Oxide films at a metal surface

Deoxidation of Al by dissociation of AI2O3 according to reaction (6.24) needs extremely low values of P02 in the furnace (for instance 10-43 Pa at the melting temperature of Al and 10 35 Pa at 900°C). Such low P02 values are not easily achieved in a furnace in which P02 is usually in the range 10-,5-10-5 Pa. Therefore, deoxidation of Al observed under high vacuum does not occur by dissociation of A1203 and has been attributed to the reduction of the oxide film by liquid Al to form volatile A120 by the reaction (Laurent et al. 1988, Castello et al. 1994)  

Qm is the molar surface area of the oxide and the flow of oxygen leaving the drop surface, fl 0, is given by  

When experiments are performed in a static neutral gas atmosphere, deoxidation depends on diffusion of volatile species into the gas. For the same temperature and P02 values, deoxidation takes longer in such atmospheres than in a high vacuum but some acceleration can be achieved by using dynamic conditions i.e., gas flow (Ricci et al. 1994). 

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