THE OXIDATION OF 2-PHASE CU-CR ALLOYS UNDER 10(-19) ATM O-2 AT 700-900-DEGREES-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 degr ees C in H-2-CO2 mixtures under 10(-19) atm O-2, i.e., below the stabi lity of the copper oxides. At variance with the results obtained by ox idizing the same materials in air, the alloys corroded quite slowly an d formed only external chromium-oxide scales rather than showing inter nal oxidation of chromium. In view of the quite small solubility of ch romium in copper, lower than the critical value calculated for the tra nsition 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 corrosio n kinetics followed the parabolic rate law to a reasonable approximati on, but the rate constants changed rather irregularly as functions of temperature and alloy composition, The scaling rates were generally la rger than those measured for pure chromium under the same oxygen press ure, but much smaller than those of the same alloys in air. These resu lts are examined with special reference to the two-phase nature of the alloys.