Dissimilar Metals and Corrosion
Why this matters
Put two different metals in contact with a little moisture and you have built a small battery that slowly eats one of them. This is galvanic corrosion, and it is behind a huge share of the connections that fail months or years after a clean install. The work looked perfect on the day. The metals were just wrong together. Knowing which pairings to avoid keeps you off the callback list for green crust at a fitting, a seized fastener, or a pinhole leak where two materials meet.
What galvanic corrosion is
When two different metals touch and an electrolyte (water, even humidity or condensation) bridges them, electrons flow from the more "active" metal to the more "noble" metal. The active metal becomes the anode and corrodes; the noble metal is protected. The bigger the difference in nobility between the two metals, the faster the active one is consumed. Three things must all be present for it to happen:
- Two metals far apart on the galvanic scale.
- Direct electrical contact between them.
- An electrolyte connecting them (moisture).
Remove any one of the three and the corrosion stops. That is your toolkit for prevention.
The galvanic order (active to noble)
This is the general ranking from most active (corrodes first, sacrificial) to most noble (protected). Pairing metals far apart on this list is the problem.
| Position | Metals (general groupings) |
|---|---|
| Most active (sacrifices itself) | Magnesium, zinc, galvanized coatings |
| Active | Aluminum, mild steel, cast iron |
| Middle | Lead, tin, common solders |
| Noble | Brass, bronze, copper |
| Most noble (protected) | Stainless steel (passive), silver, gold |
A useful field rule: the further apart two metals sit on this list, the worse the pairing. Copper touching galvanized steel is a classic bad pairing. Aluminum touching copper is another.
Where it bites in the field
- Plumbing: copper pipe joined directly to galvanized steel pipe. The steel corrodes at the joint and eventually leaks. A dielectric union (an insulating fitting that breaks metal-to-metal contact) is the standard fix.
- Fasteners: a steel bolt in an aluminum part, or a plain-steel screw in stainless. The active member corrodes and may seize or weaken.
- Roofing and flashing: copper flashing against galvanized, or dissimilar metal panels and fasteners. Runoff carries metal ions onto the downstream metal and corrodes it.
- Outdoor mechanical work: any two-metal contact exposed to rain or condensation.
The damage concentrates at the junction, which is exactly where you do not want it.
How to prevent it
You only need to break one of the three conditions:
- Break the metal contact. Use an insulating sleeve, washer, gasket, or a dielectric union so the two metals never actually touch. This is the most reliable fix.
- Keep moisture out. Seal the joint, coat it, or locate it where it stays dry. No electrolyte, no cell.
- Match the metals. When you control the material choice, pick metals close together on the list, or use compatible coatings.
- Use a sacrificial element by design where appropriate (the way a galvanized coating sacrifices itself to protect the steel underneath).
- Mind the area ratio. A small active part touching a large noble part corrodes fast and concentrated. A small stainless fastener in a large steel plate is far better than a small steel fastener in a large stainless plate.
Reading the failure
When you find corrosion at a connection, check whether two different metals meet there. If they do, cleaning and remaking the joint with the same two metals just restarts the clock. The real fix is to insulate them, seal out moisture, or change one of the metals. Note it on the work order so the next tech does not repeat the same pairing.
References
- Trade-standard practice on dielectric unions and dissimilar-metal isolation.
- Manufacturer documentation on approved fastener and material compatibility.
- See related: Why Connections Fail: Loose, Corroded, or Overheated.
- See related: Sealant vs Tape vs Gasket: Which Seal to Use.