© 2006 by Taylor & Francis Group, LLC

108 Corrosion Control Through Organic Coatings
have a coefficient of thermal expansion that is twice that of aluminium or zinc and
four times that of steel [29].
Another factor that must be considered at elevated temperatures is the glass
transition temperature (T g) of the polymer used in the binder. This is the temperature
above which the polymer exists in a rubbery state and below which it is in the glassy
state. Using coatings near the T g range is problematic, because the binder’s most
important properties change in the transition from glassy to rubbery. For example,
above the T g, polymer chain segments undergo Brownian motion. Segments with
appropriate functional groups for bonding are increasingly brought into contact with
the metal surface. An increase in the number of bond sites can dramatically improve
adhesion; wet adhesion in particular can be much better above the T g than below it.
Increased Brownian motion is also associated with negative effects, such as
increased diffusion. Above the T g, the Brownian motion gives rise to the continuous
appearance and disappearance of small pores, 1 to 5 nm or smaller, within the binder
matrix. The size of these small pores compares to the ‘‘jump distance” of diffusing
molecules — the distance that has to be covered by a molecule moving from one
potential-energy minimum to a neighboring one in the activated diffusion process.
The permeation rate through these small pores is linked to temperature to the same
degree that the chain mobility is. That is, the chain mobility of elastomeric polymers
shows a high degree of temperature dependence and thus favors activated diffusion
at higher temperatures. As the crosslink density of the binder increases, segmental
mobility decreases, even at elevated temperatures. Diffusion still occurs through
large pore systems whose geometry is largely independent of temperature. The
temperature dependence of diffusion in highly crosslinked binders is a result of the
temperature dependence of the viscous flow of the permeating species.
Miszczyk and Darowicki have found that the increased water uptake at elevated
temperatures can be to some extent irreversible; the absorbed water was not fully desorbed
during subsequent temperature decreases. They speculate that the excess water may be
permanently located in microcracks, microvoids, and local delamination sites [29].
6.4 CHEMICAL DEGRADATION
All breakdowns in polymers could, of course, be regarded as chemical degradation
of some sort. What is meant here by the term ‘‘chemical degradation’’ is breakdown
in the paint film that is induced by exposure to chemical contaminants in the
atmosphere.
Atmospheric contaminants play a more minor role in polymer breakdown than
do UV exposure, moisture, and (to a lesser degree) temperature. However, they can
contribute to coating degradation, especially when they make the coating more
vulnerable to degradation by UV light, water, or heat.
Mayne and coworkers have shown that the organic coating and the ions (e.g.,
sodium, potassium, calcium) in a solution interact, causing a gradual reduction in
resistance of the coating (the “slow change,” see Chapter 1, Section 1.2, “Protection
Mechanisms of Organic Coatings”) [30-34]. One interesting aspect is that, as long
as it doesn’t go too far, the reduction in resistance is reversible. The process is of
course accelerated by heat; raising the temperature increases the amounts of ions
© 2006 by Taylor & Francis Group, LLC