Imagine a small IT room packed with servers, monitors, and humans. CO₂ levels easily spike to over 3000 PPM since the room lacks fresh air ventilation. Two overworked mini-split units churn away, cooling the space and unknowingly creating the perfect acidic environment. Fast forward a few months, and those shiny coils now resemble something pulled from the bottom of the ocean—corroded and leaking refrigerant. The culprits? Acids. Acids are all around us and can wreak havoc on copper and aluminum HVAC coils.

What Are Acids?

Let’s take a quick trip back to high school chemistry class—but don’t worry, no pop quizzes here. We’re just going to break down the basics of pH and molecular concentrations so that we’re all speaking the same language when it comes to acids.

Acids are molecules that can donate a hydrogen proton (remember: protons are positively charged subatomic particles in an atom's nucleus). Bases are molecules that can receive hydrogen protons. We use the pH scale to measure concentrations of these hydrogen protons (pH stands for “potential of hydrogen”).

pH and the pH Scale

pH classifies how acidic or basic (alkaline) a substance is on a logarithmic scale from 0 (most acidic) to 14 (most basic). It indicates the concentration of hydrogen protons (H⁺) on the acidic end and hydroxide (OH⁻) ions on the basic end. That just means that a substance with a pH of 2 has ten times the hydrogen protons as one with a pH of 3, and the substance with a pH of 2 is ten times more acidic than one with a pH of 3. Here’s the scale: 

• 0–6: Acidic (lemon juice, vinegar, or the feeling you get when you open a condenser and see burnt compressor terminals; rough on copper and aluminum)

• 7: Neutral (pure water—the baseline that’s neither spicy nor bland, and metals like to hang out here because there’s no corrosion drama; water or H2O is amphoteric, meaning it has equal concentrations of H⁺ protons and OH⁻ ions, and is highly variable)

• 8–14: Basic (soapy, squeaky-clean vibes; great for cutting grease but not always helpful for keeping your system in balance)

If the pH dips below 7, acids start breaking down your coils, much like leaving a steel wrench outside in a rainstorm. The lower the pH, the more aggressive the acid gets; those stronger acids corrode metal a lot quicker and more severely than mild acids. Corrosion on coils creates pinholes, which leads to refrigerant leaks.

When it comes to the effects of acids on coils, it may help us to think of the lower end of the pH scale like hot sauce—on one end of the scale, you’ve got the mild stuff (neutral water), and on the other, you’ve got the burn-your-face-off levels (stomach acid).

Measuring Acid Concentrations

Let’s say you’re mixing a cup of coffee. If you add one scoop of coffee grounds, it’s light and weak. Add five scoops? Now you’ve got a strong, in-your-face brew. That’s molecular concentration—it’s how many molecules of a substance (like acids) are packed into a liquid.

We quantify the concentrations of acids in moles, which are like mega-packs of molecules. One mole is about 6.022×10²³ molecules—that’s 602 followed by 21 zeroes—basically enough to fill a stadium with individual coffee beans. When we talk about acids like carbonic or nitric acid, their concentration in water is usually measured in mol/L (moles per liter). Here’s how it applies to your systems:

• Low concentration (mol/L): A drizzle of hot sauce—annoying but manageable.
• High concentration (mol/L): A full bottle dumped into your soup—game over.

Acids Are the Silent Saboteurs

Acids don’t just show up out of nowhere; they’re formed from the very environment in which your HVAC systems operate. Let’s break down the primary offenders:

1. Carbonic Acid (H₂CO₃)

When carbon dioxide dissolves in water—like condensation on coils—it reacts to form carbonic acid. You know this form of acid as the slight tang or fizz in soda or sparkling water. It’s harmless in drinks; however, it becomes a silent but deadly corrosive villain when it shows up in your HVAC system. This weak acid reduces the local pH, making the surface more vulnerable to stronger, more aggressive acids. 

At 3000 PPM of CO₂, carbonic acid concentrations in water are typically around 1.5 x 10⁻⁶ mol/L. That’s like having a few sneaky drops of acid in your condensation—small but still enough to drop the pH to around 6.8–7.0 and open the door to corrosion. It’s just acidic enough to weaken protective oxide layers on both copper and aluminum.

2. Chlorides (Cl⁻)

Chlorides are everywhere—cleaning products, drywall, flooring, sweat, and even the salt-laden breeze of coastal areas. Many of them are salts. You’ve heard of sodium chloride—table salt (NaCl)—but other common types include potassium chloride (KCl) in our bodies and calcium chloride (CaCl₂), a salt often used to help de-ice roads. With that in mind, it’s not too surprising that chlorides could cause corrosion on aluminum and copper coils.

