Page 661 - Corrosion Engineering Principles and Practice

614 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 615

• Water: The recent regulations to reduce the emission of
volatile organic compounds (VOCs) produced by organic
solvents are forcing the coating industry to reconsider the
applicability of water as a solvent. The most common water-
borne coatings used for application to metals are air dried
or force dried at temperatures below 90°C. A wide range of
coating formulations falls into this category. The most
commonly available technologies are water-reducible
alkyds and modified alkyds, acrylic latexes, and acrylic
epoxy hybrids.

14.6 Temporary Preservatives
A type of coating use which has grown steadily in use and diversity
is temporary preservatives. The term temporary can mean intervals
from hours to months or years during which metallic components
are protected against specific indoor or outdoor environments.
Such supplementary protection is often provided to surfaces that
already have some form of permanent or semipermanent protection
such as cladding or conversion coating. Some supplementary
protection may be in the form of a fluid that can be easily applied
or removed. Other temporary preservatives are low vapor pressure
materials that will inhibit corrosion effects on systems stored in
confined environments. All these products expected to be replaced
or refreshed periodically during the life of a system. Jointing
compounds, sealants, and corrosion prevention compounds are
examples of this type.

14.6.1 Jointing Compounds and Sealants
Jointing compounds are used for protection at joints where they
act by excluding dirt and moisture and by providing a reservoir of
soluble corrosion inhibitors. Sealants are applied to joints to
prevent the escape of fluids, such as fuel, but they also exclude
moisture. Jointing compounds must remain flexible to allow easy
disassembly of parts. Various synthetic resins are used for this
purpose. The compounds harden sufficiently at edges to allow
paint adhesion, but they remain tacky within the joint so that
flexure does not cause cracking. The inhibitive sealants are very
effective when used in faying surfaces and butt joints, for wet
installation of fasteners, and over fastener patterns. They are also
effective at insulating dissimilar metals.
14.6.2 Corrosion Prevention Compounds
Corrosion prevention compounds (CPCs) are fluids that can both
prevent new corrosion sites from forming (Fig. 14.6) and, more
importantly, suppress any corrosion that has initiated. CPCs are