Selective oxide growth at a rhodium—platinum alloy under potential cycling conditions in base

Abstract

The corrosion of a platinum-rhodium alloy under potential cycling conditions (2 Hz, 0–1.50 V) in base was shown to involve the formation of a hydrated rhodium oxide surface layer — a phenomenon already reported for pure rhodium. Corrosion via a dissolution mechanism is much more marked in the case of the platinum component. Experiments carried out with the two pure metals showed that the net rate of rhodium loss, involving dissolution and oxide formation (the latter predominating with this metal), is greater than that of platinum under potential cycling conditions in base. The duplex nature of the films produced under these conditions is confirmed by the fact that while the thick outer layer showed the usual redox and electrochromic properties of hydrated rhodium oxide, the reduction behaviour of the monolayer film at the surface of the alloy is determined largely by the composition of the latter. The results are important from a practical viewpoint in that they demonstrate a new type of surface-enrichment process associated with the potential sweep technique. In most electrochemical work to date enrichment is regarded in terms of dissolution of the less noble component of the alloy. However, it now appears that, almost irrespective of nobility, the component which has the greater tendency to corrode to an insoluble oxide layer is the one most likely to accumulate at the surface and influence the rate, especially that of subsequent anodic reactions.