Composites through the release of a polymerizable

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composites through the release of a polymerizable healing agent that can bridge cracks after reaction with the appropriate catalysts. 274 The synthesis procedure to obtain this layer is critical as it should ensure that the J. Coat. Technol. Res.
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self-healing agents dispersed within this OIH layer preserve their properties and mobility after the curing treatment. Finally, a highly hydrophobic topcoat layer should be deposited at mild conditions to combine the barrier effect protection and to avoid any possible degradation of the different OIH-based layers that are beneath. The total thickness of this type of multilayer coating system should be about 16 l m considering the use of dip-coating method that allows the production of each OIH layer with an average thickness of 4 l m. The schematic representation of the proposed multi- layer coating system is displayed in Fig. 16 . Usage, weathering or loading damage Entrance of environmental aggressive agents Self–healing agents Corrosion inhibitor Topcoat 6 5 4 3 2 1 OIH Sol–gel coating doped with the OIH Sol–gel coating doped with Pure OIH Sol–gel coating self–healing agents Corrosion inhibitor Oxide Layer Metallic Substrate 6 5 4 3 2 1 6 5 4 3 2 1 6 5 4 3 2 1 a b c Fig. 16: (a) Multilayer coating system composed of five layers. 1—Metallic substrate; 2—oxide layer; 3—pure OIH sol–gel coating; 4—OIH sol–gel coating doped with a corrosion inhibitor; 5—OIH sol–gel coating doped with a self-healing agent; and 6—Topcoat. (b) Multilayer coating system after damage. (c) Expected behavior of the multilayer coating system behavior when in contact with exterior aggressive agents J. Coat. Technol. Res.
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Concluding remarks Publications between 2001 and 2013 about OIH sol–gel coatings class II based on siloxanes, which were tested to prevent corrosion on metallic substrates, were reviewed. Since the early 1970s, academic interest in OIH materials has been accelerating and since the 1990s it has become a fast-growing and very complex subject with an almost exponential increase in the number of the scientific publications. The main driving force in the development of OIHs class II based on siloxanes to prevent corrosion was the search for potential candidates to substitute environ- mentally unfriendly chromate surface treatments for metallic substrates. The analysis of the papers published between 2001 and 2013 shows that it is on aluminum-based alloys where higher search efforts for an efficient coating to replace CCCs has been done, followed by steel substrates, zinc and magnesium alloys, with copper alloys in last place. Regardless of the type of substrate, the most frequently used precursors were GPTMS, TEOS, and TMOS. Tested OIH materials present a large diversity of compositions and enhanced physical, mechanical, and morphological properties, among others. OIH materi- als show strong potential and are clearly becoming a reality as serious candidates for numerous applications such as smart coatings with ‘‘intelligent’’ self-healing properties and functional protective coatings. More-
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  • Fall '19
  • 1984, oIh, OIH coatings

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