ADHERENCE BEHAVIOR OF OXIDE GROWN IN AIR AND SYNTHETIC EXHAUST-GAS ON FE-CR-AL ALLOYS CONTAINING STRONG SULFIDE-FORMING ELEMENTS - CA, MG, Y, CE, LA, TI, AND ZR

This work evaluated the adherence of oxide grown in air and synthetic exhaust gas on Fe-20Cr-5Al alloys containing strong sulfide-forming elements: Ca, Mg, Y, Ce, La, Ti, and Zr. Results support the theory that reactive elements provide good oxide adherence on alumina-forming materials primarily by tying up sulfur as stable sulfides; however, other influences on adherence were found Highly volatile elements, such as Ca and Mg, lose their sulfur-controlling ability by diffusing out of the matrix and into the growing oxide scale. Zirconium results in the growth of an extensive network of oxide pegs into the substrate which improves adherence. Titanium segregates to the alumina scale and acts as a sink for S in the matrix. In synthetic exhaust gas (N2 + CO2 + H2O), local oxide spalling was observed and was shown to be caused by H2O in the atmosphere. The added benefits of Ti and Zr, i.e., forming oxide pegs and sinks for S, improve adherence in this environment.