Overview

Although polymer coatings have for a long time been used in anticorrosion techniques, the mechanisms of their protective effect have not been understood in sufficient depth so far. This lack of knowledge is explained by the high level of intricacy of the metal substrate-coating-medium junction as an electrochemical system and by the abundance of the phenomena that affect processes running in this system.

For a long time there existed a primitive representation of the corrosion inhibition mechanism by coatings as a physical barrier isolating a metal substrate from the hostile environment. It was believed that neither metal origin nor its chemical properties affected the corrosion kinetics of the substrate under a coating. Later on it has been found out that corrosion of metal substrates starts long before damage and peeling of the substrate surface. Besides, it has been established that the same coating protected alloyed steels (Cu, Cr, Ni) for longer periods than usual carbon steels. The period of protection turned out to be dependent on the finishing quality of the substrate surface. Passivation of the latter under the coating makes the protection last still longer. It was acknowledged finally that the polymer coating should be considered as a multifunctional element of a specific electrochemical system and that corrosion grows under the polymer film adhering to the substrate electrode.

The transfer processes of electrolyte ions in this system are not considered as macro-kinetic but at the level of molecular diffusion. The corrosion rate of the substrate depends largely on...