The anodic dissolution of iron—VI. The behaviour of variously pre-treated iron electrodes and of cold-worked cobalt electrodes in moderately acid solutions containing iodide or carbon monoxide

Abstract

In strongly acid, iodide containing solutions the anodic Tafel diagram of iron is characterized by a restricted. Tafel region at low anodic potentials followed by a very steep increase in current at more anodic potentials. It is shown that there is another Tafel region subsequent to the steep increase in current, and that the current level at the beginning of the steep increase is not related to the rate of diffusion of iodide ions to the metal surface. In a higher range of pH (2 to 3·5) it is demonstrated that a family of variously shaped Tafel diagrams (including the one obtained at low pH) may be obtained by successive changes in pH and iodide concentrations. These features are shown to be in agreement with a model of coupled, parallel reactions. Annealing or cold-working of the electrodes gives rise to different kinetic parameters for the reaction observed at low potentials, and minor changes in the parameters of the transition between the coupled, parallel reactions are also observed. The reaction order with respect to iodide is negative for the reaction at low potentials (I 1), but positive for the reaction observed at higher potentials (I 2). At low iodide concentrations the Tafel slope of the I 1 reaction is low (30 to 60 mV), and high (100 mV) for the I 2 reaction, but with increasingly high iodide concentrations this situation becomes reversed. For the I 1 reaction the reaction order with respect to pH is +2 for cold-worked iron and ca. + 1 for annealed iron, while for the I 2 reaction it is ca + 0·2 in both cases. A phenomenon similar to passivation is occasionally apparent at very high potentials. The restricted amount of data is consistent with further oxidation of a reaction intermediate (Fe(II) to Fe(III)), further uptake of hydroxyl ions and of iodide ions, but also, instead of real passivation, the possible operation of yet another dissolution process (I 3) at even higher potentials. Some less detailed experiments involving iron in solutions saturated with carbon monoxide (and cobalt in solutions containing iodide or carbon monoxide) gave results in general agreement with the above description for the case of iron in iodide solutions. It is concluded that the transition between the I 1 and the I 2 reaction follows a pattern, which has been shown to be a common feature for iron dissolution in the presence of several other electrolytes. The two individual processes follow much the same general behaviour revealed in other electrolytes, although extreme cases of behaviour are observed at high iodide concentrations. On the basis of these extreme behaviours a tentative mechanism for the I 2 reaction is suggested.