By Tanner Janesky — November 9, 2024
Corrosion in refrigeration coils is a costly problem both economically and environmentally. Refrigerant leaks from corrosion cause the system to completely stop working, necessitating service or replacement. Understanding corrosion’s causes and the methods to prevent it can greatly extend the lifespan of refrigeration systems. This article covers the types of corrosion that commonly affect refrigeration coils, why they happen, and practical strategies to prevent them.
1. What is Corrosion?
Corrosion is the gradual destruction of metals due to chemical reactions with their environment. In refrigeration coils, this typically occurs when metal surfaces, such as copper tubing, react with acids, water, and oxygen. Over time, this weakens the coils, leading to pitting and leakage.
1.1 Ant-Nest Corrosion
One common form of corrosion in refrigeration systems is ant-nest corrosion (ANC), a localized form of pitting corrosion that resembles tiny ant nests on copper surfaces. It is particularly problematic because it can weaken specific areas of the copper tubing, eventually causing microscopic pinhole leaks. Ant-cave corrosion is often the result of acidic substances and contaminants interacting with the copper surface.
1.2 Role of Water and Acids
Water acts as an electrolyte, facilitating electrochemical reactions that lead to corrosion. When acidic compounds are present, they lower the pH of the water, increasing its corrosiveness. Copper coils exposed to acids or chlorides in water are especially vulnerable to pitting and ant-cave corrosion. This is a particular problem for evaporator coils that are wet most of the time.
2. Galvanic Potentials and How They Influence Corrosion
When two different metals come into contact in an electrolyte (like water), they create a galvanic cell due to the difference in their galvanic potentials. This potential difference determines which metal will corrode. The metal with the lower potential becomes the anode and corrodes, while the more “noble” metal becomes the cathode and remains protected.
Most refrigeration coils are made of copper tubing with aluminum heat transfer fins. Aluminum and copper have different galvanic potentials (copper is more noble than aluminum), meaning that in a galvanic pair, aluminum will act as the anode (the more reactive metal) and copper as the cathode.
Electrons will flow from aluminum (anode) to copper (cathode), as aluminum has a lower potential. This electron flow causes aluminum to oxidize (lose electrons), leading to the deterioration of aluminum. The oxidation process will degrade the aluminum, and the rate of corrosion depends on factors like the size of the contact area, environmental conditions, and the electrolyte’s composition.
Copper remains relatively protected in this pairing since it acts as the cathode, where reduction reactions occur. This protection further drives the degradation of the aluminum.
But if copper tubing is in contact with another metal with a more cathodic galvanic potential like stainless steel, copper could corrode faster because it acts as the anode in the galvanic pair. The larger the potential difference, the higher the corrosion rate in the anode material.
3. Acid Formation
Acidic substances are often present in refrigeration environments, and they significantly accelerate corrosion. Here are some common acids and corrosive compounds:
3.1 Carboxylic Acids
Carboxylic acids (R-COOH) are organic acids characterized by the presence of a carboxyl group (-COOH). They can form through various natural and industrial processes. They often form in environments with volatile organic compounds (VOCs) and can settle on copper surfaces. These acids are especially corrosive because they lower the pH and cause localized pitting.
3.2 Formic Acid
Formic acid (HCOOH) is commonly found in indoor environments as a byproduct of various building materials, adhesives, cleaning products, and even some types of insulation. It can vaporize and settle on copper coils, where it condenses and forms a thin film of acidic moisture, attacking the copper surface and leading to pitting corrosion.
3.3 Acetic Acid
Acetic acid (CH₃COOH), like formic acid, is volatile and can be present in many common household and industrial materials. Sources of acetic acid include wood-based materials, adhesives, and sealants. In HVAC and refrigeration systems, acetic acid can accelerate corrosion on copper coils by lowering the pH around the metal surface, especially in warm, moist environments.
3.4 Chlorides
Chlorides (Cl-) from saltwater exposure or cleaning chemicals can significantly accelerate copper corrosion. In combination with moisture, chlorides lead to severe pitting corrosion.
3.5 Sulfur Compounds
Hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) are two sulfur-based compounds that can cause corrosion in copper. These are common in industrial areas or near sources of combustion (like furnaces or vehicle exhaust). These compounds, especially in humid or moist environments, react with copper to form copper sulfide, leading to localized corrosion and pitting.
3.6 Ammonia and Nitrates
Ammonia (NH₃) is highly corrosive to copper, particularly in moist conditions, and can lead to stress corrosion cracking. Nitrates (NO₃-), often from agricultural or cleaning products, also contribute to corrosion.
4. How to Prevent Corrosion on Refrigeration Coils
Several strategies can help prevent or slow down corrosion in refrigeration coils. These methods include creating physical barriers, neutralizing acids, and using inhibitors.
4.1 Barrier Formation
Protective coatings form a physical layer over the copper, isolating it from corrosive agents. Common barrier coatings include epoxies, polyurethanes, and specialized anti-corrosion films. These coatings are often hydrophobic, repelling moisture and acids to prevent electrochemical reactions.
