Kinetic Study of Thin Film Growth on Silver in Bromine Vapour at Low Temperatures (35°–70°c)

Abstract

The tarnishing of silver in bromine vapour in the temperature range 35°–70°c, at bromine pressures between 25·3–404 Nm⁻² (0·19–3·03 mm Hg)followed a cubic rate law. This has not been reported so far for this system. The mechanism proposed by Mott to account for a cubic growth law seems to apply to the present work. This may be because of the relatively small thickness of the film (≶ 1600Å) and the persistence of the electrical field throughout the layer. An activation energy of ∼37·6 kJ mole⁻¹ in the above temperature range supports the hypothesis that the creation of positive holes at the outer interface is the rate-determining factor. The direct proportionality between the rate constant and the partial pressure of the bromine suggests that a steady state condition is not attained at the phase boundaries because of the thinness of the bromide film.