Environmental Engineering Reference
In-Depth Information
ric acid produces chromate on the surface. Zinc chromate imparts a light yellow
color and increases the corrosion resistance in atmospheric exposures.
4.3.3 Organic Coatings
Paints, lacquers, and varnishes constitute organic coatings of which paints of
varied formulations are in use. Coal tar or asphalt is another member of this
group and probably the earliest one applied on steel to prevent rusting. It is still
in use for protection against corrosion. The cathodically protected buried pipe-
lines in soils provide an example.
Paint
is a mixture of particles of pigment
suspended in a continuous organic or aqueous vehicle. The most common natural
vehicle is a drying oil such as linseed or tung, each of which when exposed to
air oxidizes and polymerizes to solids. Modern paint formulations use synthetic
resins with a solvent—water or alcohol—as vehicles. Some catalysts are some-
times added to hasten the process of polymerization.
Lacquers
are resins dis-
solved in a volatile thinner and
varnishes
are a mixture of drying oil, dissolved
resins, and a volatile thinner.
Pigments are usually particles of inert inorganic compounds. Examples are
oxides such as TiO
2
,Pb
2
O
3
,Fe
2
O
3
, ZnO
2
, etc., and other compounds such as
ZnCr
4
, BaSO
4
, PbCO
3
, clays, etc. Metallic zinc powder or flake is also used as
a pigment. As the vehicles dry up and polymerize, they form a solid coating
along with the bonded pigment particles on the metal substrate. Pigments confer
the bulk, barrier, opacity, and color to the coating. Some pigments are inhibitors
and provide additional protection to the metal surface. Metallic zinc provides
cathodic protection to the steel substrate. Copper and arsenic, and some other
toxic compounds as well, are used as pigment in marine paints and their release
from the paint limits the growth of marine organisms.
Distinction should be made between zinc-pigmented paints and zinc-rich
paints. In the former, pigments constitute about 80% of the paint, of which 20%
is ZnO
2
. Zinc-rich coatings have loadings of zinc dust, often over 90% by weight.
Zinc-rich coatings provide excellent galvanic protection in aggressive environ-
ments. They are supplied as inorganic or organic zinc, the difference being the
vehicle in which the zinc fillers are carried. Silicates are common vehicles for
inorganics which, when reacted, produce an electrically conducting matrix in
which the particles of zinc are embedded. Chlorinated rubber, catalyzed epoxy,
polystyrene, and polyurethane are the recommended organic vehicles for zinc.
Zinc-rich coatings have a long life and are more economical than some other
high-quality three-coat systems. These are used to protect ship hull superstruc-
tures, marine structures, highway bridges, chemical process plant equipment, and
other installations exposed to high humidity and salt.
Paints are broadly classified as primers and top coats. Primers are applied
