A discussion on HVAC coil coatings and corrosion

Many HVAC manufacturers, distributors and contractors may not realize that hundreds of thousands of coil failures have occurred during the last decade from corrosion. The cause is most typically environmental pollutants, which range anywhere from salt-air, to household cleaning agents, pesticides, formaldehydes, building materials, and even off-gassing of food. Each of these contaminant sources can initiate corrosion in coil tubing in a year or less when the conditions are right.

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For example, refrigeration coils were continually failing in a South American fruit processing plant's banana room that used fruit-ripening ethylene gas generators. Gaseous by-products from the catalytic generator combined with the moisture in the ripening area formed a weak acid that resulted in pinhole leaks in the coil tubing after a year or less.

Aside from fruit processing plants, most coastal area HVAC equipment ' whether it's commercial or residential ' is also bombarded with corrosion from ocean salt.

Two Common Types of Coil Corrosion
The two most common forms of coil corrosion are pitting and formicary. These two corrosive processes can occur in as little as a few weeks after installation. More typically, corrosion will begin appearing within a one- to four-year period. The ability to distinguish pitting from formicary corrosion might help detect and eliminate the cause.

Pitting corrosion is typically caused by the presence of chlorides or fluorides. Chlorides are found in numerous items such as snow-melting crystals, toilet bowl/tile cleaners, dishwasher detergents, fabric softeners, vinyl fabrics, carpeting, paint strippers, etc. Fluorides are used in many municipal water treatment plants. Unlike formicary corrosion, pitting is usually visible on the exterior of the copper tube with the naked eye. Pitting is caused by an aggressive attack of negatively-charged chloride/fluoride ions carried to the metal surface by condensate. The negative ions attack the oxide film the metal usually uses to protect itself, essentially forming a corrosion driven battery that consumes the copper. After pits have formed in the copper, they will progress through the thickness of the copper tube until a pinhole is formed causing the coil to leak refrigerant.

Formicary corrosion is caused by organic acids such as acetic and formic acids. Acetic acids are abundant in numerous household products such as adhesives, paneling, particle board, silicone caulking, cleaning solvents, vinegar, foam insulation and dozens of other commonly found products in the home or commercial/industrial workplace. Formic acid can be found in cosmetics, disinfectants, tobacco and wood smoke, latex paints, plywood, and dozens of other materials. The corrosion caused by these substances is usually not visible to the naked eye, although black or blue-gray deposits can sometimes be seen on the surface. Formicary corrosion can form a sub-surface network of microscopic corroded tunnels within the tubing wall resembling ant nest-type structures that are substantially larger than the surface pinholes above them. Eventually one or more of these tunnels will progress to the surface of the copper and form a pinhole which quickly results in coil leakage.

Choosing the Right Coating For the Job
The first step a contractor must take when confronted with coil corrosion is to determine if it will happen repeatedly when coils are replaced. It's a difficult diagnosis to determine if the coil corrosion is a one-time phenomenon or a continuing problem in that particular location. In the case of the banana processing plant or a coastal area unit, coils most likely will continually corrode, and their replacement units should have a protective coating. In less corrosive environments, owners should attempt to eliminate in-home corrosive elements such as cleaning solvents from the return airstream by storing them in areas that are not near a return duct. These actions might eliminate the expense and need to coat a replacement unit.

Choosing the most appropriate coil coating for the application could save the project thousands of dollars and eliminate repeat treatments. Choosing the wrong coil coating could reduce heat transfer capabilities and lead to higher energy bills.

Heat transfer is a major issue to consider when coating a coil with any substance, especially in a retrofit application because the coil may no longer perform at its manufactured specification. The thinner the coating, the better the heat transfer, while thick coatings can lead to significantly diminished heat transfer. Another issue is a coating's hydrophobicity, or how well it drains condensation off the coil, to create optimum heat transfer capabilities. Ideally, water would drain quickly off of the coil to avoid reductions in efficiency.

Water accumulation is also detrimental because it can lead to the growth of mold and mildew. Most coatings do not actively resist biological growth, but their hydrophobicity can passively deter such growth.

Four basic coating types are prominent in the HVAC industry:

• Polyurethanes
• Epoxies
• Fluoropolymers
• Silanes

All four types of coatings offer differing degrees of advantages in terms of corrosion resistance, scratch resistance, flexibility, weight, thickness, hydrophobicity, and heat transfer capabilities.

