Oxidation of two ternary Cu-Ni-20 wt.% Cr alloys at 700-800 degrees C in 1atm O-2

Two ternary Cu-Ni-Cr alloys containing approximately 20 wt.% chromium, but with a different Cu and Ni content, have been oxidized in 1 atm of pure oxy gen at 700-800 degrees C The alloy containing about 60 wt.% nickel (Cu-60Ni -20Cr) was composed of a single solid-solution phase and formed external sc ales of chromium ocide with an outermost layer containing a mixture of copp er and nickel oxides. The alloy comprised of about 40 wt.%nickel (Cu-40Ni-2 OCr) contained a mixture of two metal phases and formed complex external sc ales, containing copper oxide and a nickel-chromium spinel plus a region wh ere islands of the metallic phase richer in chromium surrounded by a thin c hromia layer were mixed with oxidized islands rich in copper and nickel, pr oducing a situation out of equilibrium. With time, a very irregular and thi n but essentially continuous layer of chromia formed at the base of the mix ed internal region for this alloy, producing a gradual decrease of the corr osion rate down to very low values. The oxidation behavior of the two alloy s is interpreted in terms of their different microstructure. In particular, the fast initial oxidation of Cu-40Ni-20Cr, associated with the formation of large amounts of copper oxides, is attributed to restrictions in chromiu m diffusion in the alloy due to the simultaneous presence of two metal phas es.