Effect of Cycle Frequency on High-Temperature Oxidation Behavior of Alumina-Forming Alloys

Cycle frequency affects both high-temperature oxidation behavior and the method in which the cyclic test is conducted. Several issues are discussed using examples taken from results for Ni-base and Fe-base, alumina-forming alloys. For alloys that form adherent scales, cycle frequency has little effect on results over extended test times ( ≥500 hr). When an alloy forms a less adherent scale, reducing the cycle time often has the expected effect of increasing the mass loss per unit exposure time; however, the opposite effect is observed in other cases. Low-frequency cycle experiments can be conducted with specimens contained in alumina crucibles. This has the important benefit of collecting the spalled oxide and measuring the “total” mass gain, equivalent to the metal wastage. However, higher-frequency-cyclic tests cannot be performed with crucibles because of the large thermal mass and thermal-shock problems of alumina crucibles. The test method and cycle frequency ultimately have a strong effect on lifetime predictions.