Local conditions can have a high impact on corrosion, especially in areas that have experienced heavy coastal rains, like this year on the Eastern seaboard; those with local sources of industrial pollution, including agricultural pollution; and those with poorer water quality (i.e., well water). (Note: In cases of flooding, AHRI recommends complete equipment replacement if it was submerged.)

Service and maintenance employees can do a lot to protect their customers’ equipment from the potentially devastating costs of corrosion by thoroughly cleaning and rinsing their coils with good water, inspecting them closely for signs of corrosion, and possibly applying a protective product if conditions seem to warrant it.

Installing contractors can protect ACR coils from internal corrosion, like formicary corrosion, by making sure that new systems are made as clean as possible, especially if flux, solder, or other materials have been used. If someone else installed the system, you can’t be sure how clean they left it (beyond what a visual inspection reveals).

External Sources

According to Joshua D. Sole, Ph.D., senior mechanical engineer, Mainstream Engineering Corp., environmental pollutants could include salt air/water, household cleaning agents, pesticides, formaldehydes, building materials, and even off-gasing food. “Each of these contaminant sources can initiate corrosion in coil tubing in a year or less when the conditions are right,” Sole said.

Source control should always be considered. Look for signs of chemical storage near any mechanical system.

Corrosion caused by local water or industry may warrant the use of coatings for added protection.

The first step is determining whether the corrosion is internal or external. You can do this with your eyes.

Corrosion Types

The two most common forms of coil corrosion seen in the HVACR industry are pitting and formicary, Sole said. It can happen in as little as a few weeks after installation, he said, though “more typically, corrosion will begin appearing within a one- to four-year period.”

Pitting corrosion can be caused by chemicals found in such products as snow-melting crystals, bathroom cleaners, dishwasher detergents, fabric softeners, vinyl fabrics, carpeting, paint strippers, etc., when they combine with moist air or condensate. Pitting progresses to form pinhole leaks.

Formicary (“ant’s nest”) corrosion can be caused by organic acids, such as acetic and formic acids, which are found in household products such as adhesives, paneling, particle board, silicone caulking, cleaning solvents, vinegar, foam insulation, and dozens of other commonly found products, Sole said. These include cosmetics, disinfectants, tobacco and wood smoke, latex paints, and plywood. One of the main problems with this type of corrosion is that it usually can’t be seen with the naked eye, though sometimes black or blue-gray deposits are visible. The corrosion makes a subsurface network of microscopic corroded tunnels, Sole said. “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.”

The next determination to make, he said, is whether this was a one-off, or whether it’s likely to happen again. This can be tough with new customers; ask them whether “coil leaks” or “refrigerant leaks” have happened before, and how they were fixed. If you have other customers in the area, check their histories of coil problems to try to spot a local trend.

The question of whether to offer a coil coating depends on a number of factors. Customers without current coil problems, but whose buildings are in potentially corrosive areas, could benefit from a protective coating application as a preventive measure. Those who seem unwilling or unable to change their chemical storage locations might also benefit from such an offer.

Keep Coils Clean

Outdoor condensing coils can best fight corrosion with periodic cleaning. Water is suitable, but existing corrosion and buildup typically are removed more completely with a coil cleaner.

A lot of acid- and alkaline-based coil cleaners are available. However, proper rinsing is important to completely remove the cleaner’s chemical residue — alkaline cleaners, too. Their residues can also corrode aluminum and other materials.

Sidebar: Coating Types

According to Joshua D. Sole, Ph.D., senior mechanical engineer, Mainstream Engineering Corp., there are four basic coating types used in the HVAC industry. (Note: With all such products, read the warning labels and wear the correct PPE.)

• Polyurethane (PU) can be manufactured “as hard as fiberglass, bouncy as rubber, sticky as glue, and soft as upholstery foam,” Sole said. Many off-the-shelf PU-based coil coatings available to the HVAC trade can be applied in the field. Advantages: PU formulations are fairly inexpensive, less viscous, more flexible, and thinner (typically 25-50 microns) than other coatings. Disadvantage: They are not as resilient or long lasting as other coatings.

• Epoxy (phenolic-based) coatings might be best suited to new installations, Sole said, where heat transfer losses are accounted for in the system design. Advantages: Epoxy coatings are known for their chemical and heat resistance, and have been used for coating floors and other surfaces. Disadvantages: They are thicker coatings (approximately 50 to 100 microns), with poor flexibility and adherence characteristics; difficult to apply in the field; reduced heat transfer.

• Fluoropolymers (like DuPont’s Teflon®) are now available in many forms under a variety of trade names. Fluoropolymer coatings applied via thermal sintering or electrostatic powder coating during the manufacturing process have not gained traction in the HVAC industry. Advantages: High resistance to acids, solvents, and bases; many field-use fluoropolymer sprays are available to contractors; lower cost than the epoxy- and PU-based coating systems. Disadvantages: The field-use sprays generally have poor adhesion and their effectiveness diminishes significantly in a short period of time.

• Silanes are coupling agents used when two dissimilar materials, such as paint (an organic) and glass (an inorganic material), need to be bonded together. The proper formulation can provide a flexible, resilient, glass-like coating with good corrosion resistance and water-draining capability, said Sole. Advantages: Bonds to aluminum and copper; forms a thin coating when cured (less than 10 microns) with negligible effects on heat transfer; resilient against cracking and corrosion; hydrophobic; said to reduce airflow friction. Disadvantages: Can be difficult to apply properly in the field; may require a trained applicator, or application off site; typically more expensive than the other coatings described.

Publication date: 11/07/2011