- Also called
- Ant-nest corrosion
- Needs
- Moisture + oxygen + organic acids
- Sign
- Pinhole leaks, discoloured copper
- Help
- Coil coating, correct cleaners
What causes it
The Institute of Corrosion explains that formicary corrosion needs three things at once: oxygen, moisture and weak organic acids. The usual acids are carboxylic acids such as formic, acetic, propionic and butyric acid [1]. Sources include coil manufacturing oils, cleaning solvents and degreasers, building materials, adhesives and disinfectants [1].
Signs
- Leaks within weeks or months of installation.
- Patches of discoloured copper: dull grey or black, red-brown or purple.
- Pinhole leaks that are hard to see with the eye.
Prevention for technicians
- Ask for the pH and acid content of any cleaner you use on coils, and avoid products that leave organic acid residues.
- Rinse cleaners off as the label directs.
- Keep the indoor unit away from fresh paint, adhesives and strong solvent vapour.
- Apply a protective coil coating such as Aerosure Coil Saver after cleaning, in high-risk sites.
Products used
Quick answers
What causes formicary corrosion in AC coils?
Moisture and oxygen together with weak organic acids such as formic and acetic acid, which come from sources such as adhesives, building materials, some solvents and cleaners.
What causes formicary corrosion?
Weak organic acids, mainly formic and acetic acid from indoor sources such as some adhesives, cleaners and building materials, attack thin copper tubes and leave tiny pinholes. The Institute of Corrosion describes the ant-nest pattern.
How do I protect coils against it?
Keep coils clean, avoid acidic or unsuitable cleaners on copper, and use a protective coil coating on sites with known corrosion problems.