Pre-AP Chemistry: Handout 4.3.A. Flaking Away: From Ferraris to Fords

The old-style Volkswagen Beetle: is it a classic car or an endangered species?

The reason that Volkswagen Bugs and other older cars are dropping like flies isn’t the typical habitat loss or human encroachment that’s plaguing other endangered species.

The reason that Volkswagen Bugs and other older cars are dropping like flies isn’t the typical habitat loss or human encroachment that’s plaguing other endangered species.

Classic car fleets are constantly shrinking due to a chemical reaction that you’re no doubt already familiar with: rusting.

Read on to get the lowdown on how rust works and what measures you can take to stop corrosion in its tracks.

According to John Scully, a corrosion expert at the University of Virginia in Charlottesville, the redox reaction that forms rust needs just three ingredients to take place: an anode, or metal that readily gives up electrons; a cathode, or substance that easily accepts electrons; and an electrolyte solution, which shuttles ions between cathode and anode.

Steel has a nonuniform surface because the chemical composition is not completely homogeneous.

These defects create regions where the iron is more easily oxidized than it is at others.

Water on the surface of the steel is a solvent for ions produced when the iron metal, Fe(s), loses electrons (is oxidized to form Fe2+ ions) and the electrons are conducted through the metal and gained by water and oxygen from the air to form hydroxide ions...

Finally, iron(II) hydroxide reacts further with oxygen from the air to oxidize the Fe2+ and form insoluble iron(III) oxide, Fe2O3(s), the chemical name for rust:
4Fe(OH)2(s) + O2(g) → 2Fe2O3(s) + 4H2O(l)
.

The movement of ions through the electrolyte solution completes the electrical circuit that allows the electrons from iron to move from the anode to the cathode.

The magic ingredient that both areas share—which is missing in the basic recipe for rust—is an abundance of salt.

However, it’s also incredibly costly and impractical.

Plus, by the time most car owners notice a rust spot, a significant amount of iron has already sloughed off the car, lost to wind and rain.

One solution to stopping iron’s thermodynamic conversion, says Scully, would be to make cars out of a metal that doesn’t corrode, such as gold or silver.

“Converting to an oxide isn’t a thermodynamically favorable reaction for these metals, so they won’t corrode spontaneously,” he says.