Oxidation of two three-phase Cu–30Ni–Cr alloys at 700–800 °C in 1 atm of pure oxygen

Abstract

The oxidation of two ternary Cu–Ni–Cr alloys containing approximately 30 at.% and 40 at.% Cr, but with a similar 30 at.% Ni content, was studied at 700–800 °C in 1 atm of pure oxygen. The two alloys are composed of a mixture of three phases, where the medium gray β phase with an intermediate chromium content forms the matrix and the lightest α phase with the largest copper and lowest chromium content forms large islands and has a rather large volume fraction, while dark γ phase richest in chromium forms isolated particles dispersed in the other two phases, moreover, the γ phase particles for the alloy containing 40 at.% Cr appear to have a precipitation of very small lighter particles. The kinetic curves for the two alloys deviate considerably from the parabolic rate law and are composed of a few of stages, where they show a large initial weight increase after which the rates decrease with time, until they become very small up to 24 h. Generally, the oxidation rates decrease by increasing the Cr content in the alloy under constant temperature and increase with temperature for a constant alloy composition. The alloy containing 30 at.% Cr produced an external scale mainly composed of a rather thick and discontinuous lighter layer of copper oxides. Beneath the external scale there is a dark and continuous layer of irregular chromia in thickness, which at some locations is followed by a thin dark layer of chromia directly in contact with the alloy. On the contrary, at other locations the scale forms deep protrusions into the alloy. The alloy containing 40 at.% Cr formed a lighter thick but discontinuous layer of copper oxides containing some nickel followed by an innermost region composed of a rather thick and continuous dark layer of regular and flat chromia. The formation of an external scale of chromia for the two alloys prevents alloys from being oxidized further.