The air oxidation of two Cu-Ni-Ag alloys at 600-700 degrees C

The oxidation of two Cu-Ni-Ag alloys in air has been studied at 600-700 deg reesC, Both alloys contain a mixture of a silver-rich phase (alpha) with a second phase mostly composed of a copper-nickel solid solution (beta). Thus , these alloys are ternary two-phase systems containing two reactive metals with different affinity for oxygen plus a noble metal. Under all condition s the two alloys form complex scales containing an outermost layer of coppe r oxides followed by an inner region composed of a mixture of Cu2O plus NiO With a number of silver metal particles. 4 region of internal oxidation of nickel of the classical type is only observed for Cu-15Ni-15Ag oxidized at 700 degreesC. For Cu-25Ni-25Ag, the particles of the beta phase in the inn ermost scale region are corroded only around their surface, leaving an unox idized core surrounded by NiO. A peculiar structure observed for Cu-15SNi-1 5Ag at 700 degreesC is the presence of CuO beneath a Cu2O layer, which is a ttributed to a decrease of the oxygen pressure for the Cu2O-CuO equilibrium due to preferential dissolution of NiO in CuO rather than in Cu2O. The oxi dation rate of each alloy under constant temperature decreases generally wi th time and is only approximately parabolic, showing some short periods of accelerated oxidation. For each alloy the rate increases with temperature, while at both temperatures Cu-15Ni-15Ag corrodes more rapidly than Cu-25Ni- 25Ag. The scaling rates of the two alloys are substantially similar to thos e for the oxidation of pure copper, in spite of the presence of silver meta l and of NiO in the scales. (C) 2001 Elsevier Science Ltd. All rights reser ved.