The Influence of Chloride Ion Concentration on the Localized Corrosion of E717 Magnesium Alloy

The localized corrosion behavior of E717 magnesium alloy immersed in chloride-containing electrolyte is investigated using an in situ scanning vibrating electrode technique (SVET) coupled with time-lapse imaging. It is shown that initiation of localized corrosion in chloride-containing electrolyte is characterized by the appearance of discrete local anodes, corresponding with the leading edges of dark, filiform-like features, which combine with time to produce a mobile anodic front. The size and growth rate of these features are highly dependent on the chloride ion concentration of the electrolyte. SVET-derived current density maps reveal that the corroded surface left behind the anodic front is cathodically activated, where cathodic current density values progressively decline with increasing distance away from the anodic leading edge. The intensity of localized anodes is highly dependent on the chloride ion concentration, where progressively higher local anodic current density values are observed with increasing chloride ion concentration along with progressively higher rates of volumetrically determined hydrogen evolution. Breakdown potential, measured using time-dependent free corrosion potential transients and potentiodynamic polarization at neutral and elevated pH, respectively, is shown to vary with the logarithm of chloride ion concentration, and the time for localized corrosion initiation is progressively increased with decreasing chloride concentration. From the combination of results that are presented herein, the underlying reasons for the influence of chloride ion concentration on the localized corrosion characteristics of E717 alloy will be discussed.