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.

Mechanistic Study on the Corrosion Behaviour of Zinc and Zinc-Calcium Alloys Designed for Enhanced Metallic Coatings in the Presence of Chloride and Phosphate Ions

“…In 0.17 M NaCl, localised corrosion can readily occur on Zn as previously observed [40] due to ohmic and pH effects in the solution that…”

The Sacrificial Protection of Steel by Zinc-Containing Sol-Gel Coatings

“…An aggressive anolyte (with reduced pH and increased chloride activity) develops through a combination of electro-migration (of Cl -) and hydrolysis (of Zn 2+ ). [63][64] The galvanic current produces an alkaline catholyte by forcing the ORR onto the iron surface, leading to the evolution of passivity (and suppression of anodic dissolution) on iron. This process is visualized in Figure 9 and Figure 10 which show that the anodic current is initially concentrated at the defect/coating interface.…”

Anomalous currents determined by SVET due to composition gradients on corroding Zn surfaces in 0.1 M NaCl

“…The instability of the passive layer can explain the change from localized to uniform corrosion and also the stability of shallow pits. ZnO and Zn(OH) 2 are only thermodynamically stable at alkaline pH [8,13]. Thus, a deep occluded cell with intensively acidified local composition is not necessary to maintain the surface free from the protective layer.…”

“…The fact that HD-Zn coatings and pure Zn corrode by pitting has justified the measurement of in situ current density maps (i maps) by the scanning vibrating electrode technique [5,[11][12][13] besides its use in real-time analysis of the so-called cut-edge corrosion [1,11,14] or artificial galvanic pairs [15,16]. In the present study, we analyzed by SVET and cyclic voltammetry (CV) the slow change from localized to uniform corrosion.…”