Improving ECP Adhesion to Oxidizable Metals

An advantage of incorporating ECPs into a binder such as epoxy or polyurethane is the good adhesion that such binders provide, and the actual performance of coatings based on formulation may be due to the fact that the adhesion is better than that obtained by electropolymerization. However, this strategy leads to less contact between metal and ECP and, in view of the protection mechanisms described in Section 16.2, this may be to the detriment of the anticorrosion properties. 

Several strategies have been used to improve adhesion of ECP upon oxidizable metal. Bilayer coatings, composed of two types of ECP deposited electrochemically, are among these strategies and several groups have reported that they provide better protection by enhancing the adherence of the first layer to the metal. Many examples have already been described in Section 16.2 of this chapter. However, the interface between the metal and the first layer remains ill defined with no covalent bonds between the metal and the polymer. 

An ultrathin layer (few nanometers) of material covalently bonded both to the metal surface and to the ECP should constimte an outstanding adhesion primer. This could be 

The same applies to thiol-based pretreatments, which show some potential for specific applications, but also require quite a long time to reach complete coverage of the metal surface through the formation of a well-defined monolayer [149,150]. 

Recently, this difficulty was overcome when the starting diazonium was the aniline dimer (4-aminodiphenylamine or ADPA) [157]. In that case the grafted layer is electro-active and aniline oxidation remains possible in aqueous sulfuric acid and leads to a strongly adherent PANI film on the modified surface. The first experiments were performed on carbon and extension to an oxidizable metal has not yet been reported. However, as it has been proven that the covalent grafting of diazonium salts was also successful with iron and other oxidizable metals [178-182], we think that the same procedure might be applied in the case of iron. A strong increase of the adhesion should be expected, and therefore an improvement of the protection properties. 

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