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Chloride Ion Concentration Effects on Passivity Breakdown in Magnesium

2014; Institute of Physics; Volume: 58; Issue: 31 Linguagem: Inglês

10.1149/05831.0023ecst

2151-2051

G. Williams, Hefin Dafydd, Raman Subramanian,

Magnesium Alloys: Properties and Applications

An empirically derived dependence of the breakdown potential (E b ) of pure magnesium (Mg) on chloride ion concentration ([Cl - ]) is described. For commercial purity Mg, which undergoes spontaneous breakdown in the absence of external polarization, E b is established by following time-dependent changes in free corrosion potential (E corr ). The localized corrosion behaviour of Mg, characterized by in-situ scanning vibrating electrode studies prior to and following breakdown, is employed to explain the E corr vs. time characteristics. For high purity Mg, which remains passive at pH 11 at all chloride concentrations ≤ 2 mol dm -3 , E b values are measured 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 -0.11 V per decade. The origin of the log 10 relationship with [Cl - ] is discussed in terms of the solubility product of magnesium hydroxide.