Anticorrosive behaviour of Cr(VI)-free surface pretreatments applied on magnesium alloys

Among the anticorrosive protection technologies for magnesium alloys, the development of paint systems is a pressing need especially for the automotive and aeronautical industries. Conventional technologies are based on the use of Cr(VI) based compounds, both in pretreatments and primers, but for health and environmental reasons these technologies are being abandoned. An added problem in the painting of magnesium alloys is the lack of adhesion and the high reactivity of magnesium substrates, which may lead to underfilm corrosion and detachment of the paint system. Therefore, the selection of a suitable pretreatment is crucial.Our research seeks to evaluate the anticorrosive behaviour of Cr(VI)-free surface pretreatments applied on different magnesium alloys, AZ31, AZ91 and AM60 and to analyse whether they could be protective enough to assure the successful protection of the substrate without the application of a topcoat. The considered alternatives are four chemical conversion coatings including two coatings obtained by plasma electrolytic oxidation (PEO). A chromate conversion coating has been also applied as reference. Different corrosion tests have been carried out in both natural (marine and urban atmospheres) and accelerated environments: salt fog, condensing humidity and cyclic test (salt fog/dry/humidity). Electrochemical characterization by electrochemical impedance spectroscopy (EIS) has been also performed. The results show a notably better anticorrosive behaviour of PEO coatings compared to chemical conversion coatings for all the considered magnesium alloys in both, natural and accelerated environments. However, none of the studied coatings seem to successfully protect magnesium alloys in aggressive atmospheres (marine) without the application of a topcoat. On the other hand, although PEO coatings have shown an acceptable good behaviour in very low aggressive environments (urban), it would be also recommended the application of a topcoat.