Page 659 - Corrosion Engineering Principles and Practice

612 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 613

depends on the form of the silicate during the curing period.
Examples are
• Postcured silicates: Soluble silicates are combined with zinc
dust to form very hard rocklike films that are further stabilized
by reacting them with an acidic curing agent.
• Self-curing water silicates: In this case, the soluble alkali silicates
are combined with colloidal silica to improve the curing
speed. Once applied to a surface they develop water
insolubility by reacting with carbon dioxide and moisture
from the air.
• Self-curing solvent-based silicates: These binders are organic
esters of silica that are converted from a liquid form to a solid
by reaction with moisture from the air, forming a very hard
and corrosion-resistant binder. A major advantage of these
materials is their conversion to rain- or moisture-resistant
form shortly after their application.

Pigments
Pigments are essentially dry powders that are insoluble in the paint
medium and that consequently need to be mixed in it by a dispersion
technique. They range from naturally occurring minerals to artificial
organic compounds. Pigments contribute several properties essential
to the effective use of protective coatings. Several different pigments
may be used within the same coating, all of them contributing to the
coating’s general characteristics to perform important functions such
as providing

• Color
• Protection to resin binder
• Corrosion inhibition
• Corrosion resistance
• Film reinforcement
• Nonskid properties
• Sag control
• Increased coverage
• Hide and gloss control
• Adhesion
Zinc phosphates are now probably the most important pigments
in anticorrosive paints. The selection of the correct binder for use
with these pigments is very important and can dramatically affect
their performance. Red lead is likely to accelerate the corrosion of