Kinetics and Mechanism of Thin Chloride Film Growth on Silver and Silver–cadmium alloys at low temperatures (35°–70°C)

The tarnishing of silver and silver–cadmium alloys in dry and humid chlorine atmospheres in the pressure range 4·7–186·7 Nm⁻² and in the temperature range 35°–70°c has been studied with reaction times not more than 60 minutes, in order to limit the study to the thinner AgCl film range (>400 Å in dry Cl₂). The results below 60°c conform to a cubic growth law; those above 60°c show parabolic growth kinetics. An activation energy of 63·0 ± 8·0 kJ mole⁻¹ in the cubic range, and the fairly good agreement between the experimental cubic rate constant and that calculated using Mott's cubic equation, suggest that Motta's mechanism for a cubic growth law may be applicable to the present work as was found for the Ag/AgBr/Br₂ system in the thinner film range, the creation of positive holes at the outer interface being the rate-determining factor. The parabo lie law has been explained on the basis of Wagner's hole-diffusion-controlled process, with probably some contribution from Mott's field-induced migration. The presence of moisture changes the growth from a cubic to a parabolic type even at lower temperatures and increases the rate of chlorination in the parabolic range. The effect of cadmium on the reduction in rate in the parabolic range was appreciable, but the surface-defect-controlled cubic rate was unaffected. Cold deformation of silver caused a reduction in rate compared with that of annealed silver. The pressure dependence of the rate constant at chlorine pressures below 9·3 Nm⁻² was not in accordance with that expected under equilibrium attainment at the phase boundaries.