The Oxidation of Two-Phase Cu-Cr Alloys Under 10-19 atm O2 at 700-900°C

Abstract

The oxidation of three Cu-Cr alloys containing25, 50, and 75 wt.% Cr and having a two-phasemicrostructure has been studied at 700-900°C inH2-CO2 mixtures under10-19 atm O2, i.e., below thestability of the copper oxides. At variance with theresults obtained by oxidizing the same materials in air,the alloys corroded quite slowly and formed onlyexternal chromium-oxide scales rather than showinginternal oxidation of chromium. In view of the quitesmall solubility of chromium in copper, lower than thecritical value calculated for the transition from theinternal to the external oxidation of chromium, this result is attributed mainly to a supply ofchromium from the small Cr-rich particles present in thetwo-phase eutectic mixture. The corrosion kineticsfollowed the parabolic rate law to a reasonableapproximation, but the rate constants changed ratherirregularly as functions of temperature and alloycomposition. The scaling rates were generally largerthan those measured for pure chromium under the sameoxygen pressure, but much smaller than those of the same alloysin air. These results are examined with specialreference to the two-phase nature of thealloys.