No Cover Image

Journal article 455 views 87 downloads

The influence of Chloride Ion Concentration on Passivity Breakdown in Magnesium / Geraint Williams; Hefin Ap Llwyd Dafydd; Raman Subramanian; Neil McMurray; Hamilton McMurray

Corrosion

Swansea University Authors: Geraint, Williams, Hamilton, McMurray

Check full text

DOI (Published version): 10.5006/2328

Abstract

An empirically derived dependence of an apparent breakdown potential (Eb) of magnesium (Mg) on chloride ion concentration is reported. In situations where spontaneous breakdown in the absence of external polarization is observed, leading to the subsequent propagation of localised corrosion, Eb can b...

Full description

Published in: Corrosion
ISSN: 0010-9312 1938-159X
Published: 2017
Online Access: Check full text

URI: https://cronfa.swan.ac.uk/Record/cronfa31999
Tags: Add Tag
No Tags, Be the first to tag this record!
Abstract: An empirically derived dependence of an apparent breakdown potential (Eb) of magnesium (Mg) on chloride ion concentration is reported. In situations where spontaneous breakdown in the absence of external polarization is observed, leading to the subsequent propagation of localised corrosion, Eb can be determined by following time-dependent changes in free corrosion potential (Ecorr). Breakdown of temporary passivity is marked by a clear inflection in the time-dependent value of Ecorr, characterised by a sharp decrease in potential from a maximum value where Ecorr = Eb. Characterisation of localized corrosion behaviour by in-situ scanning vibrating electrode studies, prior to and following the point of breakdown, is employed to explain the observed Ecorr vs. time characteristics. Examples of typical behaviour upon immersion in aqueous solutions containing different chloride ion concentrations ([Cl-]) are given for commercially pure Mg and an AZ31 alloy. For high purity Mg, which remains passive at pH 11 at all chloride concentrations ‚ȧ 2 mol dm-3, Eb values are determined as a function of chloride ion concentration [Cl-] by means of a potentiodynamic method. In both cases a dependence of Eb on [Cl-] is established such that Eb = A + B log10[Cl-], where the value of B is typically -0.11 V per decade. The practical implications of the existence of a breakdown potential for Mg is considered in terms of the selection of an appropriate chloride ion concentration in standard corrosion tests.
College: College of Engineering