The air oxidation of two Cu–Ni–Ag alloys at 600–700°C

Abstract The oxidation of two Cu–Ni–Ag alloys in air has been studied at 600–700°C. Both alloys contain a mixture of a silver-rich phase (α) with a second phase mostly composed of a copper–nickel solid solution (β). Thus, these alloys are ternary two-phase systems containing two reactive metals with different affinity for oxygen plus a noble metal. Under all conditions the two alloys form complex scales containing an outermost layer of copper oxides followed by an inner region composed of a mixture of Cu 2 O plus NiO with a number of silver metal particles. A region of internal oxidation of nickel of the classical type is only observed for Cu–15Ni–15Ag oxidized at 700°C. For Cu–25Ni–25Ag, the particles of the β phase in the innermost scale region are corroded only around their surface, leaving an unoxidized core surrounded by NiO. A peculiar structure observed for Cu–15Ni–15Ag at 700°C is the presence of CuO beneath a Cu 2 O layer, which is attributed to a decrease of the oxygen pressure for the Cu 2 O–CuO equilibrium due to preferential dissolution of NiO in CuO rather than in Cu 2 O. The oxidation rate of each alloy under constant temperature decreases generally with 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 those for the oxidation of pure copper, in spite of the presence of silver metal and of NiO in the scales.