How to Prevent Corrosion of the HVAC Coils

Preventing  HVAC coils from corroding can be frustrating.  It is difficult to protect against environmental pollutants such as salt-air, pesticides or cleaning agents, all of which are responsible for the failure of thousands of coils.

Preventing corrosion is largely depending on determining what type of corrosion is occurring.  The two mostt common types being --pitting and formicary.

Pitting 

Pitting corrosion is a result of chlorides or fluorides, which are found in numerous items such as snow melting

crystals, toilet cleaners, dishwasher detergents, fabric softeners, vinyl fabrics, carpeting and paint strippers. 

Pitting is commonly visible on the exterior of the copper tube and is caused when negatively-charged chloride/fluoride ions carried to the metal surface by condensate attack the oxide film metal uses to protect itself.  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 

Formicary corrosion is caused by organic acids like acetic and formic acids, which can be found in household products such as adhesives, silicone caulking, cleaning solvents and vinegar. Formic acid can be found in cosmetics, disinfectants and latex paints. While formicary corrosion is not usually visible, black or blue-gray deposits can sometimes appear on surface.  Formicary corrosion can form a sub-surface network of microscopic corroded tunnels within the tubing wall.  Eventually  one or more of these tunnels will progress to the surface of the copper and form a pinhole which results in coil leakage 

Protecting HVAC Equipment from Corrosion 

To help prevent damage to coils by corrosion, the HVAC industry depends on the four basic coating types.  What type of coating used  depends on the cause of the corrosion. 

	Developed	Application	Advantages	Disadvantages
Polyurethane	1940s	Fiberglass, rubber, sticky, soft upholstery foam.	Inexpensive, less viscous, flexible and thin.	Not as resilient or long-lasting as other coatings.
Epoxies	1920s	Coating floors and other surfaces.	Inexpensive, excellent chemical & heat resistance, best for heat transfer losses.	High viscosity, thicker coat, poor flexibility and adherence to characteristics.
Fluoropolymers (Teflon)	1938	Cookware & non-stick products	High resistance to acids, solvents and bases.	Expensive, limited lifetime & effectiveness.
Silanes		Coupling agents—bond two dissimilar materials such as paint and glass.	Flexible, glass like, resistant to corrosion and water draining capabilities, resistant against cracking, corrosion, hydropohobic and reduce airflow friction.  Best heat transfer properties & greater lifetime.	Expensive, difficult to apply properly,

Misdiagnosing the problem can result in unnecessary costs, which is why choosing the right coating for the problem is so vital.

Recent developments have been made in coatings, one of those being a product called Surfsil.   According to the creator, Surfsil "is a hybrid compound that uses nano-silicone technology to incorporate organic and inorganic properties.  This allows the coating to chemically adhere to the substrate via a covalent bond."

This product was tested following the ASAM B-117 Salt Spray (Fog) Standard, and there was no sign of corrosion after 10,008 hours. It's chemical bond prevents corrosion from growing under the coating, it is flexible & scratch resistant in addition to be resistant to chemicals found in HVAC/R equipment.