Road salt and corrosion protection: winter work that saves spring bills

Road salt does its work invisibly: it holds moisture against metal and accelerates corrosion. The result becomes visible not in February but the following autumn — which is exactly why winter care gets postponed.

What actually corrodes

Worst affected are the enclosed cavities: inside the sills, the pillars, the reinforcements. Salted moisture gets in there and never dries.

Then: arches, suspension mountings, brake pipes, the exhaust, and seam welds.

Brake pipes are a safety topic of their own: a perforated pipe means losing the brakes, and it usually happens at a bracket under the floor.

The general overview is in the underbody rust article.

Winter washing: when it helps and when it hurts

It helps: washing the underside regularly — specifically underneath, not just the paintwork. It removes salt before it has time to work.

It hurts when:

• Washing in hard frost — water freezes in locks, seals, and mechanisms; covered in the freezing article.
• Not drying afterwards — water stays in cavities, which is worse than salt on the outside.
• Pressure washing sensor and connector areas at close range.

The right winter routine: wash above freezing or before a thaw, blow the locks out, and let the car dry before parking; after washing, drive and dry the brakes.

Corrosion protection that works

Cavity treatment is the main thing. Not an external underbody coating but injecting product inside sills and pillars through the factory access holes.

Underbody coating — an external layer protecting against stone blasting.

The main caveat: coating over existing rust seals the process in. The metal has to be cleaned back to bare, not covered in underseal — otherwise you are hiding corrosion, not stopping it.

The most effective and free measure: clearing the drains — sills, doors, the scuttle below the windscreen, and the sunroof. A blocked drain holds water exactly where it does most harm; covered in the water leak article.

Stone chips

A chip on the bonnet is exposed metal. In a salted winter it becomes a rust patch within one season.

What to do: touch it in immediately rather than waiting for spring. Fifteen minutes.

The spring inspection

After winter, an hour is well spent:

1. Wash the underside thoroughly.
2. Inspect sills and arches with a torch.
3. Tap suspicious areas — a dull sound means a bubble.
4. Check the brake pipes visually.
5. Touch in the stone chips.
6. Clear the drains.
7. Treat the seals with silicone.

What this has to do with diagnostics

Corrosion sets no codes, but its consequences do.

Corroded connectors and earths produce intermittent faults in systems nobody touched; mechanics in the ghost codes article.

Corroded battery terminals — covered in their own article.

ABS sensors in the wheel arches — a classic place for salt to do its work.

In short

• Salt works in winter and the result appears the following autumn.
• Wash the underside, not just the paint, and not in hard frost.
• Underseal over rust seals the process in rather than stopping it.
• Clear drains are the most effective and cheapest prevention.
• A bonnet chip becomes a rust patch in one salted season.

Download the app — codes across every system are read with an ELM327 adapter over Bluetooth SPP on Android.