Page 671 - Corrosion Engineering Principles and Practice

624 Chapter 1 4 P r otective Coatings 625

Also to be considered are the specialty mortars derived from the
acid-resistant cements. These mortars can be applied to steel or
concrete to provide remarkable acid, alkali, and heat resistance. They
may be reinforced with wire mesh to sustain the heavy thicknesses
normally applied. Coefficients of expansion close to that of steel have
been attained to resolve one of the major problems when applying
inorganic materials on steel. These can be used as floors, tank interiors,
in scrubbers, and other process equipment of clean interior design.
Glass, in one formulation or another, is usually applied as a slurry
and in a production line, but seldom for maintenance purposes. Once
applied and as long as they are protected from mechanical abuse,
glass surfaces are highly resistant to acidic and many mildly alkaline
corrosives. They are also easily cleaned but difficult to repair. Glass-
lined and -coated pipe, valves, pumps, and vessels of all kinds are
widely used in the chemical, pharmaceutical, and food industries. As
with the basic borosilicate glass from which the coatings are derived,
the materials may be used up to about 175°C in most acid mixtures
other than those containing fluorides.

14.7.3 Anodizing
Anodizing involves the electrolytic oxidation of typically magnesium
or aluminum surfaces to produce an oxide scale that is thicker and
more adherent than the naturally occurring film. An anodic electrical
current passing through an electrolytic bath converts the metal
surface to a durable aluminum oxide. The difference between plating
and anodizing is that the oxide coating is integral with the metal
substrate as opposed to being a metallic coating added by deposition.
The oxidized surface is hard and abrasion resistant, and it provides
some degree of corrosion resistance. Chapter 5 contains much more
details on this topic.

14.7.4 Phosphatizing
One very successful treatment of steel has been by phosphatizing,
also called phosphating or phosphate conversion coating. The
coatings consist of a thick porous layer of fine phosphate crystals,
tightly bonded to the steel. For many years, car bodies have been
given this treatment prior to coating. The steel is pickled or otherwise
well cleaned and immediately held in a bath of hot phosphoric acid
containing zinc and perhaps manganese salts, plus other selected
additives. A number of proprietary treatments such as parkerizing
and bonderizing are available for use on steel.
The coatings do not provide significant corrosion resistance when
used alone, but they provide an excellent base for oils, waxes, or
paints, and they help to prevent the spreading of rust under layers of
paint. Phosphating should not be applied to nitrided or finish-
machined steel, and steel parts containing aluminum, magnesium,
or zinc are subject to pitting in the bath. Some restrictions also apply