Resistance inhibition, which is also part of the barrier mechanism, may
supply additional protection. Inhibiting the charge transport between cathodic and
anodic sites retards the corrosion reaction. An increase in the electronic resistance
and/or the ionic resistance in the corrosion cycle may reduce the reaction rate. The
electronic resistance may, for example, be increased by the formation of an oxide
film on the metal. This is the case for aluminum substrates. The application of
organic coatings on a metal surface results in an increase of the ionic resistance.
One of the weak points of organic coatings in corrosion prevention is the fact
that these coatings are relatively easily damaged under mechanical and thermal
load. This may cause corrosion under the paint film at and near the damaged site.
The otherwise adequate barrier properties of the coating will no longer give sufficient
protection. Active pigments are then often incorporated in the polymer matrix of
the first coating layer near the substrate: the primer. These pigments (passivating,
blocking, or galvanic) provide protection through an active inhibitive mechanism
immediately when water and some corrosive agent reach the metal surface.
Again, protection can result only if the adhesion of the coating is good. Also, the
mechanical properties (e.g., the glass transition temperature) of the polymer reflect to
some extent the quality of the coating, as these determine the formability of coated
substrates (e.g., for coil-coated products) and their sensitivity to external damage.
Water Permeation
All organic coatings are to some extent permeable to water. The effective
permeability is closely related to the polymer structure and composition. The
permeability of a coating is often given in terms of the permeation coefficient P.
This is defined as the product of the solubility of water in the coating (S, kg/m
3
), the
diffusion coefficient of water in the coating (D, m
2
/s), and the specific mass of
water ρ (kg/m
3
). Different coatings can have the same permeation coefficient even
though the solubility and the diffusion coefficient, both being material constants, are
very different. Therefore the usefulness of the permeation coefficient is limited.
Water permeation occurs under the influence of various driving forces:
A concentration gradient. e.g., during immersion or during exposure to a humid
atmosphere, resulting in true diffusion through the polymer
Osmosis due to impurities or corrosion products at the interface between the metal
and the coating
Capillary forces in the coating due to poor curing, improper solvent evaporation,
bad interaction between binder and additives, or entrapment of air during
application
When a coated system is exposed to an aqueous solution or a humid atmosphere,
water molecules eventually reach the coating substrate interface. Normally, a coating
under immersion will be saturated after a relatively short time (of the order of 1 h),
depending on the values for D and S and the thickness of the layer. Typically values
for D and S are 10
13
m
2
s
–1
and 3% [7–9]. For atmospheric exposure the actual
cyclic behavior of the temperature and the humidity determines largely the periods
of saturation. In any case, situations will result in which water molecules reach the
coating-metal interface, where they can interfere with the bonding between the
two phases, eventually resulting in loss of adhesion and corrosion initiation if a
696 de Wit et al.
Copyright © 2002 Marcel Dekker, Inc.