Anticorrosive coatings including electrochemical techniques

In order to provide a long service life for an anticorrosive coating system, the protecting working mechanisms and the mechanisms of coating break down need to mapped.

An anticorrosive coating system usually consists of multiple layers of different coatings with different properties and purposes. Depending on the required properties of the coatings system, the individual coats can be inorganic, organic and even metallic.

  • The function of the primer is to protect the substrate from corrosion and ensure good adhesion to the substrate. For this reason, metallic zinc or inhibitive pigments are often formulated into coatings applied as primers for structures situated in the splash zone or in atmospheric environment. For immersed or buried service, pure barrier (non-zinc pigmented) coatings are usually used as primers.

  • The function of the intermediate coat is generally to build up the thickness of the coating system and impede transport of aggressive species to the substrate surface. Lamellar pigments, such as glass, micaceous iron and aluminum flakes are usually used to improve the barrier properties.

  • The topcoat is exposed to the external environment and must provide the surface with the required colour and gloss as well as withstand alternating weathering conditions and, in some service conditions, impact from objects.

The conceptual model of the degradation of organic coatings with no apparent defect on steel in a neutral electrolyte assumes that cathodic blistering occurs following the transport of ions through conductive pathways. The cations are believed to diffuse through the coating because of an attack by water in low molecular weight or low crosslinked regions, followed by interconnection of these regions. Subsequently, cations migrate through the conductive pathways to the metal surface, where they migrate along the coating-metal interface from the defect to the cathodic sites to neutralize the hydroxyl ions generated by the cathodic reaction. The result is blister formation and sometimes coating delamination which causes exposure of the steel substrate to the environment.

The disciplines involved in understanding the working mechanisms of anticorrosive coating systems and the break down mechanisms are mainly diffusion processes, electrochemistry and colloid and surface chemistry.

Current projects incorporate electrochemical techniques as well as non-destructive surface/interface analysis techniques for the coating characterisation and the coating performance evaluation. Through the mechanistic understanding of the coating degradation behaviour, we aim at the advancement of the development of sustainable functional coatings.