Abstract
In Part I it was observed that the growth rate of the tin sulphide film in alkaline polysulphide solution presented two distinct stages. Moreover, a simultaneous dissolution of the film was also observed.
In order to examine the mechanism of the tarnishing process certain variables have been examined in detail, notably the effects of stirring and of the concentration of potassium polysulphide and sodium hydroxide at 50°c. Below 1 g/l potassium polysulphide no sulphide film formaiion was detected while at about 7 g/l the second stage growth rate reached a maximum value. It is suggested that the dissolution rate may be controlled by diffusion of polysulphide ions to the film/solution interface and a model, based on polarisation curves, has been proposed to explain the observed behaviour.
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