Hybrid Ce-containing silica-methacrylate sol-gel coatings for corrosion protection of aluminium alloys

Abstract

The alternative we proposed for substituting chromates conversion coatings (CCC) for application on aluminium substrates treatable up to 150-200°C is based on the development of cerium doped silica-methacrylate hybrid polymer sol-gel coatings. These coatings have to combine barrier properties to delay the penetration of corrosion agents and inhibition properties to hinder the corrosion process because of the presence of a pore, crack or scratch in the coating. Cerium has the requirements for alternative corrosion inhibitors: the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors; they have a low toxicity and are relatively abundant in nature. A sol-gel coating based on silica-methacrylate using silicon alkoxides, monomers and modified silicon alkoxides offers the opportunity to design a structure with the adequate level of cross-linking to optimize the cerium diffusion in order to provide long-term corrosion protection. The coatings prepared using tetraethylorthosilicate (TEOS), 2-hydroxyethylmethacrylate (HEMA) and 3-methacryloxypropyltrimethoxysilane (MPS) provide a small barrier functionality because of the low degree of cross-linking in the structure and the high hydrophilic nature. The incorporation of cerium ions in the sols originates the increase of defect concentration due to the disruption of the structure, reducing the barrier functionality of the coatings, and a self-healing effect precipitating as yellowish oxide-hydroxide. Another signal of this inhibition mechanism is the increase of the impedance modulus at 0.01 Hz with immersion time, contrary to that observed in coatings without cerium. Several modifications, as removal of HEMA, and incorporation of commercial silica nanoparticles and ethylene glycol dimethacrylate (EGDMA), produces an increase of cross-linking and density with a significant improvement of the barrier functionality. However, the only way to combine barrier functionality and self-healing effect in this system is to develop a multilayer coating where each layer has a specific role. The results present a very good behaviour against corrosion as a barrier and signals of self-healing effect after a long immersion time in NaCl solutions.