The air oxidation of two-phase Cu-Cr alloys at 700-900°C

The oxidation in air of three two-phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700-900 degrees C. The alloys corroded nearly parabolically, except at 900 degrees C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free fr om chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu2Cr2O4, often containing particles of chromium metal surrounded by chromia and then by the double oxide, Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.