Oxidation of Co-25 w/o Cr at High Temperatures

Studies of the oxidation behavior of Co‐25 w/o Cr at temperatures between 900° and 1300°C and oxygen pressures in the range 0.2–760 Torr are reported and analyzed. The oxidation behavior of this alloy is complex and does not follow a single mechanism. The parabolic rate constant at high oxygen pressures in the temperature range of 900°–1300°C is about two orders of magnitude larger than that at low oxygen pressures . The scales formed at high oxygen pressures have a duplex structure, the outer layer being virtually pure as in Co‐10 w/o Cr (1,2), and the inner layer being mainly the spinel with small amounts of and ( is concentrated near the alloy‐scale interface). At low oxygen pressures, the morphology of the scale is dependent on the radius of curvature of the specimen; at the edges where this radius is small, the structure is similar to the structure developed at high oxygen pressures; on the large surfaces, is the dominant oxide. The mechanism of parabolic oxidation of Co‐25 w/o Cr at high oxygen pressures involves cobalt‐ion diffusion through a network in the inner layer. Initial oxidation under these conditions is nonparabolic and this is due to selective oxidation of chromium to which takes place when the specimens are exposed to the "vacuum" of the reaction furnace prior to the start of the actual oxidation run. The transition from the high‐ to low‐pressure oxidation is due to a transition to selective oxidation of chromium. The oxidation is then governed by diffusion of the reacting ions (probably chromium) through the scales.