How road salt damages your car

What is on the road in winter

Ontario roads are treated with rock salt, which is sodium chloride, and increasingly with brine sprayed on before a storm so the ice cannot bond to the pavement. Some road authorities also use calcium or magnesium chloride, which keep working at colder temperatures. The exact mix varies by road and by year, but the result on your car is the same: a salty film that sticks to every surface and dries into the white crust you see on the lower doors.

How salt speeds up rust

Rust needs three things: iron, water and oxygen. Plain water on steel will rust it slowly. Salt dissolved in that water lets electric current move far more easily between spots on the metal, and that current is what rust is. So salty water does the same damage in much less time.

Chloride salts also tend to hold on to moisture. A crusted subframe can stay damp for days after the road has dried. That is why the worst rust weather is often not the deep cold but the mild days around zero, when slush is melting and refreezing and the reaction has warmth to work with. Cars parked in a heated garage can see the same effect, since the salt on them never really freezes.

Where it collects

Salt does its worst work in places you never see and rarely clean.

• Rocker panels and the pinch welds along the bottom of the doors
• Inside the wheel arches, especially behind plastic liners where slush packs in
• Brake lines and fuel lines running along the floor
• Subframes, suspension arms and the bolts that hold them
• Exhaust hangers, heat shields and the muffler
• Bottom edges of doors, the hatch and the tailgate, where drain holes can clog
• Brake calipers and their sliding pins, and the hitch if you have one

The first signs of damage

Light orange on brake rotors after a wet night is normal and wears off in the first few stops. The signs to take seriously are different: bubbling paint at the edge of a wheel arch, flaky brown scale on the subframe, crusty brake lines, a heat shield that starts rattling, or bolts that a shop has to cut off at tire changeover.

Paint chips are where a lot of body rust starts. Gravel and sand thrown up on Highway 401 and 402 nick the front edge of the hood and the lower doors, and once the steel is exposed, salt goes straight to work. Dabbing touch-up paint on chips in the fall is one of the cheapest things you can do for a car.

What washing can and cannot do

Washing removes salt that is sitting on the car. It cannot undo rust that has already started, and one wash in March does not make up for four months of crust. What counts is how often the salt comes off, and whether the wash reaches underneath. A spray on the paint makes the car look clean while the brake lines stay coated.

At Fast Lane, the Full Wash and Luxury Wash include an under-car spray and the Quick Wash does not, which is worth knowing when you pick a wash between November and April.