Formicary (Ant-Nest) Corrosion in Copper Tubes

How it works

Formicary corrosion needs three things together: moisture on the copper, oxygen, and a low-molecular-weight organic acid. The acids form from volatile chemicals in indoor air — for example formaldehyde and acetic compounds released by building materials, adhesives, some cleaning products and lubricants. Evaporator coils, which are wet with condensate, concentrate them on the tube surface.

The attack does not thin the tube evenly. It follows narrow, branching paths through the wall, so a tube can look sound until it leaks.

Where it appears

Indoor evaporator and fan coil copper tubes, often within the first years of operation, in new or recently refurbished buildings where material emissions are highest.

Prevention

• Reduce sources: ventilate new buildings, and check cleaners and lubricants used near the coils.
• Coat the coil so the tubes, bends and joints are covered as well as the fins.
• Consider coil designs with less exposed copper where the problem is known.

ISO 9223 categories do not apply here, because the cause is indoor air chemistry, not the outdoor atmosphere.