Signs of HVAC Corrosion: What to Look For Before It Costs You

When most people hear the word "corrosion," they think of red rust. Maybe even green copper pipes. Both are fair answers. However, HVAC corrosion is more varied and much more nuanced than that. Knowing what to look for on your specific equipment can mean the difference between catching a problem early and facing an equipment replacement you didn't budget for.

HVAC corrosion is also far more expensive than most people realize. The NACE International IMPACT study estimated the global cost of corrosion at about $2.5 trillion per year—roughly 3.4% of global GDP. The same study found that 15 to 35% of those costs are recoverable through existing corrosion-control practices. That means a significant portion of what facilities and building owners lose to corrosion every year is preventable. The first step is knowing what you're looking at.

Corrosion Looks Different Depending on the Metal

Your HVAC system contains multiple metals — steel in the cabinet, aluminum in the fins, and copper in the tubing. Corrosion attacks each of them differently, and the visual signs vary accordingly.

Steel cabinets: rust stains and bubbling paint

This is the most familiar form of rust. Orange-brown staining on painted surfaces, or paint that bubbles, blisters, or flakes. When you see rust stains running down the cabinet exterior or paint lifting in patches, corrosion has worked its way beneath the surface coating. Left unaddressed, surface rust progresses to structural degradation of the metal underneath.

Aluminum fins: white powder and brittleness

Aluminum doesn't rust like steel. It actually oxidizes. Look for a white or gray powdery residue. Pay special attention to the fins of your outdoor condenser unit when looking for this form of HVAC corrosion. This is aluminum oxide. It forms a thin protective layer at first, but heavy oxidation in corrosive environments leads to fin degradation. If your aluminum fins crack, crumble, or fall off when you touch or clean them, that's advanced corrosion. It means you've already lost heat transfer surface area on your metal. The fins exist to move heat; every square inch lost is efficiency the system can never recover.

Copper tubing: pitting and formicary corrosion

Copper corrosion is the most consequential type because it directly threatens refrigerant containment. There are two primary forms to understand:

• Pitting corrosion: caused by chlorides or fluorides — often from nearby chemical storage, cleaning products, or industrial emissions — attacking the exterior surface of the copper tube. Unlike some corrosion types, pitting is usually visible to the naked eye. It progresses to pinhole leaks that allow refrigerant to escape

• Formicary corrosion: caused by organic acids (acetic and formic acids) commonly found in building materials, adhesives, and cleaning products. Formicary corrosion creates a subsurface network of microscopic tunnels that can't be seen externally — by the time it shows up as a leak, the damage has been progressing unseen for some time. Research has attributed approximately 10% of all premature HVAC coil failures to formicary corrosion alone

Both types ultimately result in refrigerant loss, reduced cooling capacity, and — if not addressed — full coil replacement.

Galvanic corrosion: where metals meet

Where copper tubing and aluminum fins make contact, galvanic corrosion is always a risk. This type of contact happens throughout the coil.  When two different metals contact each other in the presence of moisture, a galvanic cell forms and accelerates corrosion of the less noble metal. Salt air and industrial pollutants significantly increase this process because they are more conductive. Coastal facilities and those near wastewater plants or manufacturing operations face markedly higher risk.

Environmental Factors That Accelerate HVAC Corrosion

Your location matters for how quickly HVAC corrosion develops. Even though you can’t pick up and move your facility, it’s good to know and prepare for the potential impacts. Some of the most aggressive environments include:

• Coastal environments: salt air or salt spray settles on metal surfaces and then quickly increases the galvanic and pitting corrosion rates

• Industrial areas: airborne pollutants, sulfur compounds, and chemical emissions create highly corrosive conditions for both HVAC coils and cabinets

• Wastewater and agricultural facilities: ammonia and hydrogen sulfide in the air attack copper tubing and aluminum fins with particular aggression

• Buildings with high VOC exposure: formaldehyde and organic acids from building materials, adhesives, and cleaning products drive formicary corrosion from the inside out

In any of these environments, corrosion doesn't develop on a standard timeline. Wouldn’t that be nice? An HVAC system that might show its first signs of degradation after a decade in a mild suburban setting may show the same damage within a year or two in a coastal or industrial location. Maintenance schedules and protection strategies should match your specific environment, not a generic calendar.

What to Do When You See the Signs

If you already see HVAC corrosion on your equipment, your response depends on how far it has progressed. Surface rust on cabinets and early-stage fin oxidation can often be addressed with cleaning and treatment. Then follow up with a protective HVAC coating. Pitting or pinhole leaks in copper tubing typically require coil repair or replacement followed by coating to prevent recurrence.

If you don't see signs of corrosion yet, now's the ideal time to act. Protective coatings are most effective and most cost-efficient when applied before damage occurs.

This is where Energy Performance Solutions can help. CoilSafe and CoilSafe Plus form a thin, glass-like, covalently bonded protective layer on HVACR coil surfaces that repels moisture, resists corrosion, and withstands the environmental stressors that drive HVAC corrosion in the real world. CoilSafe has been validated through 6,000+ hours of ASTM B117 salt fog testing and 25,000 hours of oceanfront marine environment testing — precisely the conditions that produce the most aggressive corrosion. CoilSafe Plus adds independently verified antimicrobial protection for facilities and spaces where indoor air quality is a priority alongside corrosion prevention.

Whether your equipment is brand new or already showing early signs, the right time to protect it is before the damage gets ahead of you.

Contact our team to learn more. Send us a message or call 713-931-2735.

Citation List

1. NACE International IMPACT Study (2016), "International Measures of Prevention, Applications, and Economics of Corrosion Technologies." Cited in: Relia Magazine, "The Cost of Corrosion: Industrial Statistics, Global and U.S. Figures." reliamag.com

2. TechBullion / Rust Guard, "Why Corrosion is Costing US Industries Billions, and How New Solutions Are Changing the Game." techbullion.com

3. Mainstream Engineering Corp. / ACHR News, "Experts Share Anticorrosion Techniques." achrnews.com

4. Qwik Products, "Detecting and Eliminating Causes of Coil Corrosion." qwik.com

5. Qwik Products, "A Discussion of HVAC Coil Coatings." qwik.com

6. Manufacturing.net, "A Discussion of HVAC Coil Coatings." manufacturing.net

7. Institute of Corrosion, "Fellows Corner: Formicary Corrosion in HVAC Systems" (January 2023). icorr.org

8. Goodwind Co., "Corrosion: A Guide for HVAC Professionals." goodwindco.in