MSCA (Manufacturers Service Advisory Council) Hotline

Coil Corrosion

Q  Can you explain why the evaporator blower fins in medium-­temperature walk-in coolers have a tendency to decay and turn to a powdery residue? These fins are constructed of aluminum. I have asked several seasoned technicians about this prob­lem and I have received a number of different answers.

A  In the vast majority of installations, coils last the life of the refrigeration equipment with few prob­lems. Still, an occasional coil will corrode prema­turely and develop multiple pinhole leaks, leaving the end user to question coil quality. End users must realize, though, that if a coil operates properly at first and then leaks profusely after a year or two, the problem is usual­ly corrosion caused by the environment, not defective tubing or defective coil manufacture.

If a coil abruptly fails with multiple pinhole leaks after a couple of years, the problem can usually be attributed to something in the environment. Consider reach-in evaporators typically used in school food serv­ice applications, for example. Here, the single biggest catalyst of corrosion causing premature coil failure is acetic acid found in vinegar and salad dressings. Acetic acid attacks copper in coils, causing premature coil fail­ure. Pickles left in a jar without the lid will have the same consequences.

This acid collects between the fin and tube and attacks the tube. It doesn't normally attack the solder joint, although it can. To avoid this problem, coils can be "coated" to help prevent the atmosphere from attacking the copper tubes.

The same problem can occur in dough retard coolers (used in bakeries and pizza restaurants), but at a much slower rate - say; over two, three, or even four years. The cause here may be fermenting yeast, lactic acid (milk), or a combination of both used in the dough. A coated coil is more expensive than a standard uncoated coil, but is prefer­able due to increased life. There are several different coat­ing companies that can supply this type of protection.

Coils in banana rooms may develop pinhole leaks after a few years of operation. The problem here is a catalytic gen­erator that generates the ethylene gas used to ripen the bananas. The by-products of combustion from the catalytic generator, in combination with water, form a weak acid that causes pinhole leaks.

Pinhole leaks can also show up in installations where cor­rosion would never be suspected. Pepperoni sausage drying rooms are an example. Here, pinhole leaks can occur because of acid present during the sausage drying operation.  Again, the coil can be coated to overcome this problem.

Another major source of corrosion in coils occurs in industrial applications. Contractors and end users must be aware of potential problems here, too. Chlorine, for exam­ple, which is used in swimming pools and which evaporates into the atmosphere, is one major instigator. The chlorine in the air attacks the copper tubing. The coil should be coat­ed for protection.

Urine, which contains ammonia, causes the same type of problem. Businesses such as animal rendering facilities should be aware of this problem. Sometimes dead animals are temporarily stored in coolers for subsequent process­ing. Urine runs onto the cooler floor, evaporates into the air, and collects on the cold evaporator coil hung from the cool­er ceiling. Protected coils will last only about two years in such an environment. Unprotected coils will last even less in environments with high urine concentration.

A cooler storing live fish bait will destroy an unpro­tected coil in about a week. Fertilizer plants provide yet another example of this problem. Fertilizer is a derivative of ammonia.

Industrial areas offer no safe haven from coil corrosion. Sulfur from industrial processes such as oil refineries attacks the phosphorous in the weld joint solder rather than the copper. This problem can be remedied by using a solder with high silver content and little or no phosphorous. Another solution is to coat all of the braze joints with acrylic or epoxy to seal the joint from the atmosphere.

Sulfur can attack phosphorous in less obvious situations as well. For example, consider an ice storage house. Ice formed from well water that contains sulfur will evaporate water and sulfur into the air. If it settles on the evaporator coil, pinhole leaks will eventually develop.

Planning ahead is probably the single best protection that end users can afford themselves. With the help of con­tractors, consulting engineers, refrigeration wholesale dis­tributors, and factory assistance, end users can be made aware of the environmental issues at hand. They can then select the proper coil materials or coating that will best fit the application.

Anything that is built using metals can be susceptible to types of corrosion. Refrigeration equipment can be damaged or destroyed when used in an application or atmosphere that has some chemical properties included. Some of the chemi­cals that can and/or will attack coil surfaces are discussed above. This list is not complete by any means. The amount of corrosion depends on the atmospheric content and cannot be determined in advance by the coil manufacturer.

Question from Lowell Brooks, HVACR Supervisor, Petersburg, VA. Answer by Steve Esslinger, Vice President of Operations, Zero Zone.