Silane coatings with active species

The modification of silane coatings with species able to inhibit corrosion introduces a new functionality to organic coatings active corrosion protection. This can overcome the typically inert nature of the silane film, making it more efficient in combating corrosion processes. 

The first approaches proposed in the literature [54,55] consisted of the addition of alumina or silica particles to improve the mechanical properties of the silane coating. The addition of these particles increased the impact, scratch and wear resistance. The corrosion resistance of aluminium alloys also seemed to increase with controlled amounts of particles [54]. This effect was attributed to the formation of silicate species that delayed corrosion activity. Silane films containing silica and formed under applied potential also revealed improved anti-corrosion behaviour when applied to aluminium substrates. In this case, critical silica contents were proposed [55]. 

The addition of active ions, i.e. ions with well-known corrosion inhibition ability like cerium, zirconium or lanthanum to the silane formulations has been proposed previously by the current authors [61,62]. The aim of this procedure is to introduce a new functionality in the silane coating, making it more effective against corrosion. It is expected that these active species become trapped in the siloxane network, being released to the active corrosion sites, where they can develop its anti-corrosion ability. Furthermore, an improvement in the barrier properties of the coating is also expected due to its reduced porosity, increased thickness and decreased conductivity. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

This review is concerned with organosilanes and therefore purely inorganic Si/O/H species are not considered. Nevertheless, because of the importance of silane oxidation and of oxidized silicon coatings there is considerable interest and activity in this area. Four theoretical papers25,80-82 have been published concerning stabilities of the potentially important molecules in these processes. Needless to say there are very few experimental data in the area under consideration. 

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