Methods of Coating

Dip Coating

One 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.

One 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 conductivity.

Phenolic 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 epoxy coatings.

An 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 thermal conductivity.

Acrylics 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.

Like 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 systems.

Conversion 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.

Galvanized and other Coatings

Galvanizing 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 10° F increase in temperature and since conditions are rarely consistent, don’t assume twice as much zinc will last twice as long in providing corrosion resistance.

The metals commonly used in coil construction are aluminum, copper, cooper-nickel, carbon steel and stainless steel. Certain conditions must be avoided with each.

Aluminum 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 present.

Copper 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.

Carbon steel is normally used with ammonia, but it must be avoided around acidic conditions and around chlorides.

Stainless 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 series).

Austenitic 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.

Although 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.