High Temperature Oxidation of Fe-20Cr-4Al Alloys with Small Amounts of Sulfur

High-temperature oxidation behavior of Fe-20Cr-4Al alloys with 3, 35, 53, 104 and 171 ppm of sulfur was studied for 18.0 ks in oxygen at 1273, 1373, 1473, 1573 and 1673 K by mass change measurements, observation of surface appearance of the alloys, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The mass changes of the alloys increased roughly with increasing oxidation temperature. However, the mass changes of the alloys with 35 and 53 ppm of sulfur at 1473 K and with 35, 53 and 104 ppm of sulfur at 1573 K showed negative values. This fact was in good agreement with spalling of the oxide on the alloys. The oxides on all the alloys were only α-Al2O3 at any oxidation temperature. Oxide morphology on the alloys with 35, 53, 104 and 171 ppm of sulfur changed from convoluted to planar with increasing oxidation temperature. On the other hand, the oxide on the alloy with 3 ppm of sulfur was planar at any oxidation temperature. This result suggested that spalling of the oxide on the alloys was related to the formation of convoluted oxide morphology. After oxidation at 1473 and 1573 K, the spalling of oxide on the alloys with 104 and 171 ppm of sulfur was decreased remarkably as compared with that on the alloys with 35 and 53 ppm of sulfur. The oxide adherence of these alloys with 104 and 171 ppm of sulfur may be attributed to the formation of Cr sulfide at the oxide/alloy interface.