Polyurethane (PU), invented in the s, is used in a variety of applications. It can be manufactured as hard as fiberglass, bouncy as rubber, sticky as glue, and soft as upholstery foam. Many of the off-the-shelf PU-based coil coatings available to the HVAC trade can be applied in the field. PU formulations are fairly inexpensive, less viscous, more flexible, and thinner (typically 25 to 50 microns) than most coatings. The disadvantage is they are not as resilient or long-lasting as other coatings.

Epoxy, or phenolic-based, coatings are generally the cheapest of available coatings. Epoxies, developed in the late s, are known for their excellent chemical and heat resistance, and are well known for coating floors and other surfaces. The high viscosity of epoxy-based systems leads to thicker coatings (approximately 50 to 100 microns) with poor flexibility and adherence characteristics. An epoxy coating is difficult to apply in the field. Therefore, the coil is typically disconnected and then shipped to a factory setting for a professional treatment. Because they are thicker, epoxy coatings will reduce heat transfer from the air to the refrigerant in the coil, and thus cause a decrease in system efficiency and capacity. Epoxy coatings might best suit new installations where heat transfer losses are accounted for in the system design specifications.

Fluoropolymers, first developed in by DuPont under the trade name Teflon, are now available in many different forms under a variety of trade names. Fluoropolymers are known for their high resistance to acids, solvents and bases. They are most effectively applied to metal through electrostatic powder coating or a thermal sintering process, as is done during the manufacturing of cookware and other non-stick products. Additionally, many field-use fluoropolymer sprays are available to contractors. The field-use sprays generally have very poor adhesion and their effectiveness will diminish significantly in a very short period of time. The cost of fluoropolymer-based field-use coatings is typically less than the more advanced epoxy and PU based coating system but their lifetime and effectiveness is very limited. Fluoropolymer coatings applied in the correct manner, through thermal sintering or electro-static powder coating, have not gained traction in the HVAC industry due to the expense of such processes and their inability to be performed in the field.

Silanes are well known as excellent coupling agents where two dissimilar materials such as paint (an organic) and glass (an inorganic material) can be bonded together. A variety of silane chemistries are available, many of which are tailored to have particular characteristics such as flexibility, hydrophobicity, and scratch resistance. Thus, the proper formulation of a silane coating can provide a flexible, resilient glass-like coating with good corrosion resistance and water draining capability that bonds extremely well to aluminum and copper (an inorganic). Silanes form an extremely thin coating when cured (less than 10 microns) that has very little, if any, adverse effects on heat transfer. They are very resilient against cracking and corrosion, are hydrophobic, and reduce airflow friction. Silanes can be quite difficult to apply properly in the field unless a trained applicator is hired to do so. The coil surfaces must be cleaned thoroughly and prepared properly for a successful application and therefore, it is best if the coating is applied at an off-site application center. Although silane coatings are typically somewhat more expensive that the other coatings described, they also exhibit the best heat transfer properties and typically have a much greater lifetime as well.

To conclude, our research indicates that a silane-based coating provides the best protection from the environment, and has minimal impact on heat transfer while remaining a long-lasting barrier that protects a HVAC coil against corrosion for an extended period of time (typically five years or more).

Each of the coating technologies described carry different levels of toxicity. Service technicians planning to apply any of these coatings in the field should be outfitted with proper OSHA equipment and the appropriate breathing apparatus.

About the authors:
Joshua D. Sole, Ph.D., is a senior mechanical engineer, and Alan H. Brothers, Ph.D., is a senior materials engineer at Mainstream Engineering Corporation. Sole and Brothers led an R&D team in developing a ductwork coating for the U.S. Navy that reduces duct airstream friction and improves corrosion resistance, resulting in improved duct lifetime and HVAC system efficiency. Mainstream (www.mainstream-engr.com) is a Rockledge, Fla.-based research and development company with more than 70 patents and 30 current R&D projects with the U.S. military and NASA. Mainstream's HVAC division markets a variety of service products under the Qwik Products brand (www.qwik.com) which are available through traditional HVAC distributors.

About the Author
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Nearly 90 years ago engineers discovered how to unroll a coil of aluminum or steel, apply a primer or a finish coat, and then recoil it before shipping it to be processed.

