© 2006 by Taylor & Francis Group, LLC

116 Corrosion Control Through Organic Coatings
certainly because the amount and form of moisture varies drastically from site to
site. The global atmosphere, unless it is locally polluted (e.g., by volcanic activity
or industrial facilities), is made up of the same gases everywhere: nitrogen, oxygen,
carbon dioxide, and water vapor. Nitrogen and carbon dioxide do not affect coated
metal. Oxygen and water vapor, however, cause aging of the coating and corrosion
of the underlying metal. The amount of oxygen is more or less constant everywhere,
but the amount of water vapor in the air is not. It varies depending on location, time
of day, and season [3].
The form of water also varies: water vapor in the atmosphere is a gas, and rain
or condensation is a liquid. To further complicate things, water in the coating can
go from one form to another; whether or not this happens — and how fast —depends
on both the temperature and the RH of the air.
It is often noted that water vapor may have more effect on the coating than does
liquid water. For nonporous materials, there is no theoretical difference between
permeation of liquid water and that of water vapor [4]. Coatings, of course, are not
solid, but rather contain a good deal of empty space, for example:
1. Pinholes are created during cure by escaping solvents.
2. Void spaces are created by crosslinking. As crosslinking occurs during
cure, the polymer particles cease to move freely. The increasing restrictions
on movement mean that the polymer molecules cannot be “packed”
efficiently in the shrinking film. Voids are created as solvent evaporates
from the immobilized polymer matrix.
3. Void spaces are created when polymer molecules bond to a substrate.
Before a paint is applied, polymer molecules are randomly disposed in
the solvent. Once applied to the substrate, polar groups on the polymer
molecule bond at reactive sites on the metal. Each bond created means
reduced freedom of movement for the remaining polymer molecules. As
more polar groups bond on reactive sites on the metal, the polymer chain
segments between bonds loop upward above the surface (see Figure 7.1).
The looped segments occupy more volume and form voids at the surface,
where water molecules can aggregate [5].
4. Spaces form between the binder and the pigment particles. Even under the
best circumstances, areas arise on the surface of the pigment particle where
FIGURE 7.1 Looped polymer segments above the metal surface.
© 2006 by Taylor & Francis Group, LLC
(a)
(b)