The annealing and re-oxidation of oxidized Cu [sbnd]Ni alloys

Abstract

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu 10%Ni and Cu 24%Ni at 800°C and Cu 80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques. Oxidation of Cu 10%Ni and Cu 24%Ni produces external scales consisting of a thick, inner Cu 2O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu 80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu 2O and oxygen. With Cu 80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface. In Cu 10%Ni and Cu 24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu 2O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu 80%Ni, although to a lesser extent than for the copper-rich alloys. During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu 80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.