Localized Corrosion of Magnesium in Chloride-Containing Electrolyte Studied by a Scanning Vibrating Electrode Technique

An in situ scanning vibrating electrode technique (SVET) is used to investigate localized corrosion occurring on unpolarized magnesium (Mg) samples immersed in aqueous sodium chloride electrolyte. Corrosion is characterized by the appearance of circular, blackened areas which expand radially at a constant rate and evolve hydrogen vigorously. These are shown to consist of a cathodically active center surrounded by a wide anodic ring. Any localized corrosion currents emerging from the intact (uncorroded) Mg surface are negligible by comparison. Local anodic current density is shown to be directly proportional to the radius of the local cathode, while corresponding local cathodic current density remains relatively constant with time. Estimates of time-dependent rates of total equivalent Mg loss and evolution, obtained by numerical integration of SVET-derived normal current density distributions, indicate that corrosion rate is controlled by the area of local cathodic activity. The empirical findings are consistent with a mechanism involving cathodic evolution on the dark, film-free region which is galvanically coupled with anodic attack of the intact Mg surface. It is proposed that cathodic activation of the film-free, corroded "disk" is caused by a combination of elevated pH and enrichment in noble iron-containing impurity phases.