Water Vapour Effect on the Oxidation Mechanism of a Cobalt-Based Alloy at High Temperatures (800–1,100 °C)

A cobalt-based Phynox alloy was oxidized in the 800–1,100 °C temperature range. The alloy oxidation was consistent with a growth mechanism limited by the diffusion process in a growing Cr2O3 oxide scale. Water vapour enhanced the alloy oxidation rate and scale porosity. Thermal cycling tests at 900 and 1,000 °C showed that water vapour reduces the outer Mn1.5Cr1.5O4 subscale adherence, but the chromia scale adherence was not affected. These temperatures permited a rapid chromium supply from the substrate to form a continuous chromia scale. At 1,100 °C thermal cycling conditions led to scale spallation and chromium depletion in the alloy. In dry air, weight losses were recorded due to cobalt and molybdenum oxidation, giving CoCr2O4 and CoMoO4. In wet air, the initial porous chromia scale permited nickel and cobalt oxidation, leading to Ni5Co3O8 and CoCr2O4 formation and resulting in bad adherence during thermal cycling.