Behaviour of lead as a metal—metal phosphate electrode and mechanism of its corrosion inhibition by phosphate ions

Abstract

The potential of the lead electrode in aqueous phosphate solutions was measured as a function of pH and electrolyte concentration. Up to pH 8.7 the electrode behaves as Pb/PbHPO 4 within certain ranges of phosphate concentration which depend on the pH of solutions. At higher pH-values lead functions as a Pb/Pb 3 (PO 4 ) 2 electrode. At a certain phosphate concentration, ennobling of potential takes place as a result of inhibition of corrosion. A mechanism for corrosion inhibition is proposed, based on the adsorption of phosphate ions, on the surface of the lead phosphate layer, in a highly polymerised form. The film/solution interface acquires, therefore, a “semiglassy” structure, which hinders the transfer of lead ions to the solution. After reaching a maximum value, the potential decreases again on further increase of concentration. This was attributed to adsorption of phosphate ions on the bare cathodic areas of the electrode surface, leading to deceleration of the cathodic reduction of oxygen. This results in further inhibition of corrosion. Mixtures of primary and secondary phosphates and of secondary and tertiary phosphates, are more efficient in corrosion inhibition than individual ions. The tertiary phosphate ion showed no inhibitive effect. In large dilutions of phosphoric acid-primary phosphate mixtures, corrosion of lead is promoted. This is possibly due to acceleration of the metal dissolution by adsorbed ions on the bare anodic areas.