NiO films have been grown on high-purity Ni and Ni 0.1 weight percent (w/o) Cr alloy, at 700~ and 0.21 bar, by sequential oxidation in 1602 and 1802. The distributions of oxygen, Ni, and Cr isotopes in the films were analyzed using SIMS. For both materials, the rate controlling transport process was found to be the outward diffusion of Ni through the NiO. Inward oxygen gas transport also occurs and is responsible for the growth of new oxide within the films and for the growth of the inner layer of the well-developed duplex films which grow on the alloy. A mechanism whereby oxygen gas penetrates the films through fissures to form new oxide in available space is proposed to explain the observations and the growth of duplex films.The protective properties of oxide films formed by oxidation of metals depend on the transport of reactants (metal and oxygen) through the film and the structure of the film (including the microstructure). In general, the transport properties and structure of the oxide film are inter-related. In some cases, this relationship is relatively straightforward; for example, the oxidation of high-purity Co or Ni at high temperatures, when the classical singlelayer, single-phase compact oxide film grows by outward diffusion of metal ions through the oxide lattice. At the other extreme, the relationship is extremely complicated; for example, the oxidation of a multicomponent alloy in mixed gases at intermediate temperatures. The complicating factors include multiphase, multilayered, doped oxides containing voids, dislocations, and grain boundaries as additional potential routes for transport through the oxide.Between these two extremes is an important class of oxide film microstructures known as duplex films (or scales). Duplex films are formed under a wide variety of conditions on a large number of metal substrates. They can form on high-purity metals as well as on alloys. On high-purity metals, the inner and outer layers of the duplex film are distinguishable by their different grain size. The outer layer usually has a columnar structure, and the inner layer has an equiaxed structure of much finer grain size. The thickness of the inner layer, as a fraction of the total layer thickness, appears to depend on the system and the experimental conditions. However, the inner layer thickness never exceeds the thickness of metal converted to oxide (i.e., when the inner-layer/outer-layer boundary coincides with the position of the original metal surface). When duplex films form on dilute alloys, the inner-layer/ outer-layer boundary usually coincides with the position of original alloy surface, and the alloying elements are preferentially located in the inner layer (provided that the alloying elements have low mobility in the oxide).The transport processes which occur during duplex film growth have been studied using isotopically labeled oxygen tracers in a number of systems. The results of these experiments and their interpretation have recently been reviewed by Atkinson (1). The experiments have shown tha...

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Dedicated to Professor Dr. Alfred Rahmel on the occasion of his 60th birthdayWe have measured the diffusion of aNi radiotracer into polycrystalline NiO. nominally doped with 0.1% Cr (Cr/Ni ratio) in the temperature range 600 to 900°C. The experiments show that Cr doping increases diffusion of Ni in the oxide lattice, but decreases diffusion of Ni along grain boundaries provided that the grain boundary CdNi ratio is sufficiently large (greater than about 1%). This is believed to be due to the formation of immobile Cr-vacancy pairs which block fast boundary diffusion.The oxidation rate of Ni 0.1% Cr alloy is, however, slightly faster than that of pure Ni at 700"C, at which temperature grain boundary diffusion should be dominant. This apparent discrepancy between oxidation rates and diffusion studies, it is argued, is due to the complicating effects of low Cr mobility, a duplex film structure and inward oxygen transport during oxidation of the alloy.Aufgrund von Messungen der Diffusion von 63Ni in polykristallinem NiO (nominell dotiert mit 0,1% Cr) im Temperaturbereich 600-900"C wird durch das Dotieren die Diffusion des Mittels im Oxidgitter beschleunigt, entlang der Korngrenzen hingegen verlangsamt, sofern das CrlNi-Verhaltnis an den Korngrenzen gro6er als etwa 1% ist. Dieses Verhalten wird der Bildung von unbeweglichen Cr-Leerstellenpaaren zugeschneben, welche die schnelle Korngrenzendiffusion blockieren. Die Oxidationsrate der Ni-0,l Cr-Legierung liegt jedoch bei 700°C etwas hoher als die von reinem Nickel, obwohl bei dieser Temperatur die Korngrenzendiffusion dominant sein sollte. Diese scheinbare Diskrepanz zwischen Oxidations-und Diffusionsgeschwindigkeiten 1iBt sich der Tatsache zuschreiben, da6 bei niedrigen Gehalten die Mobilitat des Chroms durch komplexe Faktoren beeinfluat wird, da6 eine Duplex-Schichtstruktur entsteht und wahrend der Oxidation der Legierung Sauerstoff nach innen transportiert wird.

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