Why do two different metals corrode when they touch?
Two dissimilar metals plus moisture make a battery, and one of them is consumed to protect the other. The pairs you meet in a house, which one loses, and the three ways to break the couple.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 4 min
Short answer
When two different metals are in contact and moisture bridges them, a small current flows and the less noble metal corrodes faster than it would alone. Aluminium against stainless, galvanised against stainless, and steel against copper are the pairs that cause most household trouble. The fixes are to keep them dry, to insulate them from each other, or to choose metals that sit close together in the series.
This is the mechanism behind a surprising number of complaints — a new stainless bracket that has rotted the aluminium it was fixed to, galvanised screws that rusted in a year in a stainless fitting, a copper cylinder pitting a steel bracket. It is not bad luck or bad materials. It is a battery someone built by accident.
Step by step
- Understand the three things it needs.Two different metals, electrical contact between them, and an electrolyte bridging them — which in practice means water, and salty or dirty water most of all. Remove any one of the three and it stops.
- Learn who loses.Metals sit on a scale from less noble to more noble. In contact, the less noble one corrodes preferentially. Roughly, from most sacrificial to most protected: magnesium, zinc, aluminium, mild steel, cast iron, lead, brass, copper, stainless steel.
- Notice that relative area matters enormously.A small piece of the less noble metal attached to a large piece of the more noble one corrodes fast, because all the current concentrates in it. A stainless screw in an aluminium frame is the bad way round. An aluminium rivet in a stainless plate is much worse still.
- Recognise the household pairs.Stainless fixings into aluminium. Stainless fixings into galvanised steel. Copper pipe against a steel bracket or a steel cylinder. Copper gutters or flashings draining onto galvanised or zinc. And copper water running over an aluminium or galvanised surface, which corrodes it even without direct contact.
- Remember that treated timber counts.Modern copper-based timber preservatives make treated timber aggressive to plain and lightly galvanised fixings. That is why treated timber calls for hot-dip galvanised or stainless fixings, and it is a chemical attack alongside the galvanic one.
- Break the couple where you can.A nylon or fibre washer, a plastic sleeve, a strip of membrane, or a coat of paint or barrier tape between the two faces stops the electrical contact. This is why isolating washers exist and why they are worth using.
- Or keep the water out.No electrolyte, no cell. A joint that stays genuinely dry can hold dissimilar metals indefinitely, which is why the same pairing that fails outdoors is unremarkable inside.
- Or use it deliberately.Sacrificial anodes work on exactly this principle — a block of magnesium or zinc is fitted so that it, rather than the tank or the hull, is consumed. Galvanising is the same idea: the zinc is sacrificial to the steel beneath.
Tips
- Coastal air is salty and salt is a strong electrolyte, which is why the same detail that lasts inland fails within sight of the sea. It is the same reason A4 stainless exists.
- If you must mix metals in a wet joint, make the fixing the more noble of the two and keep it small relative to what it fixes.
- A run of green or white staining below a fixing is the visible sign of a couple working. Follow the stain up to its source.
Common mistakes
- Upgrading only the fixings to stainless on an aluminium or galvanised structure — It makes the fixings safe and accelerates the corrosion of everything around them. The structure is now sacrificing itself for the screws.
- Using galvanised or plain fixings in modern treated timber — The copper-based preservative attacks the zinc coating chemically and galvanically. Hot-dip galvanised or stainless is what the timber requires.
- Letting a copper gutter or pipe drain over a galvanised or zinc surface — Copper ions carried in the water corrode the zinc even with no contact between the two metals. It is a common cause of a mysterious hole in a metal roof or gutter directly under a copper detail.
If it doesn't work
White powdery corrosion around stainless bolts in an aluminium frame
Cause: Galvanic attack on the aluminium, which is the less noble metal and the larger area is not saving it at the contact — Fix: Isolate the fixings with nylon washers and sleeves, or change to fixings closer to aluminium in the series. Clean out the corrosion product first, because it holds moisture.
Rust bleeding out of a joint between two galvanised parts held with a stainless bolt
Cause: The zinc coating is being consumed preferentially around the bolt — Fix: Isolate the bolt or change it. Touching up the zinc with a zinc-rich paint helps only until the coating is consumed again.
A pinhole through a metal gutter directly below a copper pipe outlet
Cause: Copper ions in the runoff attacking the zinc or aluminium below — Fix: Redirect the discharge, or fit a sacrificial sleeve. Patching the hole without stopping the source produces another hole beside the patch.
Questions people ask
Is it a problem indoors?
Rarely, because there is usually no persistent moisture. Bathrooms, utility rooms and anywhere with condensation are the exceptions, and so is anything that gets wet in use.
Can I just use stainless everywhere?
It solves the fixing but not the couple. If the thing being fixed is aluminium or galvanised, stainless makes it worse. Choose the whole assembly, not just the screws.
Does stainless against stainless matter?
The same grade against itself is fine. A2 against A4 is a very small difference and is not a practical problem. The trouble is between families, not within one.
What about copper pipe and brass fittings?
They sit close together in the series and are a standard, proven combination. That closeness is precisely why plumbing uses them together.