Hybrid coatings are dip coated on stainless steel and

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Hybrid coatings are dip-coated on stainless steel and annealed at 300 ° C for 30 min to form relatively dense, uniform and defect free films which provide excellent corrosion protection by forming a physical barrier [ 48 ]. Sol–gel coatings sintered at 400 ° C have superior wear and corrosion resistance properties to be an effective barrier against ions diffusion from the substrate [ 49 51 ]. The hybrid coating from TEOS and MTES by acidic catalysis is obtained at a sintering temperature of 400 ° C to form continuous and crack-free coatings applied onto a corrodible surface such as carbon steel. The coatings are an effective barrier against aerated 3.5 wt% NaCl solution in 2 h. After 48 h of soaking, localised corrosion occurring in the pores propagates under the surface film, creating a more voluminous oxide that finally delaminates the coating [ 52 ]. A hybrid coating system of two layers with comple- mentary properties is developed. Each coating is heat 178 J Sol-Gel Sci Technol (2010) 54:174–187 123
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treated for 30 min at 450 ° C. The inner layer prepared using TEOS and MTES has already shown good anticorrosion properties and very low iron diffusion. The top layer pre- pared from TEOS, 3-methacryloxypropyl trimethoxysilane, and 2-hydroxyethyl methacrylate, allows produce thick coatings. The multilayered coating is an effective barrier against ion diffusion from the substrate and inhibits high temperature oxidation processes [ 53 ]. 3.5 Pigment particles Pigment particles in hybrid coatings can be classified two groups: particles forming in sols and traditional added pigments. The particles forming in sols are different from traditional pigments. There are strong chemical interaction between sol particles and organic components when hybrid coatings are dried. When the concentration of traditional pigments in the coatings exceeds their critical pigment volume concentration (CPVC), the apertures in the films accrue sharply, which degrades physical barrier properties of coatings. CPVC of traditional pigments approximates to 50–60% range. However, there is no CPVC for the sol particles in hybrid coatings, and hybrid films can be formed at random ratio of inorganic–organic components. When the average diameter of sol particles exceeds 100 nm, the transparency of hybrid coatings decreases abruptly to make the films opaque, but sol particles do not have the color aptitude and conditioning of traditional pigments. Nanosized particles with epoxy functionalities are used most for corrosion protection, and cross-linking agents are added to the solution directly before coating deposition. During the drying stage the cross-linking agent forms chemical bonds with the epoxy groups of the nanoparticles producing uniform compact crack-free coatings, which provides a fully dense protective barrier with excellent corrosion protection to metallic substrates [ 31 , 54 57 ].
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