| Popis: |
Metals, in general, critically depend on their surface oxide scales for environmental stability, particularly in aggressive oxidising atmospheres at high temperatures. The protective capabilities of oxides are dependent on many physical and chemical properties, as well as on their mechanical adherence to the metal surface. In summary, an “ideal” protective oxide would be: physically and chemically stable. An ideal oxide would not dissociate nor melt at the temperatures and pressures of interest; mechanically stable. The scale would be capable of maintaining intimate contact with the surface of the metal, particularly when sudden temperature changes occur; a barrier to diffusion. The function of a protective oxide is to separate the metal from the oxygen in the gas phase. The ideal oxide would, therefore, have a low diffusion rate for both oxygen and metal ions, otherwise the oxidation reaction would proceed at the oxide/metal or oxide/gas interfaces respectively; continuous and dense. When pores or cracks are present in the oxide scale the protective capabilities of the oxide are lost. |
