On the role of NO 2− ions in passivity breakdown of Zn in deaerated neutral sodium nitrite solutions and the effect of some inorganic inhibitors : Potentiodynamic polarization, cyclic voltammetry, SEM and EDX studies

Abstract

As a first step towards studying pitting corrosion of Zn in deaerated neutral sodium nitrite solutions (pH 6.9), we have reported the results of potentiodynamic polarization and cyclic voltammetry measurements on the passivity and passivity breakdown of Zn in these solutions. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) examinations of the electrode surface. The voltammograms involve active/passive transition prior to the initiation of pitting corrosion as a result of breakdown of the passive film by NO 2 − ions. The active region displays one anodic peak due to the formation of ZnO passive film on the anode surface. SEM examinations confirmed the existence of pits on the electrode surface. The potential at which pits initiated ( E pit) was determined, together with a pit transition potential ( E ptp) that appeared as an abrupt current discontinuity on the reverse potential scan (hysteresis loop), and a protection potential ( E prot) that appeared at the end of the hysteresis loop. The value of E pit shifted negatively as either C nitrite or temperature was increased, while it increased with the increase in potential scan rate. The effect of adding some environmentally acceptable inorganic inhibitors, as tungstates, molybdates and silicates (water glass), on the pitting corrosion behaviour of Zn in nitrite solutions has also been studied. The mechanism of inhibition was discussed.