In most states, brine is a mix of rock salt (sodium chloride) and magnesium chloride, dissolved in water so they can be sprayed on the road. “That’s a very important point, because magnesium chloride is much more corrosive than sodium chloride, the rock salt,” said Bob Baboian, an auto industry consultant and a fellow at the National Association of Corrosion Engineers.
Car rust and corrosion are caused by acid created when a salt is dissolved by the moisture in the air. Rock salt remains a crystal until the humidity reaches 70 percent, which doesn’t happen much during the winter. But magnesium chloride dissolves when there is only about 20 to 30 percent humidity. “Which means that your vehicle, if magnesium chloride is sprayed on it, is wet constantly,” Baboian said. The acid stays on your car and slowly eats away at the paint and metal.