Surface oxide films, temperature effect

SiC and SisNa powders have been studied extensively by XPS or x-ray-in-duced AES [42,43,48-57]. Powders, whiskers, and platelets were treated at elevated temperatures, and surface analysis was carried out by measuring Si 2p XPS or Si KLL AES intensities for Si02 and SiC (or Si3N4). Subsequently, surface oxide film growth rates were measured and surface oxidation activation energies were calculated [42,43,48-51]. In some cases, the effects of the presence of yttria, a sintering aid, and boron, an impurity, were studied [43,48,49]. The effects on surface oxidation chemistry caused by different powder processing routes were also studied [50]. 

It is hardly surprising that the preparation of surfaces of plain specimens for stress-corrosion tests can sometimes exert a marked influence upon results. Heat treatments carried out on specimens after their preparation is otherwise completed can produce barely perceptible changes in surface composition, e.g. decarburisation of steels or dezincification of brasses, that promote quite dramatic changes in stress-corrosion resistance. Similarly, oxide films, especially if formed at high temperatures during heat treatment or working, may influence results, especially through their effects upon the corrosion potential. 

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

Adsorption inhibitors act by forming a film on the metal surface. The action of traditional oil-based red lead paint formulations presumably involves the formation of soaps and the precipitation of complex ferric salts that reinforce the oxide film. There has been substantial interest in recent years in development of replacements for lead-based and chromate-based inhibitor systems. Adsorption inhibitors based on pol3rmers have been of particular interest. In this volume, Johnson et al. and Eng and Ishida discuss inhibitors for copper 2-undecylimidazole is shown to be effective in acid media, where it suppresses the oxygen reduction reaction almost completely. Polyvlnyllmidazoles are shown to be effective oxidation inhibitors for copper at elevated temperatures. Also in this volume, Chen discusses the use of N-(hydroxyalkyl)acrylamide copolymers in conjunction with phosphate-orthophosphate inhibitor systems for cooling systems. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

---