4.2 Acid Neutralization
Acid neutralization is a chemical process where an acidic substance is reacted with a base (alkaline substance) to reduce the acidity and bring the pH closer to neutral (pH 7). This process can be crucial in corrosion control because it prevents acids from further reacting with metals like copper, which would otherwise lead to pitting and other types of corrosion.
4.2.1 Steps of Acid Neutralization
Reaction of Acid and Base:
When an acid (with a low pH) and a base (with a high pH) are combined, they undergo a chemical reaction to form water and a salt. This is an exothermic reaction (it releases heat).
The general reaction is: Acid + Base → Salt + Water
For example, Formic Acid (HCOOH) can be neutralized by baking soda, aka sodium bicarbonate (NaHCO3). This reaction produces water, carbon dioxide, and a salt (sodium formate in the case of baking soda), effectively neutralizing the acid.
HCOOH (Formic Acid) + NaHCO3 → NaCOOH (Sodium Formate) + CO2+H2O
Effect on pH:
The reaction between the acid and base consumes the hydrogen ions (H⁺) of the acid and the hydroxide ions (OH⁻) of the base, forming water, which is neutral (pH 7).
As the acid is neutralized, the pH of the solution rises (becomes less acidic), bringing the environment closer to a neutral or even slightly alkaline state.
Formation of a Salt Layer:
Depending on the acid and base used, the neutralization reaction may produce a salt that remains in solution or precipitates out. Some salts formed in this reaction can also create a thin layer on metal surfaces, which might provide mild protection against further corrosion.
Example of Use in Corrosion Control:
In HVAC and refrigeration systems, for example, an alkaline cleaner or mild neutralizing solution can be applied to copper coils. This treatment reacts with acidic residues, forming neutral products and reducing the overall acidity of the surface.
Sodium bicarbonate (baking soda) is a common, mild base used for neutralizing acids on metals. When applied, it reacts with the acid, neutralizing it and preventing it from continuing to corrode the metal.
4.2.2 Why Neutralization Helps Prevent Corrosion
Prevents Further Acid Attack: Neutralization removes the acid that would otherwise attack the metal surface, particularly copper, reducing the rate of corrosion.
Restores Surface pH: By bringing the surface pH closer to neutral, neutralization stops the electrochemical reactions that facilitate corrosion in acidic environments.
Limits Pitting and Localized Damage: Neutralizing acids on the metal’s surface can prevent the highly localized pitting that characterizes forms like ant-cave corrosion in copper.
4.2.3 Limitations of Acid Neutralization
While acid neutralization is effective in preventing immediate corrosion, it often needs to be combined with additional protective measures like coatings or regular maintenance, especially in environments with continuous exposure to acids or volatile compounds. This combination of treatments helps to ensure long-term protection of metal surfaces.
4.3 Inhibition
Corrosion inhibitors are substances applied to copper refrigeration coils to prevent or slow down the corrosion process. They work by forming a protective layer on the copper surface, which isolates it from corrosive agents like moisture, oxygen, acids, and volatile organic compounds (VOCs). Here are some types and how they work:
4.3.1 Adsorption Inhibitors
These inhibitors function by adsorbing (binding) to the copper surface and creating a thin, protective film.
The molecules in adsorption inhibitors are designed to cling tightly to the copper surface, forming a barrier that blocks corrosive agents.
Example: Benzotriazole (BTA) is a common adsorption inhibitor for copper that forms a stable protective layer and is particularly effective against oxidation and pitting.
4.3.2 Passivating Inhibitors
Passivating inhibitors work by forming a passive layer on the copper surface, often through chemical reactions.
This passive layer acts as a shield against moisture and oxidizing agents, making the copper surface less reactive.
Example: Phosphate-based inhibitors can create a stable film on copper, protecting it from acidic conditions and minimizing the likelihood of pitting.
4.3.3 Volatile Corrosion Inhibitors (VCIs)
VCIs are inhibitors that release protective compounds into the surrounding air, where they then condense on metal surfaces like copper coils.
The released inhibitors form a protective layer, even in hard-to-reach areas of the coils, offering consistent protection against corrosion.
VCIs are particularly effective in confined environments like HVAC systems, where the protective compounds can linger and maintain a protective atmosphere.
4.3.4 Cathodic Inhibitors
These inhibitors slow down the electrochemical reactions at the cathodic sites on copper surfaces, which reduces the rate of overall corrosion.
They work by either blocking the supply of oxygen to the copper surface or interfering with the electron flow that drives the corrosion reaction.
While more common in other metals, certain cathodic inhibitors can be effective in reducing copper’s susceptibility to electrochemical corrosion in moisture-laden environments.
5. End Plate Materials
The material of the end plates on refrigeration coils can influence corrosion, especially through galvanic interactions.
