The Influence of Chloride Ion Concentration on Passivity Breakdown in Magnesium

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 localized 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, characterized by a sharp decrease in potential from a maximum value where E_corr = E_b. Characterization of localized corrosion behavior by in situ scanning vibrating electrode studies, prior to and following the point of breakdown, is used to explain the observed E_corr vs. time characteristics. Examples of typical behavior 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/dm³, 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 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.