That’s not rust. Well, that one is, but that isn’t. Different colour, different texture, and critically a different place. One’s in the middle of a panel where water sat. The other is exactly where two different metals are bolted together, and nowhere else.
Because in the right conditions a car doesn’t simply rust. It turns into a battery, a very bad one that slowly consumes part of itself. And the part it consumes isn’t random.
This is the first of four posts about cars near the sea. If your car lives in salt air (coastal, an island, or roads that get salted and aren’t rinsed) they’re for you. If you’re inland, stay anyway: the mechanism happens on every car, all the time, just slowly. Salt doesn’t cause it. Salt accelerates it, dramatically. Severity is regional. Mechanism isn’t.
I’m a certified master paint technician (B&P TEAM, 2019).
The Mechanism (It’s a Real Battery)
You need three things: two different metals, an electrical connection between them, and an electrolyte touching both.
A car is thousands of joints between different metals, all bolted, welded or clipped into electrical contact: steel panels, aluminium components, zinc coatings, stainless fixings, magnesium castings, copper. That’s two of the three ingredients, everywhere, by design. The third is the variable. Plain water conducts a little. Dissolved salt makes it conduct far better, and that’s the whole difference between a coastal car and an inland one.
With all three present, a small current flows between the metals. One metal gives up material, corroding faster than it would alone, while the other is slightly protected. So it isn’t spread across a panel. It’s concentrated exactly at the junction, and it eats one side while leaving the other looking fine.
Four Recognition Points
- Location. Ordinary rust starts where water sits and protection has failed (a chip, a drain point, a seam). Galvanic corrosion starts at a junction between different metals, even on an otherwise perfect surface. Corrosion in the middle of a panel is probably ordinary. Corrosion tightly concentrated around a bolt head, rivet, bracket mount or trim fixing points here.
- Colour and texture. Aluminium corroding galvanically goes white, grey or chalky, and powdery rather than flaky. It can swell into a white crusty build-up that lifts paint or forces a joint apart, which is very distinctive.
- Which side. One metal eaten and the other untouched is the signature. Ordinary rust doesn’t pick a side.
- Where it isn’t. If the same junction on the other side of the car looks identical and the rest of the panel is fine, you’re looking at something systematic: a design junction doing what it does.
Where to Look
Anywhere an aluminium or alloy component meets steel: roof rails and their mounting feet, bonnet and boot fittings on some cars, alloy brackets on steel subframes, and small metal fasteners through painted steel (trim and badge fixings) often with a different metal inside.
What to Do
Look, photograph, and keep it clean. Rinse salt off regularly, because removing the electrolyte removes the battery. Don’t scrape or grind at a suspect joint, and don’t paint over it: that hides the thing a professional needs to see. If a fitting is swelling or a joint is being forced apart, get it inspected.
There are no product links here. This is an “understand what you’re looking at” post, not a shopping one.
How to reduce the risk
- Wash road salt and grime off the car regularly, including the underbody and around wheel arches.
- Repair stone chips and scratches early so bare metal is not exposed.
- Keep battery terminals, earth straps and cable ends clean, tight and protected.
- When a repair joins different metals, ask the workshop to use proper corrosion protection at the join.
When to see an auto electrician
Repeated flat batteries, flickering lights, green or white crusty deposits at cable ends, or electrical faults that come and go can all point to corrosion or poor earthing. These are not good DIY jobs. A qualified auto electrician can trace the circuit, test the earth paths and repair or replace the damaged parts.
What not to try
Do not add wiring, change earth points or spray random chemicals over connections to see whether the fault goes away. A bad connection that is masked for a week is still a bad connection, and it is harder to diagnose afterwards. If you live near the coast or drive on salted roads, make regular washing and an annual inspection part of your routine.
Quick FAQ
What is galvanic corrosion on a car? Corrosion where two different metals in electrical contact are bridged by a conducting liquid like salt water, so one metal corrodes faster.
Why is there white powder around my aluminium bolt? That’s typical of aluminium corroding at a junction with steel.
Does this only happen near the sea? No. It happens everywhere, but salt accelerates it dramatically.
No product links in this post: nothing sold in a bottle fixes it, and I don’t link products I’d be guessing about.
Related posts: Undercarriage Rust and Salt · Caliper Seizing in Salt Air: Brakes Have to Move · Clearcoat Chalking: Run a Damp Towel Across the Panel
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