Oxide nanoparticle as reservoir of corrosion inhibitors for coatings with self-healing ability

Metallic substrates are pre-treated before application of an organic paint in order to improve adhesion between the polymer and the metal and to provide additional long term corrosion protection. So far, the mostly used pre-treatments contain chromates as active agents. However, chromate compounds are strongly carcinogenic due to their high potential to oxidise the organic molecules. In order to avoid this drawback the new environmental-friendly pre-treatments have been actively developed in the recent decade [1]. Most of them provide an additional barrier to the corrosive species impairing the corrosion processes. An effective pre-treatment could be the hybrid silica-based sol–gel coatings, which combine flexibility and good compatibility with the paint systems due to the organic component and good mechanical properties and adhesion to the metallic substrate conferred by the inorganic compound [2]. In spite of good barrier properties several cracks and pores can appear in the hybrid films originating sites for starting the corrosion processes. Introduction of oxide nanoparticles into the hybrid matrix leads to formation of crack-free films and can enhance mechanical durability of the films improving corrosion protection [3]. Even so nanocomposite hybrid films cannot provide an effective long term corrosion protection due to the absence of self-healing effect as in the case of chromates. One way to improve the self-repair properties of the hybrid pre-treatments is the addition of a corrosion inhibitor, which can suppress corrosion initiated at the coating defects [4]. However, the inhibitors can also have a negative effect in the sol–gel film properties, decreasing the stability and consequently the barrier properties of the film [5].