ABSTRACT
The transport of oxygen in nickel oxide scales during the oxidation of high purity nickel has been studied using 18O tracer in combination with depth-profiling and transverse section imaging SIMS. Depth-profiling demonstrates that oxygen is transported to the scale-metal interface at all temperatures, the quantity of oxide formed at the interface increasing as the oxidation temperature is reduced. The relative volume of oxide formed at the scale-metal interface represents up to about 17% of the total oxide volume in the scale produced at 600°C.
Imaging of transverse sections through the scales formed at 600 and 700°C shows that oxygen is transported via a large number of fissures through the existing oxide. The fissures are healed by the growth of new oxide within the pre-existing scale. The distribution and spacing of the healed fissures suggests that they correspond to grain boundaries in the scale. The oxide grown at the scale-metal interface is laterally continuous for scales grown at 600°C but this is not so for higher oxidation temperatures.
Re-oxidation at either 700 or 900°C of scales formed at initial temperatures of 600–900°C shows that the oxygen transport is controlled by the initial oxidation temperature for the relatively short re-oxidation times studied here (total scale thickness less than about 3 μm). It is suggested that the enhancement of oxygen transport observed for the lower initial oxidation temperatures is due to the nature of the scale-metal interface formed at the different temperatures.
