Abstract
The oxidation of two ternary Cu-Ni-Cr alloys containing approximately 30 and 40 at.% Cr, but with a similar Ni content, Cu-20Ni-30Cr and Cu-20Ni-40Cr, was studied at 973-1073 K in H-2-CO2 mixtures providing an oxygen pressure of 10(-19) atm, below the dissociation pressure of nickel and copper oxides. Both alloys are composed of a mixture of three phases, where the phase with the largest copper and lowest chromium content (alpha) forms the matrix and contains a dispersion of particles of the other two phases, one with an intermediate content of Ni and Cr (beta) and another richest in chromium (gamma). The oxidation kinetics of the two alloys at both temperatures exhibited a transition from a first stage, which was nearly parabolic except for Cu-20Ni-40Cr at 1073 K, to a second parabolic stage with a reduced rate constant. Both alloys formed a continuous external chromia scale after 24h oxidation at the two temperatures. The internal oxidation of chromium was essentially absent from the two alloys. The oxidation mechanism is discussed by taking into account the possible effects of the presence of three phases in the oxidation of ternary alloys.