Pitting corrosion of zinc in alkaline medium by thiocyanate ions

This paper describes the use of potentiodynamic anodic polarization, cyclic voltammetry and chronoamperometry techniques in order to study the pitting corrosion susceptibility of a Zn electrode in KOH solutions containing KSCN as a pitting corrosion agent. Measurements were conducted under different experimental conditions. The results demonstrated that in the absence of KSCN, the anodic voltammetric response displays two anodic peaks prior to reaching the oxygen evolution potential. The first anodic peak A₁ is related to the electroformation of Zn(OH)₂. Peak A₁ is followed by a wide passive region which extends up to the appearance of the second anodic peak A₂. The latter is assigned to the formation of ZnO₂. Addition of SCN⁻ ions to the KOH solutions stimulates the anodic dissolution through peak A₁ and breaks down the passive layer prior to peak A₂. The breakdown potential decreases with an increase in SCN⁻ concentration and temperature, but increases with an increase in KOH concentration and potential scan rate. Successive cycling leads to a progressive increase in breakdown potential. The current/time transients show that the incubation time for passivity breakdown decreases slightly with increasing applied positive potential, SCN⁻ concentration, and temperature.