Influence of yttrium on chromia scale growth on Ni-Cr alloys

The Cr2O3 scale formed on Ni-Cr alloys is generally regarded as growing by nearly exclusive cation diffusion. The scale exhibits two regions defined by their predominant defects: an inner region near the oxide/alloy interface where chromium interstitials predominate; and an outer region near the oxide/gas interface where chromium vacancies are predominant. The transport of these defects determines the oxidation mechanism and the scale growth rate. Since the total chemical potential gradient is a function of the stress gradient as well as the oxygen potential gradient, the oxidation stresses and distributions of stresses will affect the defect structure type and relative importance of anion transport as compared to cation transport. It is proposed that the total chemical potential gradient across the Cr2O3 scale in Ni-30Cr-0.5Y alloy establishes a steeper oxygen vacancy gradient and a less steep chromium interstitial gradient through the oxide scale as compared with that of the Ni-30Cr alloy, thereby enhancing oxygen anion inward diffusion along grain boundaries and reducing chromium cation outward diffusion. Therefore, the oxidation mechanism in the yttrium-containing alloy is no longer exclusive cation diffusion but includes a small, but important oxygen flux. It is also suggested that the enhanced oxygen inward diffusion is an important factor in the growth instability of the oxide/alloy interface commonly observed in yttrium-doped alloys. The convoluted metal/scale interface is often cited as an important factor in improving the scale's resistance to fracture and spallation.