Inhibition of magnesium localised corrosion in chloride containing electrolyte

An in situ Scanning Vibrating Electrode Technique (SVET) is used to investigate the inhibition of localised corrosion occurring on unpolarised magnesium (Mg) samples immersed in 5% (w/v) aqueous sodium chloride electrolyte. In uninhibited electrolyte, localised corrosion features taking the form of dark, radially expanding discs are shown to consist of a strongly cathodic interior, surrounded by a ring of anodic activity. A range of potential anionic inhibitors, selected on the basis of their ability to form insoluble precipitates with aqueous Mg2+ ions, are systematically investigated and compared with chromate additions. Of the anions evaluated only phosphate additions markedly decrease the extent of localised corrosion observed and provide a comparable level of inhibition to chromate. The onset of profound corrosion inhibition is achieved at threshold concentrations of 10−2 and 2 × 10−4 mol dm−3 for chromate and phosphate additions respectively. Phosphate inhibition efficiency increases markedly when the pH of the electrolyte is decreased. Consequently at phosphate concentrations of ≥10−3 mol dm−3, in the pH 4–7 range, Mg is able to withstand over 4 h immersion with little evidence of breakdown. From the empirical evidence presented, it is proposed that phosphate principally acts to inhibit hydrogen evolution at focal cathodic sites, where elevated pH produces sufficient PO43− speciation to form a surface precipitate by combining with free Mg2+.