The Yin And Yang Of Road Salt

Road salt is both your friend and your enemy. Those of us who live in areas with winter snow and ice have all been helped and hindered by it. It can save your life and kill your car. Most auto enthusiasts hate the stuff. Short term, it makes your car a mess. Long term, it literally rots your car away. Hit the jump for some interesting facts about road salt and some tips and tricks on how to slow down its appetite for destruction.



Let’s start with the positives of road salt, as they are obvious and easy to recount:  It keeps the roads from being icy.   Those are the positives.  I know, it’s not much of a list, but it is an important list.  It represents your physical well-being, your ability to get to work, earn a living, and support your family.  It allows cold climate communities to function equally well year round.  Unfortunately, the positives, while important, are boring.  Let’s move on to the negatives.

The main negative that just about everyone knows is corrosion.  A local car wash has an animated sign that depicts a dragon putting a car in its mouth and the caption is “Salt Eats Cars!”  It is truth in advertising.  Salt causes corrosion.  Rust is a chemical reaction that occurs under certain conditions.  When metal (usually steel alloy) comes in direct contact with air and water, the oxidation process begins.  Since oxidation is an electrochemical process (the metal loses electrons), the whole process speeds up when salt is added because salt water is a much better conductor than pure water. 

There are several different kinds of road salt.  The main one is good old fashioned salt (NaCl), just like you put on your eggs.  Salt is by far the most commonly used de-icing product.  In the last decade, however,  there has been an upswing in the use of Magnesium Chloride (MgCl2) and Calcium Chloride (CaCl2).  Generally, these are used as an anti-icer and applied to roads before a storm or freezing conditions.  They are often applied in a liquid solution that may include sugars or vegetable oil to make them adhere to the pavement.  There is much debate as to which chemical causes more corrosion, but one thing is certain, they all accelerate the process.

When you drive on a salty road, especially one that is wet, the salt solution sprays your car.  As it runs off, it deposits itself in nooks and crannies.  There, it promotes corrosion.  But it is not just an exterior concern.  Salty water is tracked inside the car on your shoes, it soaks through the carpet and rusts the car from the inside out as well.

When winter ends, you may not consider what salt can do to your car on a nice, sunny day.  If you have salt deposits left over from the winter, they will continue to work their magic year-round.  Salt is a dessicant.  It has long been used to dry foods.  In your car, it dries the air around it, and thereby keeps itself constantly wet.  On a recent trip to upstate New York, I noticed many 10-15 year old cars with huge areas of rust-through.  Here in Utah, the rust is not nearly as bad despite the fact that both areas receive considerable snowfall.  My non-scientific threory is that there is a lot more humidity in the summer in the Northeast, which keeps those salt deposits wetter and more aggressive year-round.

So what can you do?  Oddly, one remedy for a salt-exposed car is to spray acid on it.  Low pH detergents will break up the salt where neutral or high pH detergents may do nothing, or even harden salt deposits.  Many automatic car washes have incorporated low pH pre-rinse cycles and then a final neutralizing cycle.  Inquire at your local car wash if they are using this system.  (Side note:  Last summer, I took my midget out to Speed Weed on the Bonneville Salt Flats.  It was driven around the pit area enough to get covered with salt.  I had several people tell me to go home, clean it, and then spray a vinegar and water solution on it to prevent rust.  Now I understand that advice.  Vinegar is a low pH cleaner.)


Other remedies include a good coat of wax, frequent washing, and undercoating.  They may all help, but none are fail-safe.  Keeping your vehicle in the garage during inclement weather months is really the only way to prevent salt related corrosion. 

Interesting facts and figures:

  • Salt is a desiccant – it attracts water and keeps itself wet
  • Below -5* F, salt will not melt ice
  • Salt will enter your car on your shoes and cause rot from the inside-out
  • Mildly acidic detergents help to break up salt deposits
  • Road salt runoff can threaten rivers and wildlife
  • Application rates are around 250 pounds per lane-mile
  • Potassium Acetate is less corrosive, easier on the environment, and will de-ice to -60* F, but is very expensive
  • Potassium Acetate, for the reasons above is used on airport runways and certain bridges and overpasses
  • In the United States, about 70% of roads (and people) are in areas that get at least 5″ of snow annually.


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