5.1 Stainless Steel vs Aluminum Coil End Plates
Using stainless steel end plates instead of aluminum increases the galvanic potential difference with copper. In this pairing, copper will corrode more readily because it acts as the anode. Aluminum, on the other hand, has a closer galvanic potential to copper and often corrodes first, serving as a sacrificial material that protects copper to some extent.
6. Corrosion Resistant Refrigeration Tubing Alloys
In addition to preventive measures, using corrosion-resistant alloys for tubing can reduce the risk of corrosion in refrigeration coils.
Finding alloys with thermal conductivity similar to copper and more cathodic (noble) galvanic potentials is challenging because copper’s high thermal conductivity (around 390–400 W/m·K) is difficult to match. However, a few alloys and metals come close in terms of thermal performance while having more corrosion-resistant properties and nobler potentials, though they typically sacrifice some thermal efficiency compared to pure copper.
Here are some alloys and metals that may meet these criteria to a reasonable extent:
6.1 Copper-Nickel Alloys (Cu-Ni)
Thermal Conductivity: Lower than pure copper but still relatively high, around 29–50 W/m·K depending on composition.
Galvanic Potential: More cathodic than pure copper, especially in marine environments. Copper-nickel alloys tend to be more resistant to corrosion, particularly in environments with salt or seawater.
Common Grades:
CuNi 90/10 (90% copper, 10% nickel) and CuNi 70/30 (70% copper, 30% nickel) are commonly used in marine applications and are much more resistant to corrosion than pure copper.
Applications: Used in seawater piping, marine hardware, and other high-moisture environments due to their corrosion resistance.
6.2 Bronze Alloys (Copper-Tin Alloys)
Thermal Conductivity: Lower than copper but adequate for certain applications, around 60–70 W/m·K.
Galvanic Potential: Generally more cathodic than pure copper, providing increased resistance to certain types of corrosion, especially pitting and crevice corrosion.
Common Grades:
Phosphor Bronze (around 90–95% copper, with tin and phosphorus) is more resistant to corrosion, particularly in acidic environments.
Applications: Used in bushings, bearings, and environments with moderate to high moisture due to its improved durability and resistance.
6.3 Brass Alloys (Copper-Zinc Alloys)
Thermal Conductivity: Around 120–150 W/m·K, making it a decent conductor compared to copper.
Galvanic Potential: Brass alloys are generally more cathodic than copper, particularly those with a higher zinc content. They are more resistant to corrosion in some environments, but may still be vulnerable in high-chloride or acidic conditions.
Common Grades:
Admiralty Brass and Naval Brass are commonly used for corrosion resistance in water-based environments. Naval Brass contains a small amount of tin, which improves corrosion resistance further.
Applications: Often used in heat exchangers, plumbing, and marine hardware.
6.4 Aluminum Bronzes (Copper-Aluminum Alloys)
Thermal Conductivity: Approximately 50–75 W/m·K, depending on the composition.
Galvanic Potential: Aluminum bronzes are more cathodic and generally more corrosion-resistant than pure copper, especially in marine and acidic environments.
Common Grades:
Aluminum Bronze C95400 (88% copper, 10–12% aluminum, plus iron and nickel) is known for high strength and corrosion resistance, with improved nobility compared to copper.
Applications: Often used in environments with exposure to seawater, chemicals, or acidic compounds. Applications include pump parts, marine components, and industrial bushings.
6.5 Silver (Ag) and Silver Alloys
Thermal Conductivity: Very high, around 429 W/m·K, even higher than copper.
Galvanic Potential: More cathodic than copper, making silver highly corrosion-resistant.
Limitations: Silver is expensive and may not be practical for extensive applications like coils. However, silver-coated copper has been explored for applications where both high conductivity and corrosion resistance are needed.
Applications: Primarily in electronics and specialized industrial applications due to its cost.
Summary of Properties
Copper-Nickel Alloys: Moderate conductivity, high corrosion resistance, nobler potential.
Bronze Alloys: Moderate conductivity, good corrosion resistance in acidic conditions.
Brass Alloys: Moderate conductivity, reasonable corrosion resistance.
Aluminum Bronzes: Moderate conductivity, excellent corrosion resistance in marine and acidic environments.
Silver: Excellent conductivity and corrosion resistance, but limited by cost.
These alloys generally have more cathodic (noble) galvanic potentials than copper, meaning they are less likely to corrode when paired with copper or stainless steel. They’re particularly suitable in acidic or moist environments.
6.6 Cost
Copper alloys tend to be more expensive than pure copper due to the addition of other metals like nickel, tin, or aluminum. Aluminum bronze and copper-nickel alloys are commonly used in high-performance and marine applications due to their cost-effectiveness in harsh environments, while other alloys like silver are less practical due to high costs.
7. Conclusion
Understanding the causes of corrosion in refrigeration coils and applying the right prevention techniques can extend equipment lifespan and reduce maintenance costs. By selecting appropriate end plate materials, using corrosion inhibitors, and potentially employing corrosion-resistant alloys, manufacturers can significantly improve the durability of their refrigeration systems and protect against costly corrosion.
