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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 electrons;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 perfectanode 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 homogeneous.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 connectpotential anodes and cathodes, rustingwould be such a time-consuming processthat cars would virtually last forever.The water on the steel surface is a solventfor 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 oxygenfrom the air to form hydroxide ions, asshown in these equations:2Fe ➞ 2Fe 2+ + 4e - (oxidation at anodic sites)4e - + O 2 + 2H 2O ➞ 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:Fe 2+ + 2OH - ➞ Fe(OH) 2(formation of ferrous hydroxide)4Fe(OH) 2 + O 2 ➞ 2Fe 2O 3 + 4H 2O(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.A redox reaction on wheels. The bumper of this vehicle wears the product of the reaction between ironand oxygen—rust!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 Fe 3+ , whichhelps dissolve iron and accelerate corrosion.Costly corrosionScully points out that iron can’t helprusting—existing as an oxide in its thermodynamicallyfavored 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 acar, says Scully. However, it’s also incrediblycostly and impractical. Plus, by the timemost car owners notice a rust spot, a significantamount of iron has already sloughed offof the car, lost to wind and rain.One solution to stopping iron’s thermodynamicconversion, says Scully, would be tomake cars out of a metal that doesn’t corrode,such as gold or silver. “Converting toan oxide isn’t thermodynamically favorableMIKE CIESIELSKI18 ChemMatters, FEBRUARY 2006
This student investigates the role of salt on therate of rusting by putting nails in various saltwater solutions.MIKE CIESIELSKIIt’s not only cars that are rusting away. Corrosion costs the United States a whopping$276 billion per year!GETTY IMAGESreaction for these metals, so they won’t corrodespontaneously,” he says. “Archaeologistscan 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.MIKE CIESIELSKIToday’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 panelsget scratched are also in the works.Although rust is a big deal for car owners,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 militaryequipment, the Department of Defense(DOD) is one of the largest investors inantirust research, says Scully. Scientists atthe DOD hope they can keep the aging Blackhawkhelicopters 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 himselfdoesn'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.ChemMatters, FEBRUARY 2006 19
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