As little as 50–100 PPM can cause pitting corrosion on copper when combined with slightly acidic conditions. However, at 300–500 PPM, chlorides become highly corrosive to aluminum, which accelerates surface breakdown and leads to pinhole leaks. That’s because chlorides break down the natural oxide layer on copper and aluminum, exposing the metal to further attacks from moisture and oxygen.

3. Organic Acids (Formic and Acetic Acids)

If you’ve gotten into IAQ in any form, you’ve probably heard of VOCs. VOCs, or volatile organic compounds, off-gas from electronics, adhesives, furniture, varnishes or paints, carpets, and cleaning products. These VOCs degrade into organic acids when combined with moisture. Perhaps unsurprisingly, organic acids are particularly dangerous in high-humidity environments, where they can accumulate on coil surfaces and eat through protective coatings.

Even trace amounts (<50 PPM) of organic acids can cause formicary corrosion in copper, creating microscopic tunnels that lead to refrigerant leaks. Aluminum coils, while more resistant to formicary corrosion, are not immune; extended exposure to these acids can lead to surface weakening and scaling.

4. Nitric Acid (HNO₃)

Nitrogen oxides (NOₓ) react with moisture to form nitric acid. Combustion appliances produce NOₓ as a byproduct of the combustion process (that’s why some combustion analyzers have a “NOX filter”). Tyler Nelson from Sauermann talked a bit about NOₓ and nitric acid in his podcast episode about combustion analysis.

Even trace amounts of NOₓ can result in nitric acid formation, further lowering the pH of condensation to highly corrosive levels (<5.5 pH, on par with coffee). Nitric acid attacks both copper and aluminum aggressively, leading to rapid pitting and the breakdown of surface protections.

How Acids Wreak Havoc on Aluminum Coils

Aluminum coils are often considered more corrosion-resistant than copper due to their protective oxide layer, but they’re not invincible. Chlorides are particularly aggressive toward aluminum. Once the oxide layer is compromised, these chlorides accelerate pitting corrosion, often faster than pitting corrosion on copper.

We often see galvanic corrosion in systems where copper and aluminum are used together. This type of corrosion happens whenever you have dissimilar metals in contact with each other and an electrolyte (like moisture). These reactions may further weaken aluminum in the presence of acids and chlorides.

Aluminum is also more prone to scaling and surface weakening when exposed to organic acids over time. This surface degradation reduces heat transfer efficiency and may cause structural failure.

Preventing Acid-Induced Corrosion

The best way to fight acids is to stop them before they can do damage. Several principles that help improve IAQ apply here as well. Filtration, ventilation, and humidity control are the three pillars of IAQ, but they also prevent acid formation. 

Four-inch MERV 16 media filters are great at catching pollutants, even VOCs, due to their high surface area. True activated carbon filters are also good because they adsorb pollutants like VOCs; the VOCs stick right to the surface. 

Introducing fresh air via a controlled pathway (such as through an ERV or ventilating dehumidifier) dilutes CO₂ and NOₓ, both of which may lead to carbonic and nitric acid formation. Humidity control is another key aspect, and the goal is to get indoor RH below 60% to minimize condensation and keep water molecules from interacting with VOCs and CO₂. Ventilating dehumidifiers can help with that. ERVs can reduce humidity but are far less effective, especially in very hot, humid climates like Florida’s; you can use ERVs for fresh air ventilation, but try to manage your expectations for humidity control and don’t rely on them as the sole solution. (Dr. Allison Bailes always preaches against using ERVs for dehumidification.)

The rest comes down to responsible coil cleaning and inspection. As always, we tell customers NOT to use bleach or chlorinated cleaners for DIY drain cleaning because of their possible adverse effects on the coil. Instead, we should opt for neutral-pH cleaners to minimize corrosion. Really, anytime we have a good look at the coil for cleaning or service, we want to look for early signs of pitting or formicary corrosion to catch possible leak sources before they spiral into full-blown leaks.

Final Thoughts

Living in a world of acids doesn’t mean your HVAC systems are doomed. It means you need to understand the subtle (and not-so-subtle) ways these acids form and attack HVAC system components. By ventilating, controlling humidity, and using proper cleaning and filtration practices, you can save your coils from an untimely, corrosive demise. And remember, aluminum coils might seem tougher, but they’ve got their weak spots, too—so treat them with care.

Now, take a deep breath (preferably in fresh air), check your coils, and banish those acids before they take over. Your customers’ HVAC systems—and their wallets—will thank you.

—Roman Baugh