How Ontario Winters Cause Vehicle Rust

Rust is iron reacting with oxygen in the presence of water. In Ontario, road salt (sodium chloride) is the accelerant. When salt dissolves in melting snow, it lowers the freezing point of the resulting brine, which means it stays liquid — and reactive — at temperatures where plain water would be safely frozen.

That salty brine is what gets sprayed onto your vehicle's underbody every time you drive through slush. It works its way into cavities, seams and pinhole gaps in factory undercoating, and it stays wet and reactive for weeks.

Toronto sees more than 120 freeze-thaw days per year. Every cycle, moisture inside a rocker panel or wheel well freezes and expands, breaking factory sealer and creating new paths for water and salt to reach bare metal.

Vehicles parked in unheated driveways cycle through freeze-thaw all night. Vehicles parked in warm garages fare slightly better on that specific mechanism, but they can actually rust faster in other ways: warm temperatures speed up the chemical reaction between chloride and steel.

Wash the underbody frequently through winter, especially after storms and salt events. Rustproof annually. Address surface rust immediately, before it can spread. And if you own a collector vehicle, keep it off the road entirely between November and April.