Coil coatings can halt leaks and corrosion

A discussion of HVAC coil coatings: The solution to the leaking HVAC coil epidemic could be coatings.

By Joshua D. Sole, Ph.D., and Alan H. Brothers, Ph.D., Mainstream Engineering Corp.

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HVAC/R refrigeration coils are leaking at an alarming rate in industrial, commercial and residential applications. While the reasons are many, chemicals ranging from household cleaners to industrial process compounds are the main culprits that produce leaks and pitting corrosion on all types of coils.

Figure 1. Continuous contact with contaminated condensate caused this corrosion.
Figure 1. Continuous contact with contaminated condensate caused this corrosion.

Many HVAC manufacturers, distributors and contractors might not realize that corrosion caused hundreds of thousands of coil failures during the past decade. The source is typical environmental pollutants, which range from salt air, to household cleaning agents, pesticides, formaldehydes, building materials and even off-gassing of food. Each of these contamination sources can corrode coil tubing in a year or less, if the conditions are right (Figure 1).

For example, refrigeration coils in a South American fruit processing plant’s banana room were continually failing. The chamber used ethylene gas to ripen the fruit. Gaseous byproducts from the catalytic gas generator combined with the moisture in the ripening area to form a weak acid that produced pinhole leaks in the coil tubing after a year or less.

Aside from fruit-processing plants, most coastal area HVAC equipment is bombarded with corrosion caused by ocean salt (Figure 2).

Types of coil corrosion

Figure 2. This corroded coil is from a coastal area.
Figure 2. This corroded coil is from a coastal area.

The most common forms of coil corrosion are pitting and formicary corrosion. Both can occur in as little as a few weeks after installation. More typically, corrosion begins 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 (Figure 3) 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 and paint strippers. Fluorides are used in municipal water treatment. Pitting usually is visible on the exterior of a copper tube, visible to the naked eye. It’s caused by aggressive attack by chloride/fluoride ions that condensate carries to the metal surface. The cations attack the oxide film the metal uses to protect itself, essentially forming a corrosion-driven battery that consumes the copper. Once pits form in the copper tube, they progress through the tube wall forming a pinhole that leaks refrigerant.

Figure 3. This cross-section shows the results of pitting corrosion.
Figure 3. This cross-section shows the results of pitting corrosion.

Formicary corrosion (Figure 4), on the other hand, 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 used products. Formic acid can be found in cosmetics, disinfectants, tobacco and wood smoke, latex paints, plywood and dozens of other materials. The corrosion these substances cause usually isn’t visible to the naked eye, although black or blue-gray deposits sometimes can be seen on the surface. Formicary corrosion produces a subsurface network of microscopic tunnels within the tubing wall. It resembles ant nest-type structures that are substantially larger than the surface pinholes. Eventually, one or more of these tunnels progresses to the surface of the copper and form a pinhole, which quickly results in coil leakage.

Choosing the right coating

Figure 4. This cross-section shows how deep-seated formicary corrosion can penetrate a copper tube.
Figure 4. This cross-section shows how deep-seated formicary corrosion can penetrate a copper tube

The first step to take when confronted with coil corrosion is to determine if it will happen again after coil replacement. It’s difficult to determine if the corrosion is a one-time phenomenon or a continuing problem in that geographic 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, you can attempt to prevent corrosive materials, such as cleaning solvents, from entering the return air stream. Perhaps these materials can be stored in areas far from a return duct. This might eliminate the expense and need to coat a replacement unit.

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

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