Inhibition of the Localized Corrosion of Mg Alloy AZ31 in Chloride Containing Electrolyte

A scanning vibrating electrode technique (SVET) is used to investigate the influence of a selection of potential inhibitor species, including rare earth cations, along with fluoride, chromate, and phosphate anions on the localized corrosion of unpolarized magnesium alloy AZ31 in 5% w/v aqueous sodium chloride electrolyte. Of the inhibitors studied, chromate and phosphate additions are shown to produce the most efficient inhibition of AZ31 localized corrosion. A study of the influence of varying phosphate concentrations showed that inhibition is less complete than that produced on pure Mg, but that the same cathodic inhibition mechanism remains in operation. In contrast, chromate inhibits by acting as a cathodic depolarizer, producing a profound inhibition at a 10⁻² mol dm⁻³ concentration. In situ SVET analysis (with chromate) reveals intense, short lived local cathodes, which couple with a low intensity anode distributed over the majority of the exposed surface. It is proposed that the inhibition mechanism involves the replacement of cathodic hydrogen evolution by a self-limiting chromate reduction to Cr(iii) at cathodic sites, where elevated pH subsequently produces a solid Cr(iii) hydroxide film, thus blocking further electron transfer.