Reading Rust and Corrosion Patterns
Why this matters
Rust and corrosion are a written record of what went wrong, and where. A tech who reads the pattern instead of just wiping it off finds the leak, the condensation source, or the bad connection that caused it, not just the symptom. That turns a parts swap that fails again next season into a real fix. The same skill saves you on quotes: corrosion tells you whether a unit has years left or is on borrowed time.
The first question: wet or electrical
Corrosion has two broad families, and the location tells you which.
- Moisture-driven rust shows up on iron and steel where water sat or dripped: pan bottoms, the low point of a panel, the underside of a horizontal run, the threads on a fitting. It is flaky, orange-brown, and follows gravity.
- Galvanic and electrolytic corrosion shows up where two dissimilar metals touch in the presence of moisture (steel bolt in an aluminum casting, copper line on a steel bracket). One metal sacrifices itself and turns to white, green, or grey powder while the other stays clean.
Name which family you are looking at before you guess the cause. They lead to different fixes.
Trace the trail to its source
Rust runs downhill and stains as it goes. Put your eyes at the highest point of the staining, not the puddle at the bottom. The origin is almost always above the worst of the damage.
- A rust streak on a cabinet wall points up to a slow drip, a sweating line, or a failed seal.
- A ring of corrosion around one terminal in a panel of clean ones is a localized heat or moisture problem at that exact point: a loose lug, a back-fed neutral, water entry at that knockout.
- Corrosion concentrated on one side of a horizontal component means water pooled there because the unit is not level, or the drain is on the wrong end.
If the worst corrosion is at the lowest accessible point, you have probably found a collection area, not the source. Keep looking up.
Color and texture as a field key
| What you see | Likely metal and cause | What it tells you |
|---|---|---|
| Flaky orange-brown | Iron/steel, plain water | Ongoing or past wetting; check for active drip |
| Fine even surface haze | Steel, humid air, no liquid water | Environmental, slow; usually not urgent |
| White chalky powder | Aluminum oxidation or galvanic loss | Dissimilar-metal contact or salt/chemical exposure |
| Blue-green crust | Copper corrosion (verdigris) | Moisture plus often a flux, refrigerant, or acid trace |
| Black staining | Often a slow, oxygen-starved wet area | Trapped moisture under insulation or a gasket |
| Pitting (small deep holes) | Chloride or chemical attack | Aggressive water/air; the metal is thinning fast |
Pitting deserves respect. A surface that is mostly clean with a few deep pits is often closer to failure than a surface that is uniformly rusty but solid, because the pit is a stress point and a future pinhole.
Pattern shapes that name the cause
- A horizontal waterline of corrosion inside a tank, pan, or sump marks where standing water sat for a long time. The line tells you the normal level; corrosion above it suggests splashing or a higher level in the past.
- A teardrop or fan stain below a fitting, valve, or seam points to that joint as a drip source, even if it is dry when you arrive (an intermittent leak; see the related article on intermittent leaks).
- A halo around a fastener or terminal is heat plus moisture at a connection. On electrical, suspect a loose or high-resistance joint that has been warming and cooling.
- Corrosion only at the bottom of vertical metal is condensation: warm moist air hits cold metal, water beads, runs down, and collects at the base.
Decide: clean, treat, or replace
Reading the pattern is only useful if it changes what you do next.
- Surface haze, structurally sound: clean, address the moisture or air source, and document. Often not a repair at all.
- Localized corrosion at a joint or terminal: the joint is the failure. Remake the connection or replace the fitting; cleaning alone leaves the root cause.
- Pitting, scale loss, or rust-through on a pressure or structural part: this is a replace, not a wire-brush. A pressure boundary that has pitted is going to leak; do not band-aid it.
- Galvanic corrosion: fixing it means separating the metals (a dielectric union, an isolating washer, proper coating) or accepting it will return. Cleaning without isolation buys one season at most.
Always photograph corrosion before you clean it. The pattern is evidence; once you wipe it, you cannot re-read it, and the photo backs up your recommendation to the customer.
References
- OSHA general industry guidance on equipment inspection and corrosion hazards
- Trade-standard practice for dissimilar-metal isolation (dielectric unions, isolating fittings)
- Manufacturer documentation on acceptable corrosion limits for pressure components
- See related: Reading Leak Stains and Mineral Trails; It Leaks Only Sometimes (decision tree)