The oxidation of two-phase Cu-Cr alloys under 10-19 atm O2 at 700-900°C

The oxidation of three Cu-Cr alloys containing 25, 50, and 75 wt.% Cr and having a two-phase microstructure has been studied at 700-900 degrees C in H-2-CO2 mixtures under 10(-19) atm O-2, i.e., below the stability of the copper oxides. At variance with the results obtained by oxidizing the same materials in air, the alloys corroded quite slowly and formed only external chromium-oxide scales rather than showing internal oxidation of chromium. In view of the quite small solubility of chromium in copper, lower than the critical value calculated for the transition from the internal to the external oxidation of chromium, this result is attributed mainly to a supply of chromium from the small Cr-rich particles present in the two-phase eutectic mixture, The corrosion kinetics followed the parabolic rate law to a reasonable approximation, but the rate constants changed rather irregularly as functions of temperature and alloy composition, The scaling rates were generally larger than those measured for pure chromium under the same oxygen pressure, but much smaller than those of the same alloys in air. These results are examined with special reference to the two-phase nature of the alloys.