Synthesis of Cerium Doped Sol-Gel Hybrid Coatings for Self-Healing Anticorrosive Protection of Aluminium Alloys

N. C. ROSERO-NAVARRO; S. A. PELLICE; Y. CASTRO; M. APARICIO; A. DURÁN

Montpellier, Francia

Conferencia; XIV International Sol-Gel Conference.; 2007

International Sol-Gel Society

In the aerospace industry, anticorrosive coatings for aluminium alloys have to comply with a series of technical exigencies that only chromium conversion coatings (CCCs) can satisfy totally. They combine an effective barrier to ion diffusion with self-healing effect and very low costs. Nevertheless, the production of CCCs is dangerous for the human health and leaves hazardous wastes to the environment. For this reason, the new European regulations have imposed hard limitations to its use demanding the urgent development of new coatings to substitute the CCCs. Cerium compounds seem to fulfil the basic requirements for consideration as alternative corrosion inhibitors. Furthermore, combination of the sol-gel chemistry of alkoxides with the use of organic components and nanoparticles could produce an enough strong anticorrosive barrier and, at the same time, provide the necessary conditions to operate as a Ce(III) reservoir to supply self-healing effect. In this work, we present the process of synthesis of a hybrid organic-inorganic sol from the hydrolytic condensation of tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) with an aqueous suspension of SiO2 nanoparticles (Levasil 200S, Bayer) in acidic media. Ethylene glycol dimethacrylate (EGDMA) was used as organic network crosslinker through free radical polymerization initiated by thermal decomposition of 2,2’-azobis(isobutironitrile) (AIBN). Cerium nitrate was alternatively used as a source of Ce (III). A complete characterization of the process of synthesis was performed by ATR-FTIR. After catalysis by HNO3, the hydrolysis occurs rapidly at expense of the water supplied by the silica suspension generating methanol and ethanol as an indication of the hydrolysis of MPS and TEOS, respectively. At the same time, the bands of Si-O-Si and Si-OH bonds appear as a result of the condensation process. Microstructure characterization was performed by SEM and TEM. Polarization curves and Electrochemical Impedance Spectroscopy (EIS) of 4 µm dip-coated substrates in NaCl 0.05 M showed good barrier properties at initial immersion times, with passive current densities of 6∙10-8 A/cm2 and a passive region of 900 mV. The increase of the immersion time leads to a gradual reduction of the impedance associated with the sol-gel coating as a consequence of deterioration of the protection system. An increase of the impedance modulus at 0.01 Hz after an initial reduction, and precipitation of yellow deposits with significant relative increase of Ce content (EDX analysis) can be associated to self-healing behaviour.