The effect of traditional pigments on hybrid coatings

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The effect of traditional pigments on hybrid coatings is discussed in the following. When the concentration of traditional pigments is below CPVC, the permeability of coatings monotonically decreases with increases of the added pigments loading [ 58 ]. The pigments in flakes, such as montmorillonite clay, aluminum flake, should reduce coating permeability remarkably. Nanometer scale mont- morillonite clay, Cloisite 15A, is dispersed in the precursor solution of the hybrid coatings to form coating for alloy AA2024-T3. Formulation of the coating with diaminosi- lane curing agent and clay nanofillers significantly improves the coating resistance and reduces the water uptake to about 3–5% after 8 weeks of immersion in a 3.5% NaCl solution [ 51 ]. The size of traditional pigments also affects corrosion resistance of hybrid coatings. The pigments with a diameter higher than 500 nm need stabilizing additives which may alter the reactivity and the gelation capability of the sol, pigments with nano-range diameter are preferable [ 50 ]. Incorporation of nanoparticles into the hybrid sols leads to an additional improvement of the barrier properties due to enhanced thickness and low crack ability of such com- posites and enhancement of the mechanical properties [ 59 ]. 4 Corrosion inhibitors When hybrid coatings are integral, hybrid coatings on metal substrates form a physical barrier which is a uniform, defect-free and relatively dense film. However, corrosive ions still diffuse through micro-pores in the hybrid coatings and erode the metal elements at the interface between the coating and substrate [ 60 ]. Inorganic and hybrid SiO 2 coatings on steel substrates are compared by XPS. Inor- ganic coatings prepared from TEOS as only precursor allows certain extent of Fe diffusion due to film porosity, but the hybrid coatings prepared from MTES/TEOS lim- ited the diffusion of iron [ 61 ]. Therefore, the hybrid coat- ings can not eliminate film porosity completely. Hybrid coatings doped with corrosion inhibitors provide active corrosion protection, especially when integrity of the coating is compromised. Corrosion inhibitors can be added to the sol–gel system during a synthesis procedure for pre- treatments or at the stage of film formation and cross- linking. Many kinds of inorganic inhibitors with low toxicity have been used to increase the corrosion protection of hybrid coatings. Inorganic inhibitors, such as phosphates [ 62 ], vanadates [ 63 ], rare earth elements [ 64 , 65 ], have shown positive influence on the corrosion protection of aluminum alloys. Cerium ions with relatively low cost and abundance satisfy the basics requirements for being con- sidered an alternative inhibitor to chromates: the ions form insoluble hydroxides and present low toxicity. Moreover, cerium dopants that unlike NaVO 3 and Na 2 MoO 4 do not decrease of the sol–gel network stability in epoxy–zirconia films [ 66 ]. The main role of Ce 3 ?
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