To pre-treat roads, a liquid salt “brine” mixture of 23% salt and 77% water is sprayed as an anti-icing measure and helps to prevent dangerous road conditions. (Lisa Bolton/The Washington Post)

Your car — an investment second only to your home (unless you’ve paid college tuition) — is rotting out from under you.

You know why, and if you’re prudent you will line up at the carwash with everyone else after this weekend’s meteorological mayhem. But there’s something more insidious than the white salt caking your car from the window level on down.

Brine wants to eat through your car like a school of hungry piranhas, and more than 2 million gallons of the stuff has been sprayed on roads this winter in Northern Virginia, Maryland and the District.

Are you just too sick of winter to read another story about it? Before you go, there are some things you’ve got to know about brine:

●The reasons you should love brine are the reasons your car hates it.

●You can invest in the carwash this week or a new muffler sometime down the road.

●Until you hit the carwash, don’t use an underground parking garage or the one attached to your house.

●Never drive behind a plow truck spreading salt and brine.

●Avoid dusty dirt roads in the summertime.

●Think about moving to Arizona. (After this winter, you probably already are.)

Stick with the story, and you’ll learn why.

Brine, which is used to pre-treat roads in the hope that snow will melt on contact, gets into cracks and crevices in which a chunk of rock salt can’t lodge. It’s both cheaper and more effective. The Michigan Department of Transportation (Want to see corrosion? Go to Michigan.) determined a while back that 40 percent of rock salt spread on the roads bounced off to the shoulder and did no good.

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.

For your car, however, it’s not better than rock salt. That has to do with something you don’t much think about in the wintertime: humidity.

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.

So, avoid warmer garages. On a bitter cold morning a few days ago, the temperature two levels down in The Washington Post’s underground garage was 44 degrees. That temperature can support a relative humidity above 30 percent. If your car is coated with magnesium chloride, it will turn wet at that temperature: corrosion city. Chilly but not frigid in your garage at home? Same issue.

“Inside that garage is a 100 percent time of wetness and a very corrosive situation,” said Baboian, who has written two books on automotive corrosion. “Sometimes the corrosion rate can be 100 to 1,000 times faster in the higher humidity and the higher temperature.”

You can tell when a roadway has been sprayed with brine before a storm. The stuff streaks the pavement with white lines.

Driving along behind a spray truck will cause you nothing but trouble. But it’s an issue even later unless the temperature is in the teens. Remember: Magnesium chloride above 20 degrees can form acid.

“The salt brine already is a frozen liquid, so the automobiles driving over that will get sprayed in the undercarriage and all the crevices, the wheels where the brake pads are and all of that,” Baboian said.

Unless you want to buy a new muffler soon, get the under­carriage wash.

A little-known fact, Baboian said, is that before something called the “bare pavement policy” came around in the 1960s, not much salt was used on roads. With the brutal winter this year, he figures more than 20 million tons will be used nationwide. So far, upward of 325,000 tons of rock salt have been used in Northern Virginia, the District and Maryland.

The District uses plenty of brine, but it doesn’t mix it with magnesium chloride.

“Ours is sodium chloride and we use this because of the known corrosive effects of magnesium chloride,” said Department of Public Works spokeswoman Linda Grant.

You can avoid all of this by moving to Arizona, Baboian said. In Tucson on Monday, the temperature is forecast to reach 70, and the average humidity will be below 50 percent. “The cars in Arizona are pristine after many years,” he noted.

But be wary of dusty dirt roads there, or anywhere else once summer rolls around.

“In the summertime, they spray a brine solution to cut down on the dust,” he said. “People think it’s water that they’re spraying. You follow directly behind that and you’re getting that sprayed on your vehicle.”