The following is an excerpt from an EPA paper titled Detecting and Eliminating Causes of Coil Corrosion By Alan H. Brothers, Ph.D. You can find the whole paper here.
While the possible causes of coil corrosion can stem from poorly manufactured copper, chemical residue from coil manufacturing, and other problems that initiate the corrosion process long before the coils arrive on a jobsite, the majority of problems occur when environmental acids corrode coils from the outside in.
The following list illustrates sources of coil corrosion that could be overlooked:
- fermenting yeast (lactic acid from milk) in a bakery walk‐in cooler
- chlorine from an indoor swimming pool or aquatic process
- urine (ammonia) from dead animals in meat processing plant coolers
- sulfur from well water used in cleaning coils or rooms with coils
- fertilizer (ammonia) in agricultural building evaporative coolers
Two Types of Environmental Corrosion
The two most common forms of coil corrosion are pitting and formicary. Pitting 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. Formicary corrosion is associated with pinholes in the copper tube walls. Although this type of pinhole corrosion is not usually visible to the naked eye, some black or blue‐gray deposits often can be seen on the surface. Formicary corrosion is caused by organic acids such as acetic and formic acids. Acetic acids or the derivative acetate 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.
Keeping Coils Clean to Fight Corrosion
Outdoor condensing coils can best delay corrosion with periodic cleaning. Water is suitable, but existing corrosion and buildup typically are removed more completely with a coil cleaner. Numerous acid‐based and alkaline‐based coil cleaners are available. However, proper rinsing is important to avoid the coil cleaner chemical residue that could initiate the corrosion process. Some alkaline cleaners tout the fact that they are “non‐acid” to capitalize on the belief that acids cause corrosion. However, alkaline cleaners also need to be rinsed thoroughly just like acid‐based cleaners because alkaline residues can also corrode aluminum and other materials. Outdoor coils located in areas where corrosives are prevalent, such as heavy industrial areas with acid rain or coastal areas where ocean salt is a factor, should have frequently scheduled, periodic cleanings.