1998 Volume 47 Issue 1 Pages 42-48
Cyclic oxidation behavior of Fe-20Cr-4Al alloys with 3, 35, 53, 104 and 171ppm of sulfur was studied in oxygen at 1473K. Cyclic oxidation consisted of a 18.0ks exposure and was performed by cycling between 1473K and room temperature. This oxidation-cooling cycle was repeated up to five times. Mass changes of the alloys with 3, 104 and 171ppm of sulfur increased roughly with increasing cycle and showed positive values after each cyclic oxidation. However, mass changes of the alloys with 35 and 53ppm of sulfur showed negative values after each cyclic oxidation, and was consistent with spallation of the oxide. Amounts of spalled oxide of the alloys with 35 and 53ppm of sulfur increased after 1 and 2 oxidation cycles, and then decreased after 3 or more oxidation cycles. Observation by a scanning electron microscope of both sides of the spalled oxide indicated that spallation maybe related to morphology of the surface oxide and to the morphology and volume of cavities present in the oxide/alloy interface. Many particles of chromium sulfide were observed at the oxide/alloy interface of the alloys with 104 and 171ppm of sulfur after 5 oxidation cycles. It is postulated that the existence of many particles of chromium sulfide probably plays a role in improvement of the oxide adherence.
