182 Fundamentals of Corrosion
may result. Do not permit application to take place below the temperature
recommended by the manufacturer.
6.1.4 Cure of the applied Lining
Proper curing is essential if the lining is to provide the corrosion protection
for which it was selected. Each coat must be cured using proper air circula-
tion techniques. To obtain proper air circulation, it is necessary that the tank
has at least two openings, one at the top and one at the bottom.
Because most solvents used in lining materials are heavier than air, the
fresh air intake will be at the top of the vessel and the exhaust at the bottom.
The temperature of the fresh air intake should be higher than 50°F (10°C)
with a relative humidity of less than 8%. If possible, the fresh air intake
should be fed by forced-air fans.
A faster and more positive cure will be accomplished using a warm,
forced-air cure between coats and as a nal cure. This will produce a dense
lm and tighter crosslinking, which provides superior resistance to solvents
and moisture permeability.
Before placing the vessel in service, the lining should be washed down
with water to remove any overspray. For linings in contact with food prod-
ucts, a nal warm, forced-air cure and water wash is essential.
It is important that sufcient time be allowed to permit the lining to obtain
a nal cure. This usually requires 3 to 7 days. Do not skimp on this time.
When the tank is placed in service, operating instructions should be pre-
pared and should include the maximum temperature to be used. The outside
of the tank should be labeled “Do not exceed X°F/X°C. This tank has been
lined with Y. It is to be used only for Z service.”
6.1.5 Causes of Coating Failure
Most types of failure are the result of the misuse of the tank coating, which
results in blistering, cracking, hardening or softening, peeling, staining,
burning, and/or undercutting. A frequent cause of failure is overheating
during operation. When a heavily pigmented surface or thick lm begins to
shrink, stresses are formed on the surface and that results in cracks. These
cracks do not always expose the substrate and may not penetrate. Under
these conditions, the best practice is to remove those areas and recoat accord-
ing to standard repair procedures.
Aging or poor resistance to the corrosive can result in hardening or
softening. As the coating ages, particularly epoxy and phenolic amines,
it becomes brittle and may chip from the surface. Peeling can result from
improperly cured surfaces, poor surface preparation, or a wet or dirty sur-
face. Staining results when there is a reaction of the corrosive on the surface
of the coating or slight staining from impurities in the corrosive. The true
cause must be determined by scraping or detergent-washing the surface. If