Concentration Effects on the Spatial Interaction of Corrosion Pits Occurring on Zinc in Dilute Aqueous Sodium Chloride

Abstract

The scanning vibrating electrode technique is used to study the localized corrosion of unpolarized zinc in near-neutral aqueous sodium chloride electrolyte of varying concentration [NaCl]. As [NaCl] is reduced from 1% (w/v) to 0.0005% (w/v) the morphology of attack changes from large, irregular, areas of anodic zinc dissolution and cathodic oxygen reduction, to a regular array of anodic pits set in an otherwise cathodic surface. In dilute electrolyte ([NaCl] < 0.01%) the total (area averaged) corrosion current density, J, (A · m −2), obtained from a numerical area integral of SVET anodic current density data, is shown to vary with approximately the square root of [NaCl]. The number density of pits, n (m −2) is shown to decrease with increasing [NaCl]. The mean individual pit current, i pit , (A = J/n) varies with approximately the square root of [NaCl]. A simple geometric analysis, based on a calculation of the radial dependence of electrical resistance exhibited by a hemispherical shell of electrolyte concentric with the pit, is used to explain the relationship between ohmic potential drop in the external solution, solution conductivity, and the radial distance away from an existing active pit. Zinc provides sacrificial protection to a number of substrates and consequently enjoys widespread technological application. Pitting corrosion is observed on Zn in the presence of aggressive anions, such as chloride, 1-12 but can also occur in very dilute environments, such as distilled water. 4 Zn is only passive over a very limited range of pH and consequently exhibits passivity breakdown in a wide range of electrolytes over a wide range of concentrations. Galvele concluded that: i) in unbuffered solution the pitting potential of Zn is close to the equilibrium potential for Eqaution 1 and ii) pitting starts as a result of local acidification of the metal surface. 1 It has been shown elsewhere that repassivation of the Zn surface only occurs when pH ≥ ca. 8.5. 11