Kinetic study of silver iodide film growth on silver at low temperatures (30-73° C)

Abstract

Studies on tarnishing of silver in iodine atmosphere both for thinner (<6000A) and thicker (5500 to 30000A) silver iodide film formation in the temperature and pressure ranges of 30° to 73°C and 0.019 to 1.241 mm Hg, respectively, show the validity of a parabolic growth law. While the rate constant value for the thinner halide film range has been found to be proportional top 1.5 I2 the same for the thicker film growth follows a modified √pI2 relation. These unusual pressure dependences suggest a situation of non-attainment of steady state conditions at the phase boundaries of the films. An activation energy value of 6.3 ± 1.5 kcal mole-1 in the above temperature range atp I2 = 0.039 mm Hg is in accord with that obtained by analyzing others’ data, thereby supporting the hole diffusion controlled process of Wagner. The presence of moisture in the atmosphere has been found to reduce the rate of iodination. Comparison of tarnishing behavior between annealed and cold deformed silver samples demonstrates a reduction in rate of iodination with percentage cold work.