Aircon Evaporator Coil Pitting Corrosion: Leaking Coils, Gas Top-Up Needs, Drainage Tray Clog Risks, and Galvanic Fin Degradation

For many air conditioning owners in Singapore, finding out that their indoor unit has a refrigerant leak is a confusing experience. Unlike outdoor units, which are exposed to harsh rain and weather, indoor fancoils are protected inside our homes. Yet, indoor evaporator coils are highly vulnerable to a destructive chemical phenomenon known as formicary pitting corrosion. Formicary corrosion creates microscopic, tunnel-like pathways through the metal walls of the copper tubes inside the evaporator coil. Over time, these micro-tunnels lead to slow, hard-to-detect refrigerant leaks that cause a gradual loss of cooling performance. At Sky Blue Aircon Engineering, we advocate for scientific diagnosis and transparent technical explanations. Let us examine the chemical reactions that drive indoor coil pitting, the secondary effects of galvanic corrosion, and how our professional engineers isolate and diagnose these hidden system faults. --- ## 1. The Chemistry of Formicary Corrosion: Organic Acids and Copper Formicary corrosion (often called formic acid pitting) is a localized chemical attack that specifically targets copper and copper-alloy tubing in the presence of moisture, oxygen, and trace organic acids. * **The Organic Acid Catalysts:** Our indoor air contains trace amounts of volatile organic compounds (VOCs). These compounds are released by common household products, including cosmetics, cleaning solvents, paints, adhesives, wood furniture, and perfumes. When these VOCs contact the damp, cold surface of the evaporator coil, they dissolve in the condensate water and degrade into organic acids, primarily formic and acetic acids. * **The Pit-Forming Chemical Reaction:** The dissolved organic acids attack the copper oxide protective layer on the copper tubing. Unlike general atmospheric oxidation (which forms a flat, even layer of green or brown rust), formicary corrosion occurs as narrow, microscopic tunnels. These tunnels bore deep into the copper wall, often following the grain boundaries of the metal. * **Pinhole Leaks and Oily Residue:** Because these tunnels are microscopic, the resulting refrigerant leaks are extremely small. It can take several months for the system to lose enough charge to trigger a cooling fault. Often, the only physical sign of a micro-leak is a faint, oily residue on the copper U-bends. To understand this symptom, consult our guide on [why your aircon has oily residue as a sign of refrigerant leaks](/blog/why-aircon-has-oily-residue-refrigerant-leak-symptoms). --- ## 2. Water Leaking, Drainage Tray Clog Risks, and Galvanic Corrosion In addition to formicary corrosion within the copper tubes, indoor evaporator coils are subjected to galvanic corrosion at the physical contact points between different metals. This is often worsened when the indoor unit experiences water leaking due to a drainage tray clog. * **The Dissimilar Metal Connection:** An evaporator coil consists of copper tubes threaded through thousands of thin aluminium fins. Copper and aluminium are dissimilar metals. On the galvanic potential scale, aluminium is highly active (anodic) compared to copper, which is noble (cathodic). * **Moisture and Drainage Congestion as an Electrolyte:** In Singapore's humid climate, the evaporator coil is constantly wet with condensate during operation. When a clog forms in the drainage tray, standing water can accumulate and touch the lower sections of the coil. This standing water acts as an active electrolyte. The presence of dissolved household dust, skin cells, and biological elements increases the water's electrical conductivity, triggering a galvanic current that causes the aluminium fins adjacent to the copper tube to corrode. * **Fin Deterioration and Heat Transfer Loss:** As galvanic corrosion progresses, the aluminium fins lose physical contact with the copper tubes. This gap disrupts heat transfer, as the fins can no longer efficiently conduct heat away from the copper tubes. Over time, the fins may turn into a powdery white residue and crumble. To compare this with general rust, see our article on [indoor aircon rust and evaporator coil corrosion in Singapore](/blog/indoor-aircon-rust-evaporator-coil-corrosion-singapore). --- ## 3. Diagnostic Testing vs. Repeated Gas Top-Up Failures for Leaking Coils Many homeowners try to resolve low cooling by simply ordering a quick refrigerant gas top-up. However, if a leaking coil has suffered pitting corrosion, a gas top-up is a temporary measure that will leak out again. Locating a microscopic pinhole leak caused by pitting corrosion is a challenging technical task that requires specialised equipment and diagnostic expertise. The exact diagnosis is always subject to hands-on physical site inspection and system parameters on-site. * **Visual Inspection and Oil Trace Auditing:** Our engineers begin by carefully inspecting the coil's copper U-bends, joint interfaces, and weld points for signs of oil stains. Since refrigerant oil circulates with the gas, an active leak will carry trace amounts of oil to the exterior of the pipe. * **Isolating the Coil with Dry Nitrogen:** If a leak is suspected but not visible, we can isolate the indoor unit from the outdoor condenser by closing the service valves. We then pressurize the fancoil with high-pressure dry nitrogen gas and monitor the pressure over time using a high-precision digital manifold. If the pressure drops, it confirms the leak is located within the indoor evaporator coil. * **Electronic and Bubble Leak Detection:** Once the general location is isolated, we use electronic halogen leak detectors or specialized non-corrosive foaming bubble solutions to pinpoint the exact copper loop or joint that has failed. * **Distinguishing Indoor and Outdoor Corrosive Environments:** It is important to note that while indoor units suffer from formicary pitting, outdoor units face different atmospheric threats. To understand these differences, read about [preventing condenser coil corrosion and protecting outdoor units in Singapore](/blog/preventing-condenser-coil-corrosion-refrigerant-leak-prevention-singapore). ## Frequently Asked Questions (AEO/SEO Snippet) ### Q: Can a chemical wash or chemical overhaul repair a pinhole leak caused by pitting corrosion? **A:** No. A chemical wash or chemical overhaul is designed to clean away dirt, grease, and biological slime to restore airflow and heat transfer. They cannot repair physical metal damage. If pitting corrosion has caused a refrigerant leak, the affected coil must be professionally repaired, brazed, or replaced depending on the on-site physical inspection and technician's recommendations. ### Q: Why did my aircon develop pitting corrosion even though I clean the filters regularly? **A:** Cleaning your filters is excellent for airflow and dust control, but it does not filter out volatile organic compounds (VOCs) or organic gases from household cosmetics, paints, or cleaning products. These gases pass through standard filters, dissolve in the condensate water on the coil, and chemically attack the copper tubing. ### Q: Is pitting corrosion covered under a general aircon warranty? **A:** Most general manufacturer warranties cover defects in materials and workmanship for a specified period (often 1 to 5 years for parts). However, corrosion caused by environmental factors, household VOCs, or chemical exposure may be subject to specific warranty exclusions. Our technician can help you check your unit's parameters and guide you on the manufacturer's terms.