Since those early days of prepainted metal, many stampers and fabricators have found that it makes sense to form parts from prepainted metal rather than from unfinished metal and paint the parts afterward. Stamping shops forming prepainted material can eliminate staffing and in-house painting operations and reduce the costs associated with adhering to environmental regulations'a good starting point for those wanting to "go green."

Converting to Prepainted Coil

Prepainted coil is a finished product, so it must be managed differently than raw metal. However, only minor process changes are required when converting from a postpainted operation to a prepainted operation. Implementing the following time-proven prepaint practices can help smooth the process and prevent damage to coils and formed parts.

Specifications

The coil coater can help determine the best material specification for a product. It is important to consider the current and proposed manufacturing processes, the life cycle of the product, and the expectations of the consumer.

Handling

There is no need to be fearful of handling prepainted metal. Sound, traditional manufacturing techniques should prevent damage in normal production. However, it is important to remember that prepaint is a surface-finished material, and it should be handled as any finished material would be.

Prepainted coils can be shipped on flatbeds and moved with forklifts and still retain a flawless surface (see Figure 1). In the plant, forklift forks and masts should be padded to protect coil edges. Padding on hooks and other handling devices help keep coils in good shape.

V-shaped coil cars with nonmetallic surfaces are recommended. Turnstiles with modular round supports of hardened steel for coil support are most suitable. Cushioning materials like feltboard and polyurethane help protect coils while they are moved, placed in inventory, or shipped.

If transit or installation damage does occur, repair paints usually are available from the original coating manufacturer.

Welding and Assembly

Dry lubricants provide a way to enhance stamping performance without adding wet or oil lubrication. Current dry coating products and application methods are yielding new results for stampers and metal formers. Because they don't contain oil, dry films provide an excellent surface for welding and other assembly methods, including adhesive bonding.

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Storing

It's important to leave stocked coils banded until they are needed for processing (see Figure 2). Coil bands should be removed with care so they do not snap back on the coil, and proper personal protection equipment, such as long sleeves, gloves, and eye protection that would be used in any sheet metal operation, should always be used. Clips must be on the side of the coil'not the bottom'when the coil is set down.

Storage in a climate-controlled, weatherproof building is recommended. (It's not advisable to store galvanized steel outside, either.) The coil should be protected from direct contact with the ground or with transport vehicles. Floor pads or cushioning materials should be used, and protective films also may be applied at slitting or sheeting if required.

Wrapping coils with stretch wrap, coated paper, fiber-based wrap, or particleboard helps protect them during storage. However, wraps should not form an airtight seal, as this can create a humidity chamber that could lead to condensation damage.

Press Adjustments, Tooling

Prepainted coil is coated on both the top and bottom. Generally this adds 0. to 0. in. (25 to 40 ) to the total sheet thickness, depending on the coating system specified (see Figure 3).

The most common adjustment required is die clearance adjustment. In some cases, die clearance must be increased to accommodate that extra thickness of the coating.

As with any forming operation, total substrate thickness and tooling tolerances are important considerations that need to be understood for proper material feeding and processing. If tolerances are too tight, binding, release problems, scraping, or burr formation can occur through the cycle. When tolerances need to be adjusted to accommodate the prepainted metal's increased thickness, usually only minor tooling changes are necessary.

Tooling should be dedicated to prepainted material or thoroughly cleaned between runs. Dies stamping bare metal can pick up roll oils and metal fines from the raw material. If the tooling is not cleaned before it is used on prepainted metal, it can transfer those oils and fines to the prepainted surface, potentially damaging the coating.

Every point throughout a coil processing line at which tooling or anything touches a finished surface should be taken into account. Using clean equipment and protective films and papers can decrease the chance of damage during stamping.

Padding and bonding strips can be added to shears, press brakes, punch presses, carry tables, and benchtops for protection.

Quality Assurance

Implementing a quality assurance program for press setup and handling and storing coated coils can help ensure customers receive high-quality, uniform prepainted metal stamped components and assemblies. Each substrate and each coating is different and requires unique settings depending on gauge and processing needs.

A good prepainted metal quality assurance program can be established by working closely with the prepainted metal supplier (the service center or coil coater) to ensure optimal press setup for the particular precoated product being processed.

