HVAC Coil Corrosion: Should You Be Concerned?

Coil corrosion is an expensive problem in the HVAC industry, leading to coil replacement or entire system replacement. Corrosion results in failure, and is responsible for about 40% of equipment failures in industrial applications, according to CED Engineering.

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As the authors of an article in Reliable Plant explain, coil corrosion comes in the form of either pitting or formicary deterioration. Corrosion may occur as quickly as a few weeks after an installation or it may take up to four years to present itself.

Pitting corrosion is most often caused by exposure to fluoride or chloride. Fluoride is present in municipal water supplies, while chloride is found in a variety of products including snowmelt, detergents, cleaners, carpeting, and fabrics. Pitting occurs when chloride or fluoride ions are transported to the metal via condensate. The ions attack the metal, forming pits that form pinholes, causing the coils to leak refrigerant.

Formicary corrosion is typically caused by exposure to acetic or formic acids. These acids are present in a host of household products including cleaning solvents, insulation, adhesives, paints, plywood and many other materials. This type of corrosion is not always immediately visible and sometimes presents itself as black or blue-gray deposits. Formicary corrosion creates tunnels within the tubing that result in pinholes forming in the coils, again often leading to a refrigerant leak.

Facilities located in more corrosive environments including near saltwater or in industrial sites are particularly prone to experiencing coil corrosion. Other environments that may contribute to a higher amount of corrosive materials being expelled include areas around pools, laundry facilities, water treatment plants, sewers and high traffic areas. In such highly corrosive areas, coils have been known to fail in less than one year, according to CED Engineering.

A potential cause of coil corrosion is Chinese drywall, also known as odorous wallboard. Chinese drywall is also hazardous to your health. According to an article in G3 Environmental & Industrial Hygiene, an environmental and industrial hygiene service company, the drywall was imported from China from 2004 to 2007 and installed in both residential and commercial buildings. The drywall emits sulfur compounds, which corrode metal, including air conditioning coils.

The Centers for Disease Control and Prevention confirms that exposure to Chinese drywall can cause certain health problems such as irritated eyes and skin, respiratory problems and headaches. Exposure to this type of drywall can also exacerbate asthma symptoms.

You can identify the drywall by looking at the back. It should contain a label with the words: “Made in China,” “China,” or “Knauf.” Some Chinese drywall is not marked. In that case, metal corrosion including in plumbing, electrical, and natural gas lines, combined with a rotten egg odor may signify a Chinese drywall installation

To combat the problem of coil corrosion, coil manufacturers are applying coatings prior to production. In addition, there are companies offering aftermarket coatings. In a brochure, Aeris Technologies Ltd., a manufacturer of aftermarket coil-protection coatings, explains that the consequences of coil corrosion can include reduced efficiency, unattractive surface deterioration and equipment failure.

Additionally, reduced heat rejection may occur, resulting in an increasing compressor temperature and lower cooling capacity, which in turn increases the power usage. Lower cooling capacity means the compressor doesn’t cycle as intended, which means increased power consumption. As systems work harder they become more stressed, experience more breakdowns and have higher maintenance costs. A lower cooling capacity may also mean occupants aren’t very comfortable, resulting in loss of business or productivity. Preventing coil corrosion is much more effective and cheaper than replacing coils or the entire system.

There are basically four types of coil coating materials: polyurethanes, epoxies, fluoropolymers, and silanes, according to Reliable Plant. Choose carefully as the wrong coating, especially with aftermarket products, can reduce heat transfer and result in more expensive operating costs. An aftermarket application may affect the manufacturer’s specifications.

Thinner coatings have better heat transfer while thicker coatings restrict heat transfer. Hydrophobicity, or how effectively water drains from the coil, can affect heat transfer. Water buildup may also cause mold or mildew growth. The advantages of the four types of coatings differ as to how they resist scratching and corrosion; their weights/thicknesses; hydrophobicity; and heat transfer abilities.

Polyurethane is inexpensive, flexible and thin, but it’s not as long-lasting as other coating options. Epoxy is cheap, but it’s usually a thicker coating and can’t be applied in the field; the coils must usually be shipped to a factory for professional application. Fluoropolymers are highly resistant to acids and solvents and are inexpensive, but the sprays generally adhere poorly and their effectiveness wanes quickly.

Silanes form a thin coating that affects heat transfer very little and they typically last longer than other coatings. But they’re more expensive and proper application is best done by a professional and usually off-site. Each coating type has varying toxicity levels. Technicians applying the coating should wear equipment as specified by OSHA and an appropriate breathing apparatus.

Whether coated or not, continuing maintenance of coils is the primary way to combat the effects of corrosion. Twice annual cleaning with an alkaline coil cleaner or ph balanced coil cleaner will clear away any accumulated deposits, keeping coils safe from corrosion but also positively impacting the efficiency of your system.

