of the most widely used coil coating methods is dipping -- where the entire
assembled coil is submerged in a coating, either solvent or water based.
Conventional liquid coating must "wet" the substrate to achieve good
adhesion. Electrodeposition ("E" coat) requires a "clean"
surface. This cleanliness requires a multi-stage cleaning process prior to
E-coating. The substrate is positively or negatively charged and the liquid is
opposite charged. The substrate is rinsed and then baked to cure the coating.
E-coat coatings provide excellent adhesion and corrosion resistance, but they
are expensive (materials and operating equipment). Coil performance degradation
varies from hardly noticeable to significant deterioration due to type of resin
(various thermal conductivity) and coating thickness.
of the best dip coatings is epoxy. Epoxy coatings (moderate to high-priced) have
excellent adhesion properties and corrosion resistance, but are often brittle.
Epoxies are usually two-part coatings that have a very short "pot"
life. They also are typically very thick. Epoxies may cause a noticeable
reduction in coil performance due to coating thickness and to a low thermal
coatings (moderately priced) provide excellent adhesion and chemical resistance
(except against alkalies (bases) and chlorides) with medium film thickness.
Phenols are usually very thick and difficult to apply to finned coils. Heresite
is an example of phenolic coating. Coil performance is slightly better than with
enamel coating cures by chemical cross-linking of its base resin. This curing is
usually initiated by higher temperatures or by moisture. Enamels are not
resoluble in their original solvent after curing. Enamels -- a moderately priced
coating -- provide good to excellent adhesion, weathering, and chemical
resistance depending on the base resins (usually polyesters or polyurethanes
with thin to medium film thickness). Enamels reduce the coil performance less
than epoxies or phenols because of the thinner coating thickness and a higher
and alkyds provide good corrosion protection at moderate cost. Acrylics provide
excellent weathering characteristics, adhesion, corrosion resistance and
hardness. Alkyds provide good weathering characteristics, low cost, ease of
application and thin film. Acrylics and alkyds have similar coil performance
effects as enamels.
E-coatings, the substrate is positively or negatively charged and the coating is
opposite charged. The coating may be a liquid or a powder and is sprayed onto
the substrate. Electrostatic coatings do not perform well on evenly spaced, irregular
shaped surfaces, such as between fins. This is the "Faraday Effect."
Therefore, electrostatic coatings are not recommended for corrosion protection
where the "Faraday Effect" may be a problem. The coated substrate is
then baked to cure the coating. Like E-coatings, electrostatic systems require a
multi-stage cleaning process, but provide excellent adhesion and corrosion
resistance. However, they are expensive (materials and operating equipment) and
do not provide complete coverage inside finned coils. Powder coating
applications also require a clean, humidity-controlled environment.
Electrostatic coatings may be phenol, enamel, acrylic or alkyd-based resin
coatings can be classified as fairly corrosion resistant. The coatings are
formed from the base metal by a chemical reaction with some other material. The
natural aluminum oxide that forms from weathering of aluminum is one example,
but is not considered a conversion coating because of the extended time required
for the chemical reaction. Some examples are irriditing, anodizing, etc., of
aluminum. These are similar processes for creating an aluminum oxide coating on
the aluminum surface. Because of the processes involved, only aluminum can be
anodized. Therefore, the aluminum fins must be anodized before coil assembly.
and other Coatings
of carbon steel is a sacrificial coating -- the coating is applied to the steel
to be sacrificed instead of the steel being corroded. The amount of coating is
designated by G60, G90, etc., to specify the amount of zinc (galvanized) in
ounces/sq. ft. of sheet (including both sides of sheet metal). The more zinc
that is present, the longer the resistance to corrosion. However, since chemical
reactions double in reaction rate with every 10į F increase in temperature and
since conditions are rarely consistent, donít assume twice as much zinc will
last twice as long in providing corrosion resistance.
metals commonly used in coil construction are aluminum, copper, cooper-nickel,
carbon steel and stainless steel. Certain conditions must be avoided with each.
will corrode when galvanic conditions exist with the other metals, especially
with copper or cooper-nickel, since aluminum is an active metal. Also, aluminum
must be avoided in highly alkaline and in highly acidic environments because of
chemical corrosion. But because of its naturally occurring oxide protection,
aluminum is widely used in the HVAC industry. It can be safely used with air,
water, slightly alkaline or slightly acidic environments. It can also be used
with copper in dry or wet environments if a corrosive electrolyte is not
and cooper-nickel will not normally corrode when galvanic conditions are present
since they are passive metals. However, ammonia, amines, strong acids, sulfides
and moist carbon dioxide are very corrosive to copper. Copper, since it is a
soft metal, is subject to erosion with abrasive substances. Even water at a
velocity of greater than 6 ft./sec can erode copper. Copper and cooper-nickel
may be used in refrigerant, water, oil, or air service as long as the velocity
is not excessive. Copper may also be used in weak acidic environments.
steel is normally used with ammonia, but it must be avoided around acidic
conditions and around chlorides.
steel is normally considered a corrosion resistant metal, but it is very
susceptible to stress corrosion cracking, especially in chloride environments.
Stainless steels are normally divided into three groups: austenitic (AISI 200
and 300 series), ferritic (AISI 400 series), and martensitic (also AISI 400
stainless steels are nonmagnetic and are used when corrosion resistance and
toughness are primary requirements. Ferritic alloys are magnetic and are
typically used where moderate corrosion resistance is required and where
toughness is not a major need. These alloys are also used where chloride SCC may
be a problem because they have high resistance to that type of corrosion failure.
Martensitic steels are in the AISI 400 series and are also magnetic. They are
less resistant to corrosion than the austenitic or ferritic grades. Martensitic
stainless steels are used where strength and/or hardness are of primary concern
and where the environment is relatively mild from a corrosive standpoint. All
stainless steels are subject to; corrosion in an oxygen free environment, SCC
and hydrogen embrittlement. Super stainless steels or duplex stainless steels
have been developed for extremely corrosive environments. These alloys have
enhanced levels of chromium, nickel and molybdenum. For example, Alloy AL 29-4C
has been developed for the condensing furnace industry, a highly corrosive and
high temperature environment.
several environments have been mentioned that are safe or must be avoided, every
application should be evaluated individually. Slight changes in environments,
such as addition or combination of certain ions or chemicals, may drastically
alter the corrosion and the corrosion rate of the system materials. The customer
must be aware of the application and its environment and must communicate these
to the manufacturer.