Why Do Iron Nails Rust?

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Rust in iron and steel costs our economy billions of dollars each year.
Rust in iron and steel costs our economy billions of dollars each year. (Image: rusted image by Stephen Orsillo from Fotolia.com)

Iron nails and any other object made entirely or mostly of iron will rust in the presence of oxygen and water. According to Haverford College’s Knight Foundation science website, rust is a chemical reaction that rearranges four iron atoms and three oxygen molecules to form two molecules of iron oxide, each consisting of two iron atoms bonded to three oxygen atoms. The rust reaction needs water to transport the electrons that accomplish this atomic rearrangement of iron and oxygen. Putting some salt or acid in the water makes rusting go much faster by increasing water’s conductivity.

Iron in Our Lives

Iron is the base element of steel, and both metals are the major structural materials in our society, according to the Haverford College website, making up everything from buildings and bridges to cars and cans. Iron oxide is physically a much weaker substance than elemental iron, which is why heavily rusted iron and steel objects crumble under stress. In essence, rust is decayed iron.

Cost of Rust

Rust costs the U.S. economy about $560 billion a year, or about 6 percent of the U.S. gross domestic product, according to CorrosionCost.com, a website affiliated with the U.S. Transportation Department. The economic areas most heavily burdened by rust-related costs are water and sewer systems, motor vehicles, national defense and highways and bridges.

Stopping Rust

The main method for stopping iron and steel surfaces from rusting is to coat them with something that will keep oxygen and water from reaching the metal, according to the University of Illinois Department of Physics website. The most common coatings are paint and oil. Paints and oils are subject to wear and tear and must periodically be renewed to maintain rust protection.

Plating the Iron

Another way to stop rust is to plate the iron or steel with a thin layer of a rust-resistant metal, such as chromium, nickel or gold, according to the University of Illinois website. The metal plating keeps oxygen and water away from the iron. This is how auto bumpers are protected from rust.

Rust-Resistant Alloys

Instead of plating iron with rust-resistant metals, you can melt the metals together with iron to form a rust-resistant alloy. Stainless steel is a rust-resistant iron alloy made of iron, chromium and nickel melted together.

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