Montana Viewpoint


Current winter road de-icer may make roads safer, but it has some serious downsides

January 2, 2004

What’s your choice: safer winter highway conditions or a rust-free car? Because I’ve had a lot of complaints about the corrosive effects of the anti-icing treatment used by the Montana Department of Transportation (MDT) I asked the Legislative Auditor’s Office to look into the controversy. I also did a lot of research on the internet, and now know way more than I want to about highway deicers. Now you can too.

It used to be that the MDT used a combination of sand and salt to dissolve road ice and aid in traction. (They still use salt in sand, but just to keep the sand piles from freezing solid.) That has been replaced by the chemical magnesium chloride. There’s no doubt that mag chloride is more effective in fighting winter road ice. Salt melts ice only down to 20 degrees but mag chloride goes way down to zero. The move away from sand occurred for several reasons; damage to vehicles from sanding materials (chipped paint and windshields), a more aggressive approach to winter road maintenance, lower cost of maintenance, and concern for air and water quality due to dust or sedimentation.

A few years ago, there was a visible difference on Montana 200 where you crossed over the line separating the Thompson Falls Maintenance District from the Plains District. The Plains side was often snow free, while the T. Falls side was snow packed. As a result of questions from drivers, MDT found that Plains was spraying mag chloride before winter storms, while T. Falls used it only as a pre-wetting agent for sand (I don’t know what pre-wetting is either). Plains used much more of the mag chloride than T. Falls, but even though they had a higher cost for materials, they had a lower cost for labor and equipment. A four-year study found that the winter maintenance cost per mile of highway (both lanes) was $1095 for Plains and $1751 for Thompson Falls. There has not been a system wide cost analysis.

That sold the MDT. Now, mag chloride treated with corrosion inhibitor is sprayed on highways just before a storm and sand is added only where needed. Better than sliced bread from a highway safety and maintenance point of view, but what about the stories from auto mechanics about corroded wires and pitted metal? I was constantly being told that mag chloride is less corrosive than salt! That’s what the guys who sell it say, and it’s true—all things being equal. But they’re not.

While salt is less corrosive, it gets washed off the car in the course of driving on wet roads. Water dilutes it. Mag chloride, however, forms a film on the vehicle that doesn’t readily wash off. In fact, getting it wet reactivates it, and the corrosion inhibitor goes south from old age. It’s also deliquescent, which means it can liquefy by drawing humidity out of air. That’s why it’s also used for dust suppression. The advice from the manufacturers and state departments of transportation; wash your car, wash it often, and don’t forget to wash the undercarriage. Don’t forget to wear something warm and waterproof either, maybe a wet suit.

Mag chloride is not as non-controversial as the manufacturers and state highway departments claim. According to the Rural Utilities Service of the U.S. Department of Agriculture, mag chloride used near utility lines can cause power outages because it builds up on insulators and conducts electricity where it’s not supposed to go. Some Colorado utilities are giving their insulators a periodic scrub with soap and water. It also corrodes steel and aluminum poles and pole components.

The American Trucking Association reports that fleet managers are seeing increased corrosion on electrical components, chrome and aluminum, and most seriously, brake shoe metal, which is mighty hard to wash. They have asked the Federal Highway Administration to support an in-depth study of de-icers.

Oh, just one more thing; it’s suspected that it corrodes concrete things like pavement and bridges. If that’s true, it becomes a less cost effective method from the MDT’s point of view. You’ll have to judge if the corrosion it might cause to your rig is worth the touted increased highway safety.

Jim Elliott
Phone: 406-444-1556
Mail: State Senate Helena, MT 59620