Page 655 - Corrosion Engineering Principles and Practice

608 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 609

Some of the main variables used to design corrosion protective
coatings are
• Impermeability: An ideal impermeable coating should be
completely unaffected by the specific environment it is
designed to block, be it most commonly humidity, water, or
any other corrosive agent such as gases, ions, or electrons.
Such coating should have a high dielectric constant and also
have perfect adhesion to the underlying surface to avoid any
entrapment of corrosive agents.
• Inhibition: In contrast with coatings developed on the basis of
impermeability, inhibitive coatings function by reacting with
a certain environment to provide a protective film or barrier
on the metallic surface.
• Cathodically protective pigments: As with inhibition, cathodic
protection in coatings is mostly provided by additives in the
primer. The main function of these additives is to shift the
potential of the environment to a less-corrosive cathodic
potential. Inorganic zinc-based primers are good examples of
this concept.
For serious corrosion situations, the coating system approach
(primer, intermediate coat, and topcoat) provides all the ingredients
for a long-lasting solution.

Primers
The original term “primer” implied a coating first applied to a
substrate because it had a singular adhesive affinity for it and/or
because it provided better adhesion for a subsequent coat than the
subsequent coat could achieve if applied directly. A primer also
functioned as a carrier for an inhibitor or for an anodic metal
loading such as zinc, both of which will be discussed further in
later sections of this chapter.
In modern usage and for many applications, primers usually are
thin films (75 m m or less), and may be used on both metal and wood
surfaces. On wood, their function is to seal the grain or to provide a
smooth base for topcoats. On concrete, they should be compatible
with the alkaline surface and help improve both adhesion and the life
of subsequent coats.

Secondary or Intermediate Coatings
A second coat may be either the top or final coat, or an intermediate
coat added when multiple thin films are required. The intermediate
coat must serve as a tie coat between the primer and the subsequent
coats, and may be of a different composition than either of them. When
the three are of different formulations, it is the intermediate coat that
is often used to provide the major film thickness for the coating.