Page 624 - Corrosion Engineering Principles and Practice

CHAPTER 14

Protective Coatings

14.1 Types of Coatings
The coating of surfaces is so ancient that its beginnings are lost in the
mists of history. Paleolithic cave drawings exist which are reputed to
be tens of thousands of years old. They were applied by fingers, a
splayed twig, or, as some believe, were sprayed by blowing pigments
through a hollow reed. The first evidence of coatings used for
protection can be traced back to the Egyptians when they began using
varnishes around 4000 BC. Polychrome Greek statues became
common by 300 BC, and evidence exists that the Romans used
coatings for both decorative and protective purposes. Coated Chinese
artifacts are even more ancient than those in Western Europe.
The use of organic or metallic barrier coatings between a possibly
aggressive environment and a material surely has become the most
widely used method of protecting most engineered materials. There
are three general classes of coatings: organic, inorganic, and metallic
coatings. However, protective coatings often refer to integrated
multifunctional systems that may combine more than one type of
coatings.
Organic coatings protect more metal on a weight basis than any
other means of corrosion protection. Besides providing protection by
creating a physical barrier between the metal and the environment,
these coatings may also contain corrosion inhibitors or other additives
to stifle corrosion processes. Organic coatings include paints, resins,
lacquers, and varnishes.
According to the U.S. Department of Commerce Census Bureau,
the total amount of organic coating material sold in the United States
in 1997 was 5.56 × 10 L, at a value of $16.56 billion. The total sales can
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be broken down into architectural coatings, product original
equipment manufacturers (OEM) coatings, special-purpose coatings,
and miscellaneous paint products, with approximately one third of
the total sales for the main purpose of corrosion protection [1].
Inorganic coatings include enamels, glass linings, and conversion
coatings. Porcelain enamel coatings are inert in water and resistant to

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