It is widely known that corrosion causes direct and indirect costs amounting to about 6% of the gross domestic product (GDP) of a developed country. Organic coatings are the major protective measure accounting for up to 90% of expenditure on corrosion protection [1].

In order to increase the service life of organic coatings it has been good practice for many decades to add special pigments to the primer (the first coating layer on the substrate) which should increase the duration of corrosion protection. The purpose of these so-called active pigments is actively to influence the electrochemical processes taking place during the initiation of corrosion at the primer–substrate interface and at all later stages of degradation of the coating.

It must be accepted that it is impossible to provide permanent protection to thermodynamically unstable low-alloyed steel. In all cases, it is only a question of time until the particular atmospheric conditions lead to irreversible water uptake, blister formation, delamination, crack formation and finally rust (Fig. 1.1 shows the ‘final stage' of coating failure).

Corrosion protection with organic coatings uses a wide range of different strategies and materials, e.g. multi-coat systems such as in Fig. 1.2 or high-build single coats of more than 1 mm thickness. Solvent-based materials still dominate for heavy duty corrosion protection such as outdoor applications in coastal zones but water dilutable coating materials or powder coatings are gaining increasing importance when controlled application conditions are available, e.g. in specialised paint workshops.

In all branches of the industry, including the manufacture of cars and other forms of transportation (bus, truck, and rail), construction companies, shipyards and chemical plants, particular strategies for corrosion protection have been developed and all use, to some extent, organic coatings with active pigments. Some of them, for example, in steel construction, use coating systems similar to that shown in Fig. 1.2, where zinc particles act as an active pigment.

Although active pigments have been used for decades, not much is known about the physico-chemical mechanism of their protective properties. This is true except for zinc dust primers, red lead oxide primers and chromate primers, where the basic mechanisms have been investigated and largely agreed by the scientific community. Unfortunately, red lead oxide and chromate pigments can no longer be used due to environmental concerns and safety at work restrictions (due mainly to their toxic and carcinogenic properties). The protection mechanisms for highly pigmented zinc dust primers have been quite well developed and will be presented below.

In recent years, it has become increasingly popular to talk about ‘self-healing' in the context of active pigments and in fact, new concepts and materials are under investigation and development to complement the range of usable substances. These new concepts will briefly be explained in Section 1.8 of this paper.

Edited by: L. Fedrizzi, W. Fürbeth, F. Montemor