Behaviour of tin as metal-metal phosphate electrode and mechanism of promotion and inhibition of its corrosion by phosphate ions

Abstract

The potential of the tin electrode in aqueous phosphate solutions was measured at 30o as a function of both pH and the electrolyte concentration. In concentrated solutions the electrode behaves reversibly with respect to certain tin compounds. Thus, in the pH range, 2.05–4.6, the electrode reaction in 0.2–1 M solutions involves the formation of HSn(HPO4)2−, which is considered as the bi-ion of “phosphatostannous” acid. In the pH range, 6.5–8.7, the formation of the normal ion, Sn(HPO4)22−, governs the electrode potential in 0.05–1 M solutions. At higher pH, a tertiary phosphate layer persists on the electrode surface. In dilute phosphate solutions, the results indicated either promotion or inhibition of corrosion of tin. The former effect was observed in the pH range, 2.05–4.6, whereby the potential decreased by 8–15 mV per unit increase of log C. Corrosion inhibition was observed, on the other hand, at higher pH-values, whereby exceptionally high potentials were developed up to about 0.01 M. With further increase of concentration, the potential decreased sharply towards the values required by the above mentioned reversible couples. Corrosion inhibition, manifested by the high potentials, was attributed to the adsorption of phosphate ions, on the surface of a persistent tin phosphate layer, in a highly polymerised or condensed form. The layer/solution interface acquires a “semiglassy” constitution, which inhibits the transfer of tin ions to the solution. Adsorption on the bare cathodic areas as the concentration is further increased, was suggested to deaccelerate the cathodic reduction of oxygen, and hence, the potential decreases sharply leading to further corrosion inhibition. In the absence of a tin phosphate layer, adsorption of phosphate ions on the bare anodic areas accelerates the ionisation of the metal, perhaps through the reduction of the energy of activation of the process. This causes the corrosion of tin to be promoted. Simple tertiary phosphate ions exhibit no inhibiting effect on corrosion. Therefore, above pH 12.5 (in which case the solutions contain only very small amounts of HPO42−), the electrode behaves as Sn/Sn3(PO4)2 over the whole concentration range studied. The free energy of formation of tin tertiary phosphate was calculated as −607.8 kcal.