Why Do Coil Copper Tubes Develop Pinhole Leaks?
Key facts
| Cause | Where and what you see | Typical setting | Does coating help? |
|---|---|---|---|
| Formicary | Tube wall between the fins; tiny pinholes, branching tunnels in cross-section | Indoor evaporators in new or refurbished buildings | Yes |
| Sulphide, ammonia | Return bends and tube ends; black or dark-blue film on the copper | Wastewater plants, livestock sheds, fertilizer stores | Yes |
| Waterside attack | Clean outside, pitted or grooved inside; chloride pitting or erosion near inlets and bends | Water coils with untreated or high-chloride water | No |
| Brazing or wear | One leak at a brazed joint, or a flat spot worn by vibration at a tube sheet | Any coil, early in service | No |
How to tell the causes apart
Start with where the leak is and which side of the tube wall is damaged.
Leaks in the straight tube between the fins of an indoor coil, with no visible corrosion product, point to formicary corrosion. Leaks at return bends with a dark film on the copper point to sulphide or ammonia in the air. A tube that looks clean outside and is pitted inside has been attacked by the water, not the air. A single leak at a joint in the first months of service is usually a brazing fault.
What to send to a laboratory
When several units fail the same way, a laboratory examination pays for itself. Send a section of the leaking tube with the leak marked and uncleaned, and ask for a cross-section of the tube wall at the leak. Add a sample of any deposit on the tube. For water coils, add a water analysis that includes chloride, pH and hardness.
The cross-section settles most cases: branching tunnels mean formicary corrosion, pits open to the inside mean waterside attack.
When a coating helps
Formicary, sulphide and ammonia attack all start on the outer surface of the copper. A film that covers the tubes and return bends keeps the moisture and the chemical off the metal.
Coverage matters more than the coating name. Spray applied from the face of the coil does not reach tubes deep in the fin pack. Dip coating, or a coating applied to the bends and exposed tubes during manufacture, reaches the parts that leak.
When a coating does not help
A coating on the outside of the tube does nothing for attack from the inside. If the cooling water is high in chloride, or the water velocity is eroding the bends, the fix is water treatment, lower velocity or a different tube material.
Brazing defects and vibration wear are corrected at the factory or during installation.
What this means for your project
Before specifying a coating to stop leaks, confirm that the leaks start on the air side. If they do, specify protection that reaches the copper tubes and return bends, not only the fin faces.
Related questions
Does thicker copper tube solve pinhole leaks?
It delays them. A thicker wall takes longer to perforate, but the attack continues at the same rate. Removing the cause, or keeping it off the copper, is what stops the leaks.
Do gold fin or blue fin protect the copper tubes?
No. Pre-coated fin stock is coated before the fins are stamped. The copper tubes, return bends and brazed joints are added afterwards and stay bare.
Can a leaking tube be repaired and then coated?
A single leak can be brazed and the area recoated. Several leaks from the same cause usually mean the rest of the tube wall is close behind, and replacement is the safer choice.
