The air oxidation of two-phase Co-Cu alloys at 600-800 degrees C

The oxidation of three two-phase cobalt-copper alloys containing approximat ely 25, 50 and 75 wt% Cu has been studied at 600-800 degrees C in air. The scaling kinetics follow only approximately the parabolic rate law, with rat e constants increasing with the copper content under constant temperature. All the alloys form composite scales containing complex mixtures of copper and cobalt oxides, where copper concentrates preferentially in the external regions. Beneath the external oxide scales, there is, in most cases, a reg ion where cobalt oxide is mixed with copper metal: for alloys containing up to 50 wt% copper this zone corresponds to internal oxidation of cobalt, wh ile, for the alloy containing 25 wt% copper, it contains a dispersion of is olated copper particles in a cobalt oxide matrix. At variance with the beha vior of solid-solution binary alloys, cobalt is not depeleted in the alloy beneath this region in spite of its preferential local oxidation. The pecul iar structure of the scales formed on these alloys is interpreted by consid ering their two-phase nature. (C) 2000 Elsevier Science Ltd. All rights res erved.