Salt rocks – which are just that, rocks of salt – are compromised of salt or for you nerd, sodium chloride. Salt brine, on the other hand, is sodium chloride and magnesium chloride dissolved in water. “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 to the Washington Post.

“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.”

Think hiding your prized car away in the garage will save you? Think again. As the salty brine serves its purpose in reaching tiny nooks on paved highways, the solution is also finding its way into crevices of your car not reachable by rock salt. Add humidity and a decent temperature to the mix, and you’ve got this brine on steroids – eroding 100 to 1,000 times more than it did outside. You’d have to have your garage chilling in teen-temps for the brine not to act so aggressively. This means – get the whip a washdown including the undercarriage before putting it away for the night.

So why have the “DOTs” opted for the car-corroding solution? According to the WaPo, “brine is sprayed on as a liquid. It doesn’t bounce, lands where it’s directed and is 100 percent effective. Safer for you as a driver, and better for you as a taxpayer.” In fact, Maryland has used 40% less salt rock this year opting for the brine instead. According to Maryland’s State-Wide Salt Management Plan, road salt that’s used in winter can add up to high concentrations that can harm plants and waterways. ““This is the first winter where … each county will have one [secondary] route that they designate as liquid-only. There’ve been several winters that we’ve tried it in certain areas. Down in our La Plata shop, down in Charles County, they’ve been using it for several years,” David Coyne said in a WTOP article covering the changeover. Also noted, “the Michigan Department of Transportation… determined a while back that 40 percent of rock salt spread on the roads bounced off to the shoulder and did no good.”

That said, Washington DC has opted for a brine solution that doesn’t use the magnesium chloride in an effort to save what little car we have left after its pot holes.

So in summary: