THE OXIDATION OF COBALT‐CHROMIUM‐CARBON ALLOYS

1. Introduction The oxidation of cobalt‐chromium alloys has recently been reviewed in some detail by Wright. In the temperature range 950–1350°C (1740–2460°F), alloys containing up to 15% Cr oxidize faster than pure cobalt, forming double‐layered scales: the outer layer is virtually pure CoO and the inner one is composed of Co‐Cr spinel particles in a chromium‐doped CoO matrix. For oxidation in air or in oxygen, if the specimens are heated in the gas, a sharp minimum in the rate is observed at about 25 %Cr, with the development of a continuous protective CT2O3 scale. However, if the alloys are rapidly exposed to oxygen at pressures above 100 Torr, then only a shallow minimum in rate is observed at about 25 to 30 %Cr, and the higher‐chromium alloys form a two‐layer scale similar to that found on the dilute alloys. Above 35 %Cr, the scale is always a thin single layer of Cr2O3.