Kinetics of Al2O3-Scale Growth by Oxidation and Dissolution in Molten Silicate

Abstract

During the high-temperature oxidation of an alloy, the growth of a protective oxide scale selectively consumes one of the alloying elements. This can be accelerated if a molten deposit is present and dissolves the scale. The kinetics of Al2O3-scale growth and dissolution in Na silicate were studied by considering the Al mass balance at the alloy/scale and scale/silicate interfaces. The analysis was applied to a single-phase γ-NiCoCrAlY alloy exposed to a SiO2–Na2SO4 deposit at 1100 °C, using an oxidation rate constant and effective Al diffusivity measured after deposit-free oxidation of the same alloy, and independent measurements of Al solubility and diffusivity in Na silicate. The simulated Al2O3-scale thickness and Al depletion profile matched well with the experimental data. The model was then used to assess Al2O3-scale failure on two alloys with particularly large Cr contents (27 and 35 at.%), which underwent internal oxidation when exposed to the SiO2–Na2SO4 deposit.