Road salt is a common sight just anywhere that winter brings snow. But putting salt on a road's surface creates a massive amount of salty water that corrodes bridges and (as any car owner in a winter region knows) cars. Bridge corrosion is no small matter: The 2007 bridge collapse on Interstate 35 in Minneapolis that killed 13 people and injured 145 was partially caused by corrosion in the structure.
Since road salt contributes to the problem, chemists have sought a way to make the road salts less corrosive, or at least slow the process down. The Montana Department of Transportation is experimenting with using a biologically based chemical based on corn sugar.
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From a company called Rivertop, the additive, Headwaters, is made from sodium gluconate, which is derived from gluconic acid. Gluconic acid occurs pretty widely in nature –- in wine for example. By adding about one gallon for every thirteen of ordinary brine (a solution that is about 23 percent salt and 77 percent water), corrosion can be slowed by up to 70 percent, Dr. Dave Wilkening, product manager of corrosion sciences at Rivertop told Discovery News.
Rust happens because salt in water makes the water into an electrolyte, allowing electrons to move more freely. This is just like what happens in a battery. The problem is that moving the electrons in metals –- primarily iron -– makes the metal react more easily with the oxygen in the air. Rivertop’s chemical works because it’s negatively charged molecules have an affinity with the positively charged iron atoms that are missing electrons, essentially cutting off the electron flow and preventing the oxidation.
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There are lots of additives that have been developed over the years. But they have often been expensive and more importantly, add to the already environmentally dubious effects of salting roads. For example some contain phosphorus, which causes algal blooms when it gets into streams. Other corrosion inhibitors such as potassium acetate can leave behind acids that damage concrete. Therefore, Wilkening said, any corrosion inhibitor should be biodegradable and as importantly, cheap to make.
If nothing else, widespread use of corrosion inhibitors could cut down the cost of maintaining bridges, guard rails and signs -- and boost safety. But corrosion is a slow process, which can make it harder to convince local authorities to use inhibitors. "A guy can sign off to pay for the inhibitor or not, but by the time [corrosion] becomes a problem 15 years later the guy who is signing off on it is retired on a beach somewhere," Wilkening said.
Image: Wikimedia Commons
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