Part of the book: Recent Researches in Corrosion Evaluation and Protection
Polymethylmethacrylate–silica hybrids have been prepared using the sol–gel route by the radical polymerization of methyl methacrylate(MMA) using benzoyl peroxide (BPO) as a thermal initiator and 3-(trimethoxysilyl)propyl methacrylate(MPTS) as a coupling agent, followed by acid-catalyzed hydrolytic condensation of tetraethoxysilane (TEOS). Carbon nanotubes (CNTs) were first dispersed either by surfactant addition or by functionalization with carboxyl groups and then added at a carbon (CNT) to silicon (TEOS and MPTS) molar ratio (CCNT/SiHybrid) of 0.05% to two different hybrid matrices prepared at BPO/MMA molar ratios of 0.01 and 0.05. Films of 2–7 μm thickness deposited onto carbon steel by dip-coating were characterized in terms of their microstructure and their mechanical, thermal and anticorrosive behavior. Atomic force microscopy and optical microscopy confirmed that there was a homogeneous dispersion of CNTs in the nanocomposites and that the surfaces of the films were very smooth. X-ray photoelectron spectroscopy (XPS) confirmed the nominal composition of the films while nuclear magnetic resonance showed that the connectivity of the silica network was unaffected by CNT loading. Thermogravimetric analysis and mechanical measurements confirmed an increase of thermal stability, hardness, adhesion and scratch resistance of CNT-loaded coatings relative to those without CNTs. Electrochemical impedance spectroscopy measurements in 3.5% NaCl solution interpreted in terms of equivalent circuits showed that the reinforced hybrid coatings, prepared at the higher BPO/MMA molar ratio used in this work, act as a very efficient anticorrosive barrier, with an impedance modulus up to 109 Ω cm2.
Part of the book: Carbon Nanotubes
A variety of organic-inorganic hybrids have been designed to act as anticorrosive coatings of metallic substrates. Among them, epoxy-silica and poly(methyl methacrylate) (PMMA)- silica hybrids, prepared by the sol-gel process and deposited onto steel or aluminum alloys, have demonstrated high anticorrosive efficiency combined with high thermal and mechanical resistance. Lignin, carbon nanotubes, and graphene oxide have been incorporated into PMMA-silica hybrids as reinforcement agents, and cerium (IV) as corrosion inhibitor. Both hybrids were characterized in terms of their structural and thermal characteristics using different pectroscopies, microscopies and thermogravimetric analysis. Both hybrids present homogeneous nanostructure composed of highly condensed silica nanodomains covalently bonded to the polymeric phase. The transparent coatings with a thickness of 2–7 μm have low surface roughness, high adhesion to metallic substrates, elevated thermal stability, and excellent barrier behavior. Electrochemical impedance spectroscopy showed for coated samples a high corrosion resistance of up to 50 GΩ cm2 and durability >18 months in saline solution. Further improvement of corrosion resistance, thermal and mechanical stability was achieved by incorporation of lignin, carbon nanotubes, and graphene oxide into PMMA-silica matrix, and a self-healing effect was observed after Ce(IV) addition. The results are compared and discussed with those recently reported for a variety of hybrid coatings.
Part of the book: New Technologies in Protective Coatings
Organic-inorganic coatings based on poly(methyl methacrylate) (PMMA)-silica and PMMA-cerium oxide hybrids provide effective and active corrosion protection of metallic surfaces. For both hybrid materials, the covalent conjugation of inorganic silica or ceria nanodomains with the PMMA matrix, provided by molecular coupling agents, leads to homogenous and highly cross-linked nanocomposites, which act in the form of coatings as an efficient diffusion barrier. The addition of lithium salts (500–2000 ppm) into PMMA-silica hybrid and optimized ceria fraction in PMMA-cerium oxide coatings results in active corrosion inhibition by the self-healing effect. Results of electrochemical assays of aluminum- and steel-coated samples, performed in a 3.5% NaCl solution, show an excellent corrosion resistance (impedance modulus up to 100 GΩ cm2) and durability (up to 350 days) of the 10-μm-thick passive barrier layer. Time-of-flight secondary ion mass and X-ray photoelectron spectroscopies evidenced the self-healing ability of coatings induced by lithium/cerium ion leaching toward corrosion spots or artificial scratches, which are restored by a protective layer of precipitated phases. Results presented in this book chapter evidence the active role of lithium and cerium species in improving the hybrid structure and providing through self-healing a significantly extended service life of metallic components.
Part of the book: Corrosion