Novel corrosion-resistant films for Mg alloys

A novel method for the corrosion inhibition of Mg alloys, based on the formation of a thin polyacrylic–polypyrrole film, is presented. The film was created after simple immersion of the samples in a specially formulated emulsion of polyacrylic and polypyrrole polymers. The synthesis conditions were studied, as well as the resulting corrosion resistance of coated magnesium foils (AZ31) in terms of corrosion rate in neutral aqueous NaCl solutions. The relative ratio of polyacrylic polymer to polypyrrole was found to be important for the film properties. Furthermore, only in situ polymerisation of pyrrole in the polyacrylic emulsion can provide advanced corrosion properties in relatively thin films (2–5 μm). Blending of polypyrrole powder with the polyacrylic emulsion gives rise to thick films (20 μm) with reduced corrosion-inhibition properties. The corrosion rate was only weakly dependent on the film thickness of these novel coatings. We argue that the mechanism of corrosion inhibition is related to the formation of a thin passive layer of chemisorbed polypyrrole polymers or oligomers on the native magnesium oxide of the surface. The structure is further stabilised by polyacrylic polymer chains interacting as dopants of polypyrrole. The method is environmentally friendly: non-toxic chemicals are used, no rinsing is required, etc. The advanced corrosion-inhibition properties of the films, as well as the simple application process on the metal surface, provide significant potential for the industrial application of the method.