Contractor Dave Mitlyng, owner of Mitlyng Electric & Refrigeration, Montevideo, Minn., came across an odd pattern of evaporative coil failures. There were pin holes on the outside of the coils, which allowed refrigerant to escape and let air and other noncondensables get into the system.

A respected colleague told him that contractors down South call the problem “ant dust,” adding that the higher pressure of R-410A could be making the problem worse. Mitlyng askedThe NEWS: “Is he feeding me some ant dust?”

The problem of “ant nest” coil corrosion is not really a new one. It has little to do with R-410A in the system. Formicary (literally ant nest) coil corrosion has been occurring throughout the industry for years, on all manufacturers’ brands.

According to Robin Boyd, a territory sales manager for Goodman Manufacturing, “While the higher pressures of R-410A may cause a coil already weakened by formicary corrosion to leak a bit sooner, there is nothing in 410A, or the related POE oil used with it, that would cause pin hole leakage.”

“The only way there could be corrosion issues from inside of the coil is if there are corrosives such as moisture in the system,” he said. Formicary corrosion works from the outside of the coil’s surface.

“We refer to the problem as champagne leaks because the bubbles that emanate from the leaks when the coil is checked under water look like champagne bubbles,” said Steve Hancock, senior principal engineer, Trane.


“Formicary coil corrosion is caused by contaminants in the air that collect onto the condensation that forms on indoor coils during a cooling cycle,” Boyd explained. These contaminants turn into acids when they are exposed to the moisture, “etching the copper of the coils in a manner that looks like tunnels in an ant farm,” he said.

“Eventually, these etched areas of the copper become thin enough to start leaking as the copper becomes so thin that it becomes porous.”

From the outside of the tube, and to the naked eye, it does not look like an ant farm. According to Peter Elliot of Corrosion Materials Consultancy Inc., and Richard A. Corbett of Corrosion Testing Laboratories, the coil’s copper surface “will be discolored adjacent to the corrosion pit(s). Surface films can vary from dull gray-black to red-brown or purple, depending upon the specific environment.”

“The corrosion initiates from the tube surface and progresses rapidly into the tube wall,” they wrote. The damage progresses rather quickly. “Perforation usually occurs in weeks or months, not years.”

According to their paper, “Ant nest corrosion will only occur when oxygen, moisture, and a specific corrodent, usually an organic acid, are simultaneously present on a copper surface. The damage is worse when stagnant fluids are retained against the copper surface, or when crevices are part of the unit design.

“The most common cause of ant nest corrosion is the presence of chlorinated organic compounds or hydrolysis products produced by the decomposition of, for example, esters or aldehydes to carboxylic acids, such as formic or acetic.”


Contractor Fred Kobie, owner of Kobie Kooling, Ft. Myers, Fla., pointed out that the problem is not brand-specific. Formicary corrosion leaks are also tougher to find than more common refrigerant leaks.

“The leaks do not always present during a leak test,” he said. “They can be tricky to detect. Using a component isolation test is the better method to identify the leaks, using both very low negative pressures and very high nitrogen pressures.”

Formicary corrosion has been common on R-22 systems for over a decade, said Hancock. “There have been various attempts to address the problem, including coating the copper tubes and trying different tube alloys,” he said. “Trane’s current approach is abandoning copper for aluminum, which is not susceptible to this form of corrosion.”

“The resolution to this problem,” said Boyd, “is to reduce contaminants in the air by using less chemically laden household and personal hygiene products, having a controlled outside air mix in the home, and decreasing the coil surface temperature to increase the amount of air being brought to below the dew point for more moisture removal.

“Some of these factors reduce calculated system efficiency, but can dramatically increase comfort level,” he said. “In many cases I have been able to set up systems so that they are technically less efficient but in practical use, they reduce system operational cost, greatly increase the comfort level of the air, and reduce the circumstances that cause formicary corrosion.”

“Ant nest corrosion is a real phenomenon associated with the premature failure of copper tubes used principally for refrigeration or air conditioning applications,” stated the NACE paper. “The fact that the phenomenon is less well known is alarming because of the high number of failures that may be attributed to other forms of corrosion by those less informed.”

The authors agree that “damage will be reduced or eliminated if copper surfaces are properly cleaned and kept dry.”

Publication date:06/11/2007