Coil Corrosion: Replace Now vs Monitor Condition Decision Tree

Why this matters

A corroded coil is not automatically a dead coil. Surface oxidation that scares a homeowner can run for years, while a coil that still looks clean can be days from a pinhole leak. The job is to separate cosmetic corrosion from structural corrosion, and to call replace-now only when the metal has actually lost integrity or is actively leaking. Get it wrong in the conservative direction and you condemn serviceable equipment; get it wrong the other way and the customer loses charge mid-season and pays for emergency refrigerant on top of the coil.

This is a condition-based call. The decision rests entirely on observable corrosion type, wall integrity, and leak status, not on the age of the unit or how the coil photographs.

Symptom presentation

Three corrosion patterns show up at the coil, and they do not carry the same risk:

• Galvanic/surface oxidation: white powdery aluminum oxide or green-blue copper carbonate on fin surfaces and tube bends. Cosmetic in most cases.
• Formicary (ant-nest) corrosion: copper tubing pitted with microscopic branching tunnels, often under intact-looking surfaces, driven by organic acids (cleaning products, off-gassing). This is the leading cause of premature evaporator failure.
• Pitting/uniform wall loss: visible craters, flaking, or thinned tube walls, common on coastal/salt-exposed condensers.

Customers report slow charge loss over weeks, a coil that "looks bad," or repeated low-charge service calls with no obvious mechanical leak.

The reason the corrosion type drives the decision is that the failure mechanisms differ. Surface oxidation sits on the fin and tube exterior and does not by itself breach the pressure boundary. Formicary corrosion attacks from the inside of the copper wall outward along microscopic tunnels, so it perforates tube walls in the field of the coil, away from any joint, which is why it cannot be repaired by re-brazing. Pitting from salt exposure thins the wall until it perforates. Only the latter two cross the pressure boundary, so only they condemn.

Quick checks

1. Wipe a representative section clean and inspect the tube wall, not the fins. Fin corrosion alone is cosmetic.
2. Run an electronic leak detector (heated-diode or infrared) slowly over U-bends, return bends, and the hairpin field. Confirm any hit with bubble solution.
3. Check refrigerant history: a documented loss without a found joint or Schrader leak points toward formicary in the tube wall.
4. Press a fingernail or pick gently at a suspect tube. Sound copper resists; tube that flakes or dents under light pressure has lost wall.

Decision thresholds

Call REPLACE NOW when any one of these is true:

• Confirmed active leak in the coil tube field (not at a serviceable joint or Schrader), verified with detector plus bubbles or a documented pressure-decay test that holds nowhere on the coil.
• Formicary corrosion confirmed: pinhole leak located in a tube wall away from any joint, with characteristic blue-green crystalline deposit at the seep point.
• Through-wall pitting visible, or tube wall flakes/perforates under light mechanical probing.
• More than one confirmed leak site in the coil, or a leak that returns within one season after a brazed repair.

Call MONITOR when ALL of these hold:

• No leak detectable after a thorough heated-diode sweep and a standing pressure test that holds.
• Corrosion limited to fin surfaces and tube exteriors with no measurable wall loss (tube sounds solid under probe).
• Charge has been stable across at least one cooling season with no unexplained top-offs.
• Capacity and subcooling/superheat read within spec.

Schedule a recheck at the next seasonal service when monitoring, and document the corrosion location with photos so drift is measurable next visit.

Confirming diagnosis

Before condemning, prove the leak is in the coil and not elsewhere:

• Isolate the coil. A pressure-decay (standing) test on the isolated coil that loses pressure with the rest of the system valved off confirms the coil itself.
• Use nitrogen-pressurized bubble testing at the suspect zone after the detector flags it; a detector hit alone is not condemnation.
• For formicary specifically, a leak that sits in the middle of a clean tube run, not at a braze, is the signature. Joint leaks are repairable; mid-wall formicary is not.

Do not condemn on appearance. A coil coated in white oxide with no leak and stable charge is a monitor, not a replace.

Remediation

• Replace-now coils: evacuate and recover per EPA Section 608, replace the coil (or the affected slab/condenser), pull a deep vacuum to 500 microns and confirm a rising-rate hold, then weigh in the nameplate charge and verify subcooling.
• Monitor coils: clean fins with a pH-neutral, non-acid coil cleaner only. Avoid acidic cleaners on aluminum and avoid harsh solvents that accelerate formicary. Document baseline charge and corrosion extent.
• Reduce future formicary risk by addressing indoor air sources of organic acids (storing cleaning chemicals away from the air handler, correcting return-air contamination).

References

• U.S. EPA, Section 608 of the Clean Air Act, Stationary Refrigeration and Air Conditioning regulations (refrigerant recovery requirements).
• AHRI Standard 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.
• ASTM G46, Standard Guide for Examination and Evaluation of Pitting Corrosion.
• ASHRAE Handbook, HVAC Systems and Equipment, chapter on heat exchanger materials and corrosion mechanisms.
• ACCA Standard 4, Maintenance of Residential HVAC Systems (inspection and condition-assessment intervals).