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

Abstract

AbstractThe 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 A 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...