he old-style Volkswagen Beetle: Isit a classic car, or an endangeredspecies? The answer depends onwhere you live. Although there arefew classic cars hanging aroundthe northern states or on the

coastlines, plenty of vintage automobiles stillexist in the mild southern climates and inAmerica’s interior states.

The reason that Volkswagen Bugs andother older cars are dropping like flies isn’t thetypical habitat loss or human encroachmentthat's plaguing other endangered species.Classic car fleets are constantly shrinking dueto a chemical reaction that you’re no doubtalready familiar with: rusting.

But why does rust unequally strike carsin the snowy states and coastal towns butleave vehicles elsewhere virtually untouched?And more importantly, how can you keepyour beloved grocery-getter safe, no matter

what parking place you call home? Read onto get the lowdown on how rust works andwhat measures you can take to stop corro-sion in its tracks.

Electron swap meetLike all types of corrosion, rust is

actually a chemical bargain, withtwo reactions in one: reduc-tion, in which someatoms gain electrons,and oxidation, inwhich other atomslose electrons. Withall those electronsflowing from oneplace to another,rust-making is alsoconsidered an electro-chemical reaction.According to John Scully, a

From Ferraris to Ford Pintos,almost every car is fighting a losing battle to rust.

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18 ChemMatters, FEBRUARY 2006

corrosion expert at the University of Virginia inCharlottesville, the redox reaction that formsrust needs just three ingredients to take place:an anode, or metal that readily gives up elec-trons; a cathode, or substance (in this case,oxygen) that easily accepts electrons; and anelectrolyte solution, which shuttles ionsbetween cathode and anode.

Most cars are made mostly of steel, atough mixture of iron, carbon, and smallamounts of other ingredients like manganese,silicon, phosphorus, and sulfur. It’s the ironpart of steel that corrodes to make rust.

Iron doesn’t hold onto its electronsvery tightly, says Scully, making it the per-fect anode for an electrochemical reaction totake place. Other metal atoms in the steelmixture, or even another point on the pieceof iron, make excellent cathodes. Steel has anonuniform surface because the chemicalcomposition is not completely homoge-neous. Also, physical strains leave stresspoints in the metal. These defects createanodic regions where the iron is more easilyoxidized than it is at others (cathodicregions). However, without a bridge to con-nect potential anodes and cathodes, rustingwould be such a time-consuming processthat cars would virtually last forever.

The water on the steel surface is a sol-vent for ions produced when the iron metal atthe anodic region loses electrons (is oxidizedto form ferrous ions) and the electrons areconducted through the metal to the cathodicregion where they react with water and oxy-gen from the air to form hydroxide ions, asshown in these equations:

2Fe ! 2Fe2+ + 4e- (oxidation at anodic sites)

4e- + O2 + 2H2O ! 4OH-

(reduction at cathodic sites)

The ions in this electrolyte solution canmigrate together and react to form ferroushydroxide, which reacts further with oxygenfrom the air to oxidize the ferrous ion andform insoluble ferric oxide, the chemical namefor rust, as shown in these equations:

Fe2+ + 2OH- ! Fe(OH)2

(formation of ferrous hydroxide)

4Fe(OH)2 + O2 ! 2Fe2O3 + 4H2O(oxidation to ferric oxide or red ‘rust’)

The movement of ions through the electrolytesolution completes the electric circuit thatallows the electrons from iron to move fromthe anode to the cathode.

But all this still doesn’t explain whycolder climates and coastal areas get anunfair share of rust. The magic ingredient thatboth areas share—which is missing in thebasic recipe for rust—is a high abundance ofsalt. Coastal areas have plenty of salt sailingthrough the air from ocean spray, and witheach cold, snowy winter, northern statessmear tons of rock salt on roads to lower thefreezing point of water and help keep roadsfree or ice and snow. [See “Salting Roads:The Solution for Winter Driving” in this issue]

Salt speeds rust’s redox reaction alongby making water a better conductor. “Saltallows the anode and cathode to be in toucheven better,” says Scully, making corrosionhappen even quicker. Also, chloride ions formvery stable complex ions with Fe3+, whichhelps dissolve iron and accelerate corrosion.

Costly corrosionScully points out that iron can’t help

rusting—existing as an oxide in its thermody-namically favored state. In fact, the metalrarely exists in a pure state in nature. Before itbecomes a side panel in your car, engineersmust convert rusty iron ore into a pure metal.

With rust being iron’s favored state, it isof little use trying to fix rust after it hasalready happened. By putting energy intorust, it’s possible to plate metal back onto a

car, says Scully. However, it’s also incrediblycostly and impractical. Plus, by the timemost car owners notice a rust spot, a signifi-cant amount of iron has already sloughed offof the car, lost to wind and rain.

One solution to stopping iron’s thermo-dynamic conversion, says Scully, would be tomake cars out of a metal that doesn’t cor-rode, such as gold or silver. “Converting toan oxide isn’t thermodynamically favorable

A redox reaction on wheels. The bumper of this vehicle wears the product of the reaction between ironand oxygen—rust!

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reaction for these metals, so they won’t cor-rode spontaneously,” he says. “Archaeolo-gists can dig up gold coins that have survivedthrough the ages without corroding.”

But while driving a gold car might makeyou feel like a million bucks, making such avehicle would cost substantially more. Not tomention the fact that these soft metals wouldbe unable to support the car’s weight andwould squash like putty in a collision!

Even using noncorroding metals that arecheaper than gold or silver, such as stainlesssteel that Deloreans are made of, is still moreexpensive than using the plain steel that mostcars are made of today.

The best way to prevent corrosion is stillthe cheapest. A good coating of paintremoves the connection between anode andcathode by preventing water from makingcontact with steel. Without water, rustingslows down to a snail’s pace.

Today’s high tech paints have evolvedfar from being just a simple barrier, saysScully. Researchers are currently working onpaints that release rust inhibitors on demandwhen paint’s seal on steel is breached, forexample, when the paint on a car is scrapedor scratched. Other “smart paints” that oozetogether to close gaps whenever a car’s pan-els get scratched are also in the works.

Although rust is a big deal for car own-ers, it's an even bigger deal for industries thatrely on machines with metal parts, rangingfrom farm tools to factory equipment tofighter jets. According to Scully, corrosioncosts the United States a whopping annualtoll of about $276 billion in lost goods andservices. With the exorbitant cost of new mil-itary equipment, the Department of Defense(DOD) is one of the largest investors inantirust research, says Scully. Scientists atthe DOD hope they can keep the aging Black-hawk helicopters and B52 bombers that arecurrently in use running smoothly fordecades to come—a cost savings of millionsof dollars per machine.

But one military asset sometimesharmed by corrosion is extremely difficult toput a price on, says Scully. “If a soldier goesto war and the rifle he’s using to protect him-self doesn't fire when he needs it to, how doyou estimate the cost of corrosion then?”

Christen Brownlee is a contributing editor toChemMatters. Her article “Super Fibers” alsoappears in this issue.

This student investigates the role of salt on therate of rusting by putting nails in various saltwater solutions.

It’s not only cars that are rusting away. Corrosion costs the United States a whopping $276 billion per year!

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