Mechanism of corrosion-inhibition and corrosion-promotion of zinc by phosphate ions

Abstract

The potential of the zinc electrode has been measured at 30°C in 5×10 −4 -2 M aqueous phosphate solutions of different pH values. The results indicate that zinc does not behave as a metal/metal phosphate electrode. At pH≧4.6, a shift of potential to a less negative value takes place at a definite phosphate concentration which depends on the solution pH. This is attributed to the formation of a protective semi-glassy interface, due to adsorption of phosphate ions in a complex form on the surface of a persisting zinc phosphate layer. After reaching a maximum value, the potential decreases on further increase of concentration owing to adsorption of phosphate ions on the bare cathodic areas of the metal; this decelerates the reduction of oxygen and hence the corrosion-rate is further diminished. Primary-secondary and secondary-tertiary phosphate mixtures are more efficient in corrosion inhibition than individual ions. In Na 3 PO 4 −NaOH mixtures, both OH − and PO 4 3− ions take part in forming the semi-glassy interface. In the presence of 0.5 N NaOH, no corrosion inhibition occurs (owing to the dissolution of zinc phosphate as zincate), except above 0.5 M Na 3 PO 4 , when sodium zinc phosphate persists. In H 3 PO 4 −NaH 2 PO 4 mixtures (pH 2), corrosion of zinc is promoted. Below 0.005 M , promotion is restricted to the ionisation of zinc. This is probably due to reduction of the activation energy of the process through the primary phosphate ions adsorbed on the anodic areas of the metal surface. At higher concentrations, the cathodic hydrogen evolution is also catalysed. From corrosion-rate determinations, in these solutions as well as in pure phosphoric acid solutions, this catalytic action was suggested to be due to the existence of H 4 PO 4 + ions, from which hydrogen ions are preferentially discharged. Activated corrosion of zinc in formic and acetic acids was observed and was analogously related to the existence of HCOOH 2 + and CH 3 COOH 2 + ions.