Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 245collector, the rest lose their charge by collision with the rods. A hot filament anda Penning type source have both been used as a means of producing ions. Theinstrument operates over the pressure range 5 x torr to lo-'' torr. Resolutionis practically constant over the mass range 1-200 and adjacent mass numberscan be resolved.2.3 NON CATALYTIC REACTIONS2.3.1 Oxidation of metalsThe first stage in the interaction of oxygen with a metal must be chemisorptionof oxygen. Since the standard free energy of all metals (except gold) is negative,the chemisorbed layer should be formed very rapidly. A study of the initial stagestherefore normally requires ultra high vacuum conditions to eliminate contaminantsand to allow a sufficiently low partial pressure of oxygen to be used. The formationof the chemisorbed layer can be followed by the methods described in Section 2.1.Diffusion of oxygen and/or metal atoms near the surface then occurs and theinitial oxide layer is formed. The further growth of this layer involves two steps:(a) reaction at the metal/oxide and oxide/oxygen interfaces, and (b) transportof material through the oxide layer. As the oxide layer gets thicker, the rate ofgrowth is controlled by (b) which becomes the slower of the two steps. If the volumeof the oxide produced is less than that of the parent metal, the simple rate law foran unimpeded reaction is observedwhere y is the film thickness at time t. The oxidation of light metals, sodium,calcium etc. follows the above equation. With most metals, the volume of oxideis greater than that of the metal and a protective layer is formed, resulting in alogarithmic or parabolic rate law as indicated belowTheories of oxidation have been developed by Wagner278s279 and by Mott280*28'.In general the logarithmic rate law applies to very thin oxide layers which formprotective coatings and the parabolic rate law to thick oxide layers. More recentreviews of the subject have been given by GrimleyzEZ, Kubaschewski and Hopand by Wyn RobertszE4.References pp. 270-278