1. Who Is Using Coated Coil?

More than 4.5 million tons of coated steel and aluminum coil are produced and delivered to processors in North America each year. Manufacturers in the appliance, automotive, transportation, building construction, office furniture, lighting fixture, and machinery industries use prepainted metal to manufacture products such as ranges, refrigerators, agricultural equipment, heating and cooling units, metal roofs, metal buildings, entry doors, garage doors, awnings, carports, and beverage cans.

2. How Is the Coil Painted?

Coil coating lines use a continuous process to unroll each coil, clean it, treat it, apply a primer and finish coatings, cure the paint, and then reroll the coil for shipment for downstream processing. This process applies finishes to coils with widths up to 72 in. at speeds of up to 700 feet per minute. Typically, the finish is applied to both sides of the substrates (seeFigure 4).

3. Which Coatings Are Applied?

The basic types of coating chemistries applied to coil include acrylics, epoxies, fluorocarbons, plastisols, polyesters, siliconized modified polyesters, urethanes, waterborne emulsions, zinc-rich coatings, and treatment and primer combinations.

Acrylics. Acrylics have exceptional flow and film clarity when they are cured, resulting in a very hard, glossy surface that has excellent dirt and stain resistance. They readily accept screen printing, making them highly suitable for screen-printed signs and for trailer side panels in the truck-trailer industry.

Epoxies. Epoxy-based coatings inherently have exceptional adhesion'both to substrates and to other coatings, called intercoat adhesion. They also have excellent chemical resistance and are hard, recoatable, and bondable. Subsequently, an epoxy is one of the first choices for backers, but it can also be used for primers. This paint technology is also used to manufacture beverage cans and food cans.

Fluorocarbons. Fluorocarbon coatings are known for their nonstick and weathering properties. This technology, created more than 50 years ago, is highly suitable for applications that require extreme weathering, stain, dirt, and chemical resistance. Subsequently, these coatings are used on interior and exterior wall, roofing, and other construction materials for which resistance to the elements and longevity are of prime concern. Fluorocarbon prepainted metal can be found in automotive trim applications too.

Polyesters. Polyester coatings are used on both aluminum and steel and can be formulated for both interior and exterior applications. This chemistry is the most versatile coating technology and provides relatively high performance and cost efficiency. Polyesters show good surface hardness, flexibility, and resistance to metal marking, marring, and staining. They are used primarily for appliances, building products, truck-trailer sheet, office furniture, mobile home sheet, awnings, RVs, garage and entry doors, and rainware.

Plastisols. A thick film usually applied at thicknesses from 4.0 mils to 10 mils, plastisols are used primarily in specialty markets and for residential trim and siding. They can be formulated to have a texture or a wood-grain look, known as striated plastisols. Because of the thick film, these coatings also provide excellent chemical resistance.

Siliconized Modified Polyesters. Siliconized modified polyesters are most often used for building construction applications and other exterior exposures. They are specifically designed and formulated for good flexibility, weathering, and chalk and fade resistance. To produce the necessary requirements, coating formulators use ceramic pigments whenever possible to resist the color fading, which results from ultraviolet light exposure.

Treatment, Primer Combinations. Combinations of treatments and primers are used when extra protection against chemical and salt spray corrosion is required to help prevent weathering in aggressive environments, such as industrial and seacoast areas. These combinations are seen on architectural applications such as storefronts, building panels, curtain walls, and roof panels.

Urethanes. Urethane coatings are very flexible and resistant to postbend fracturing. They are preferable as high-performance primers. When they are formulated with corrosion-resistant materials, urethanes provide excellent adhesion and chemical resistance. While most urethanes are used for primers in prepaint applications, they can be formulated as a finish coat, providing excellent scratch and mar resistance, as well as improved flexibility over other paint types. They generally are reserved for the most severe forming situations when other coatings simply do not work.

Waterborne Emulsions. Waterborne emulsions and acrylic-based resin systems are used on steel substrates such as hot-dipped galvanized, Galfan®, Galvalume®, and Zincalume®. These chemistries are low in volatile organic compounds (VOCs). They provide superb adhesion and resistance to water staining and softening and have excellent color and gloss retention. They are often used for commercial building panels, agricultural buildings, industrial construction, pre-engineered buildings, and post-frame structures.

Zinc-rich Coatings

Zinc-rich coatings provide excellent corrosion and weathering protection and are very resistant to salt water and solvents. They are often used in chemical plants, refineries, and coastal or offshore installations.

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