There are a number of things which happen to
coatings at high temperatures or cyclic high temperatures. Chloride containing
polymers, such as vinyl chloride and chlorinated rubber, tend to break down over
a period of time (depending on the temperature), releasing hydrochloride acid
which can severely aggravate a corrosive condition.
There are other polymers that do not contain a chloride atom in the molecule
which may tend to depolymerize and even appear to evaporate from the surface.
Some styrene polymers exhibit this characteristic . Other cross-linking polymer,
such as epoxies, often over-cure at high temperatures and become very brittle,
shrink and loose adhesion.
The inorganic zinc compounds are quite stable to temperature in the area of 370
C (700 F) and have been used continuously at such temperature quite effectively.
Silicone polymers also have excellent heat resistance in the same temperature
range. For relatively short time exposures (e.g. days or weeks, but not
continuously), inorganic coatings coated with silicone aluminum topcoats have
been exposed to 520 C(968 F) and have maintained their effective coating
characteristics. Some nonzinc coatings have also shown good resistance to higher
temperatures and alternate high and low temperatures.