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The Influence of Chloride Ion Concentration on Passivity Breakdown in Magnesium

2017; Volume: 73; Issue: 5 Linguagem: Inglês

10.5006/2328

1938-159X

G. Williams, Hefin ap Llwyd Dafydd, Raman Subramanian, H. N. McMurray,

Aluminum Alloy Microstructure Properties

An empirically derived dependence of an apparent breakdown potential (E b ) 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, E b can be determined by following time-dependent changes in free corrosion potential (E corr ).Breakdown of temporary passivity is marked by a clear inflection in the time-dependent value of E corr , characterised by a sharp decrease in potential from a maximum value where E corr = E b .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 E corr 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 ≤ 2 mol dm -3 , E b values are determined as a function of chloride ion concentration [Cl -] by means of a potentiodynamic method.In both cases a dependence of E b on [Cl -] is established such that E b = A + B log 10 [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.