Waterborne sol-gel coatings doped with corrosion inhibitors for the protection of Mg-Al alloys

Resumen

Novel organopolysiloxane films for protection of magnesium alloys have been designed in this study. Such films are based on the physical entrapment or nanoencapsulation of corrosion inhibitors in a hybrid organic-inorganic matrix obtained by the sol-gel processes. Two commercial magnesium alloys have been used, AZ31 and AZ61. The hybrid sol-gel films were synthesized through co-hydrolysis and polycondensation of two precursors, ¿-methacryloxypropyltrimethoxysilane and tetramethylorthosilicate. For properly preparing the organopolysiloxane films, hexa(methoxymethyl)melamine (HMMM) has been added to the solution during the sol stage; p-toluenesulphonic (p-TSA) was also added to film formulation just before coating. HMMM acts as a crosslinking agent and p-TSA as a blocked acid catalyst. In a first phase of the study, the resulting thin films have been modified by adding to the sols variable amounts of benzotriazole (BTA). This heterocyclic compound is a reference corrosion inhibitor widely used for the protection of metal surfaces. In a second stage, BTA has been replaced by L-cysteine (L-Cys). The corrosion behaviour of the resulting metal/film systems was studied by immersion tests in NaCl aqueous solutions at variable exposure time. The films obtained by the sol-gel process were evaluated as autonomous protective films as well as a pretreatment prior to a waterborne acrylic topcoat. The obtained results show that the combination of XPS, SEM/EDX and AFM with global and local electrochemical impedance spectroscopies (EIS and LEIS) allow not only to assess the protective ability of these new sol-gel films but also the influence of inhibitors added to the films on the kinetics and mechanisms of corrosive processes of the metal substrates. The sol-gel films obtained from mixtures of organopolysiloxane precursors with HMMM and BTA were especially effective as pretreatments for a final water-based acrylic coating, but controlling appropriately the formulation of sol-gel films, L-Cys could be used as an environmentally friendly inhibitor for potential replacements to benzotriazole.