Have you experienced coil corrosion? How did you combat the problem? Or did you choose a cooling system with coil coating protection included? And have you been happy with the product?

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  • Really interesting and great blog post on coil corrosion and what may be leading to it! Wasn’t sure how to reach you, but wanted to send you a message.

    I work with Owens Corning and thought I’d send you information about our updated HVAC duct solutions site. As you know every little thing can hurt or help air quality and your HVAC system as a whole. Would love for you to share this with some of your fans.

    Thanks! Good luck and keep writing!

    Owens Corning

    August 10, 2012
  • Great post. An issue not many people talk about but I am glad you gave some great advice on how to deal with this.

    August 13, 2012
  • Great information in this post guys.

    We come across coil corrosion quite often with our service. Continued maintenance is so important to keep your HVAC system clean, and we are seeing more cases as people try to save their money.

    October 21, 2014
  • Wonderful post, everything is very well explained. Thanks for the information.

    April 6, 2015
  • Too informative, this is a common issue. Most of us face HVAC coil corrosion and your post will be helpful in dealing with the problem. We deal with best HVAC from branded companies which use latest technologies which reduce corrosion problems.

    July 11, 2015
  • Very well explained. Really people don’t put much attention to it to save money and pay much more later. Thanks for sharing this information!

    July 23, 2015
  • Kenneth Smith

    I have a outside wood burning boiler. with a water to water 50 plate water to water heat exchanger going to a inside gas boiler. The outside wood burner will NOT heat properly. It has been setting for 3- years without being used. I think the 50 plate water to water heat ex-changer is plugged up. Do you have anything that I could put in the 200 gallon of water that would clean the deposits out of the heat ex-changer?

    December 21, 2015
  • Great post.

    March 15, 2016
  • Tim

    ScaleBreak is a liquid descaler that can help remove mineral deposits if that’s what you believe to be clogging the exchanger. We also have a variety of mechanical solutions that can help remove if tubes are plugged. Best bet is to call us and we can talk through the options.

    April 18, 2016
  • This is a great post that clearly explains the concerns of coil corrosion for both the HVAC industry and consumers alike. It was very enjoyable to read as you really did your research well. For example, you explained there are different types of coil coating materials and how choosing the wrong material can result in more expenses.

    Thank you for your Informative post.

    May 5, 2016
  • Phil Brinlee

    My dogs have destroyed some of the coils/heat transfer media on my outdoor uni,t exposing the pipes. I can’t afford to replace the coil or entire condense unit as suggested by a local company. I don’t have any leaks in the pipes, but heat transfer efficiency must ne way down. Is there something (like steel wool) I can pack around the pipes to increase heat transfer efficiency and get another few years of life out of the coil? Thanks. I know that this is not the “right” way to fix this. I’m just looking for a good “McGiver”.

    May 27, 2016
  • Tim

    Something we’ve heard of many times. To be honest, you probably are leaking refrigerant , you just don’t know it…yet. If not, it’s just a matter of time. At this point, as long as the fan is going strong, and you are keeping the coils free of debris, the heat exchange process is going fine. Stuffing it with anything will actually limit the air flow, which is the worst thing.

    July 11, 2016
  • Eva

    Our freon keeps leaking out and we are trying to get our home warranty to cover but they are trying to say that the coils have corrosion from dog urine. Im trying to fight them on this because our dog is female and urinates out in the yard not on the coil. They are saying it has a foaming look and corrosion when you look down inside near the bottom on one side. I think it looks like freon has leaked and the dirt is sticking to the coils on the inside at the bottom not foam. Is this possible? The first guy they sent out could not find a leak but did hose off the coils and show my husband how to keep them clean from dirt. Then he added the needed freon. I think after he hosed it the dirt probably caked on to the leaked freon and it didnt show up til they sent out a second opinion when the air broke down 2 weeks later. How do I prove to them its not urine? I feel they should cover it and trying to get out of it by blaming us for the problem which is simply false.

    September 24, 2017
  • As an HVAC contractor, it is challenging to explain to clients the unfortunate fact that coil corrosion can happen without them realizing it. And you’re right, corrosion can start just after a few weeks after installation and it’s a real headache when you need to explain why it’s happening when the unit is just weeks old.

    January 14, 2018
  • Jill

    The coils in our HVAC are rusted. It’s a 14 yr old unit and I’ve lived here for just over 1 yr. thee are obvious signs on patch ups and repairs done over the years. Landlord’s mgmt company is blaming me for the erosion saying it’s because I’m not changing the filter (I am) and I’m using the wrong size filter (I’m not) and they are not paying for the latest repair they had to do and won’t replace the unit. I highly doubt I caused 14 yrs worth of erosion. I used to own a house on this street built the same time and I had to replace my ac unit after 6 yrs. the latest repair (just to get it to actually cool the house) was $400 and they recommend replacement because the rust is so bad on the coils they can’t touch them to clean it. Mgmt is trying to send me the bill.

