Abstract
Concentration profiles of nickel in the (Co, Ni)O scales grown on the Co-low Ni alloys (0.45, 2.46, and 7.23 wt%Ni) over the temperature range from 1000 to 1300°C were determined by means of EPMA and then analyzed according to the analytical equation of Wagner and the differential equations of Dalvi et al.
By making use of both computation methods, the concentration profiles similar to the observed ones were obtained, but the agreement between them was found to be insufficient in the inner part of the scale. The values of P(=DNi⁄DCo) and n in PO21⁄n determined by the present analytical method were within the range of the literature values. These values of P and n depended on all other parameters, in particular, strongly on ξ′(NiO content at the alloy-scale interface) and k′(=k⁄DCo0), respectively. In contrast with the presentation by Dalvi et al., both P and n values changed with NiO content in the oxide.
The mass balance equations at both scale interfaces were used to judge the validity of the numerically obtained solutions. The resulting solutions did not satisfy the mass balance equation at the scale-gas interface. Therefore, it was concluded that the perfect solutions could not be obtained by using the equations of Dalvi et al. This result may be ascribed to the change in the P and n values with NiO content in the oxide.
On the other hand, the equation of Wagner is suitable for an approximate estimation of the cation distribution and diffusivities in the oxide scales because of its simplified form.
