KOVAR OXIDATION IN AIR

Kovar oxidation in air, which starts at about 580-degrees-C, follows a parabolic law between 640 and 780-degrees-C, with an apparent activat ion energy of E = 125 +/- 5 kJ mol-1. At the beginning, only iron cont ributes to the oxidation reaction, which leads to nickel and cobalt en richment of the inner interfacial zone of the, alloy. Later on, slight nickel and cobalt diffusion is observed in the oxide layer near the o xide-alloy interface. The oxide layer, mainly composed of Fe2O3 (outsi de) and Fe3O4 (inside), has a scattered porosity. Nickel and cobalt ox ides were not individually identified. The reaction mechanism, which i nvolves a diffusional limiting process, is complex and could not be cl arified. Globally, some analogies of behaviour with pure iron exist, b ut they are limited because of the lack of iron monoxide and the poros ity of the oxide layer.