Corrosion Type: What Caused It - A Decision Tree
Why this matters
Corrosion is rarely just "old metal." The pattern of attack tells you the mechanism, and the mechanism tells you whether replacing the part fixes the problem or the new part will rot just as fast. A green crust at one connection, a pitted line, a rusted bracket, and a perforated pan each have different root causes. Reading the corrosion type lets you fix the cause - the dissimilar-metal contact, the trapped moisture, the stray current - instead of swapping parts on a loop. This applies wherever metal meets moisture, which is every trade.
Start here: where is it, and what color
Two observations frame the diagnosis: the location pattern (one isolated spot, a joint between two metals, a low collecting point, or everywhere uniformly) and the color and texture of the product. Corrosion product color is a fingerprint of the metal and the chemistry attacking it.
If it is concentrated at a junction of two different metals
- If you see heavy corrosion exactly where two dissimilar metals touch (one badly eaten, the other clean): this is galvanic corrosion. When two different metals contact in the presence of moisture, the more active metal sacrifices itself - it corrodes fast while the nobler metal is protected. The fix is not just a new part; it is breaking the contact with a dielectric union, a non-conductive separator, or matching the metals. Replace like-for-like into a galvanic couple and it rots again.
If it is localized pits or holes in otherwise good metal
- If you find deep, isolated pits or pinholes while the surrounding metal looks fine: pitting corrosion. A small breach in a protective layer concentrates the attack. Causes include chlorides (from certain water or cleaners), trapped debris holding moisture against the surface, or a coating defect. Pitting is dangerous because it perforates with little overall metal loss - a line can look healthy and still spring a pinhole leak.
- If pits cluster under a deposit, gasket, or crevice: crevice corrosion. Stagnant moisture trapped in a tight gap loses oxygen and turns aggressive. Look for where water cannot drain or dry.
If a low or collecting point is eaten through
- If the corrosion is worst at the bottom of a pan, sump, or trap where water sits: standing-moisture attack. Water that never dries keeps the metal wet and oxygenated at the waterline. The cause is poor drainage, a clog, or a low spot. Fix the drainage, not just the pan.
If the whole surface is uniformly rusted or oxidized
- If the metal is evenly coated with product over its whole exposed face: general (uniform) corrosion. The expected, slow result of metal exposed to air and humidity over time. Least urgent of the types because it is predictable and progresses evenly. The fix is protection - coating, sealing, or controlling humidity - and it is often just age.
If you suspect electrical current is involved
- If a buried or grounded metal part corrodes unusually fast for its environment: stray-current or electrolytic corrosion. Leaking DC current forces metal into solution where it exits the part. Common around improperly grounded systems. The fix is electrical (find and correct the current path), not metallurgical. Replacing the part without finding the current just feeds it new metal.
Read the corrosion product color
| Product color/texture | Likely metal and meaning |
|---|---|
| Reddish-brown flaky rust | Iron or steel oxidizing; general or moisture attack |
| Green or blue-green crust | Copper or copper alloy; often at connections or with moisture and chlorides |
| White powdery / chalky | Aluminum or zinc (galvanizing) sacrificing itself; common in galvanic couples |
| Black scale | Iron in low-oxygen water, or high-heat oxidation |
| Bright clean metal next to heavy attack | The protected (noble) side of a galvanic pair |
The color confirms which metal is losing. White powder on a galvanized or aluminum part next to a clean copper one is the classic galvanic signature - the zinc or aluminum is doing its job and dying for the copper.
Safety note on electrical and pressurized parts
Corrosion on energized or pressurized equipment is a hazard, not just a cosmetic flaw. De-energize before handling a corroded electrical connection - the corrosion itself raises resistance and can arc. A corroded pressurized line can let go when disturbed; relieve pressure before working it.
Confirm the root cause before you replace
Name the mechanism, then verify the condition that drives it still exists: the dissimilar-metal contact, the trapped water, the chloride source, the stray current, the bad drainage. Correct that condition as part of the repair. A part that corroded once in place will corrode again if you only swap the metal and leave the cause.
References
- OSHA electrical safety standards - de-energize corroded electrical connections before service.
- NACE / trade-standard corrosion references on galvanic, pitting, crevice, and stray-current mechanisms.
- Manufacturer material-compatibility and dielectric-union guidance for the specific system.
- See related: Residue or Buildup Color Diagnosis, Unexpected Heat Diagnosis.