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The corrosion of silver, copper, palladium and gold in dimethyl sulphoxide and water

Benari, Max (1989) The corrosion of silver, copper, palladium and gold in dimethyl sulphoxide and water. PhD thesis, Murdoch University.

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Abstract

The mechanisms of the corrosion reactions of Ag, Cu, Pd and An with CuII and FeIII in calcium chloride in DMSO and calcium bromide in DMSO have been studied. These results have been contrasted with studies carried out in comparable aqueous media. To determine the mechanisms, it was necessary to study the corrosion reactions, as well as the anodic and cathodic half reactions.

For the study of the anodic half reactions in calcium chloride in DMSO the metals dissolve in their low valent state, i.e. AgI, CuI PdII and AuI. It was also found that the anodic dissolution was favoured by the reduced nobility of the metals in DMSO calcium halide solution compared with aqueous calcium halide solution. The effect for gold was pronounced. The anodic reaction occurred at potentials 0.7 V less positive in calcium chloride in DMSO than in aqueous calcium chloride. The anodic study for silver showed that silver chloride film formation is suppressed in DMSO solution compared with aqueous solution. For a silver rotating disc, current densities were up to an order of magnitude greater in calcium chloride in DMSO than in comparable aqueous solution. For the anodic dissolution of copper in calcium chloride in DMSO, CuCl film formation was also suppressed, compared with aqueous calcium chloride solution. For the anodic reaction of palladium in calcium chloride in DMSO, oxide films were found to inhibit dissolution at low overpotentials.

Anodic sweep data for Ag, Cu, Pd, Au and Pt in calcium bromide in DMSO were found to be similar to those in calcium chloride in DMSO. The main distinguishing feature is that calcium chloride in DMSO is more resistant, by 0.25 V, to solvent oxidation than calcium bromide in DMSO.

The electrochemical properties of the cathodic half reactions Cu II + e Cu1 and FeIII + e Fe II were studied in calcium chloride in DMSO, calcium bromide in DMSO and lithium perchlorate in DMSO. Formal potentials, diffusion coefficients and heterogeneous rate constants were measured and contrasted with results from comparable aqueous solutions. Formal reduction potentials were less positive in DMSO than in water. Diffusion coefficients and heterogeneous rate constants were an order of magnitude less in DMSO than in water. Cyclic voltammetric studies in DMSO showed that the rate of reduction of FeIII in calcium chloride and lithium perchlorate is controlled by electron transfer. In calcium chloride in DMSO, the rate of CuII reduction is controlled by a slow prechemical step involving loss of chloride ion from the CuII complex ion.

The corrosion reactions of Ag, Cu, Pd and Au were studied using a rotating disc electrode. Data on the effect of rotation rate on corrosion potentials were combined with information on the kinetic behaviour of the two half reactions, to determine the rate controlling steps. The rate of silver corrosion by Cu II and FeIII in calcium chloride in DMSO is solution diffusion controlled. The same applies for silver corrosion by Cu11 in calcium bromide in DMSO. The corrosion rate in DMSO media is two orders of magnitude faster than in comparable aqueous solution. In aqueous halide solution the rate was found to be under mixed solid state and solution diffusion control. The mixed control is due to the changing concentration profiles of silver ion in the silver chloride film and the oxidant in the solution diffusion layer. The corrosion of copper and palladium by CuII in calcium bromide in DMSO is essentially cathodic diffusion controlled. When calcium chloride is used the reaction mechanisms are complicated by film formation. Gold is corroded by CuII in both calcium chloride in DMSO and calcium bromide in DMSO. In calcium chloride in DMSO the rate is slow and is electrochemically controlled. In calcium bromide in DMSO the rate is considerably faster and is solution diffusion controlled. For practical purposes. leaching of Ag, Cu, Pd and Au should be with CuII in calcium bromide in DMSO. The reactions are solution diffusion controlled and the metals will dissolve rapidly.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Hefter, Glenn and Parker, Jim
URI: http://researchrepository.murdoch.edu.au/id/eprint/51619
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