Fe₃O₄. With increasing strain, the preformed Cr₂O₃-rich scale changed drastically to a multilayered scale with an alternating oxide layer-sulfide layer structure.