    April 5, 2018
  • John D

    This article reminds me of the age-old saying from my HVAC-101 course:

    “If a condenser coil corrodes and the RTU’s EER sinks, does it make a sound?”

    The answer? A resounding YES! So happy there are other like-minded individuals fighting the fight against coil corrosion and the corrupt OEMs. Let’s bring back the good way of doing things here in ‘Merica the *right* — build it to last! God bless!


    May 30, 2018
  • Bill Lucas

    Unfortunately, it is an uphill battle trying to fight an authorization representative for a home warranty company. If the technician they dispatch to your home thinks the coils are chassis of the unit were damaged by dog urine, the warranty company will automatically deny the claim. I argued with my warranty company that the a/c tech was not a degreed metallurgist and or a certified corrosion specialist. I also argued that my condenser unit is 14 years old and that most outside condenser units have a life span of about 10 to 15 years. The warranty companies (American Home Shield) authorization representative would not even consider my arguments because all Home Warranty Companies use arbitration agreements in their contracts to keep themselves from having to explain their decisions to a judge or jury during a lawsuit. As a retired attorney, I never ran across a case where a person was able to overcome the arbitration clause of a home warranty company, however I have ran across numerous cases where landlords have sued tenants over pet damages to a/c condenser unit. In almost every instance the age of the condenser unit and a accurate estimate to how long a dog had been pissing on it left the Tennant about 65-70 percent liable for the damage.

    August 15, 2018
  • Mark D

    The houses on my street have experienced coil failure one after another. All have units installed in the attic, all with sprayfoam insulation. I suspect the offgassing from the insulation (homes less than six years old) is the culprit.

    October 10, 2018
  • How about Stainless steel jacket around it if it can resist it from rusting.
    Thanks for the nice article.

    December 9, 2018
  • Nice Article.Thanks

    December 9, 2018
  • Suzy

    Has anyone ever heard of aluminum coils (located in the attic) being damaged from having cats? The tech is claiming that the ammonia from the litter boxes is what’s causing the damage to a less than 2 year old system and that it will need to be replaced within 3 years. Is there anything that can be done to prevent this from happening?

    July 8, 2019
  • Very helpful article.
    following link can be visited for furnace troubleshooting

    April 30, 2020
  • Informative Post!
    HVAC coating for coils in corrosive environments is extremely important. We all know energy efficiency and reliability are key aspects when it comes to choosing chillers or equipment in general for an HVAC project. Protexion is a protective coating and solution provider company specializing in corrosion protection for HVAC and Refrigeration Coils.
    Visit following link for more information http://www.protexion.in/protective-coatings-for-hvac-coils.php

    May 27, 2020
  • Don

    Good article and some guidelines to help fight it. Home warranty corruption also good from Attorney. I think in the past with lower R22 pressures and lower SEER ratings the quality of the copper was thicker and higher. I never had these problems in the 1970s but of late plenty. The same goes for roofing shingles. No tar just glass, lower quality they only last a short time compared to past. What we are seeing is just capitalism at it’s best eating out the middle class capital. You have mandated 500 psig R410A and EPA combined with a more monopolized HVAC industry to insure the profits are flowing. This is just industry designed to fail manufacturing. Ever have a coil problem in a window shaker ? Never. The higher EPA requirements result in higher profits. It takes more energy to generate more AC systems that fail than the energy savings. I guessing on that but the stats would be interesting. It would be interesting to compare copper coil wall thickness of new and old systems.

    June 29, 2020
  • Great Article, choose wisely is your first step,we have been use EpoxSil water base, has the highest ASTM B117 salt spray test data in the industry = 15,000 hours as well as ASTM G85 A1 acid salt spray test of 4,000 hours.https://www.bpcoils.com/epoxsil-coatings.html

    October 21, 2020
  • Joe

    With corrosion issues per the article, if purchasing a new ac should I try avoiding copper tubes and aluminum fins and with all aluminum tubes and fins such as Trane for example

    April 18, 2021
  • Martin

    This elaboration is very helpful more so to our client who really know and believe that copper doesn’t rust but we all know that our evaporator could face volitaile organic compounds (VOC’s) in form of rust which does weaken the copper coils this causing leakages. Thanks for the peice
    Martin from Uganda

    July 19, 2021
  • I need a product to halt a very small, 1” section of corrosion on the aluminum tubing inside a deep freezer, and a product also for the small pipe going to the condenser or compressor, on the outside of the deep freezer at the bottom which has a 1” section of pipe with corrosion.

    January 24, 2022

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