Oxidation and Rusting

Oxidation is the chemical reaction that takes place when a substance combines with oxygen. Some oxidation reactions are extremely fast (like burning a candle or exploding a firework). Others are very slow (like iron rusting on a fence or copper turning green on a statue). All of them share the same basic chemistry: oxygen atoms join up with other atoms to form new compounds called oxides. Oxidation is everywhere, from the apple browning on your plate to the dying battery in your phone.

  • What it isSubstance + oxygenForms an oxide
  • Fast oxidationBurningFire, explosion, flame
  • Slow oxidationRusting, browningTakes hours to years
  • Rust formulaFe2O3.xH2OHydrated iron oxide
  • Why steel rustsIron + water + oxygenAll three needed
  • Yearly cost worldwide$2.5 trillion3% of global GDP lost to rust

What is oxidation?

In its simplest form, oxidation is a chemical reaction in which atoms of an element combine with atoms of oxygen. The new compound is called an oxide.

  • Iron + oxygen -> iron oxide (rust)
  • Copper + oxygen -> copper oxide
  • Magnesium + oxygen -> magnesium oxide
  • Carbon + oxygen -> carbon dioxide

Some oxidation reactions release lots of heat (burning a piece of magnesium ribbon produces a blinding white flame). Others are so slow that you cannot tell they are happening at all without careful measurement.

Rust: the most famous oxidation

Rust is the brown-orange flaky substance that forms on iron and steel left in damp air. It is a special kind of iron oxide: hydrated iron(III) oxide, with the chemical formula Fe2O3.xH2O.

Rusting needs three things:

  • Iron (the metal being attacked)
  • Oxygen (from the air)
  • Water (even a tiny amount of moisture is enough)

Take away any one and rust stops. Iron in dry desert air does not rust, even after decades. Iron underwater (without oxygen mixed in) does not rust either. But iron in damp air or salty sea spray rusts quickly. Salt water makes rusting even faster because the dissolved ions help the reaction along.

Fact Rust is bad news for engineering. It is brittle, flaky and expensive to remove. The world spends an estimated 2.5 trillion US dollars every year fighting corrosion: protecting bridges, pipes, ships, cars and steel buildings from oxidation. That is about 3 per cent of the worlds entire economy.

How to prevent rust

Engineers use many tricks to stop iron and steel from rusting:

  • Painting: a layer of paint keeps water and oxygen away from the metal. Cars, ships and bridges all rely on this.
  • Oiling and greasing: a thin film of oil blocks water. Used on tools, hinges and bike chains.
  • Galvanising: coating steel with zinc, which corrodes much more slowly and even protects the iron underneath chemically.
  • Stainless steel: an alloy of iron with chromium and nickel. The chromium reacts with oxygen to form a thin, invisible, protective layer that stops further oxidation.
  • Cathodic protection: attaching a piece of more reactive metal (zinc or magnesium) to the steel. The reactive metal rusts first, sacrificing itself to protect the steel. Used on ship hulls, oil pipelines and metal jetties.

Oxidation in everyday life

You see oxidation happening all over the place:

  • Apple slices turning brown: when you cut an apple, oxygen in the air reaches enzymes inside the fruit. The enzymes oxidise certain chemicals into brown pigments. Squeezing lemon juice on the apple slows this down because vitamin C is an antioxidant.
  • Copper turning green: the Statue of Liberty is made of copper. After many years exposed to damp salty air, the copper oxidises to form a green coating (copper carbonate and copper sulfate), called a patina.
  • Silver tarnishing: silver jewellery slowly turns black because of a reaction with tiny amounts of sulfur in the air, forming silver sulfide.
  • Bronze going green: bronze statues outdoors slowly grow a green coating, similar to copper.
  • Iron tools rusting: garden tools left in the rain go brown and crumbly.
  • Cooking oil going rancid: oils exposed to oxygen for a long time develop a bad smell and taste as the fat molecules oxidise.
Did you know? Your body uses controlled oxidation to release the energy in food. When you eat sugar, your cells gradually combine it with oxygen, breaking it down into carbon dioxide and water, and releasing the energy stored in the chemical bonds. The overall reaction is exactly the same as burning sugar, but it happens slowly, in tiny steps, controlled by enzymes, so the energy comes out in a useful form rather than as a sudden burst of heat.

Antioxidants

An antioxidant is a chemical that slows or prevents oxidation. They are useful in food (to stop fats going rancid), in materials (to stop rubber and plastics breaking down) and in the human body (to mop up harmful by-products of normal oxidation).

Common natural antioxidants include vitamin C, vitamin E and many of the colourful pigments in fruits and vegetables. Diets rich in fruit and vegetables provide lots of antioxidants, which may help protect the body from some diseases of ageing.

The wider meaning of oxidation

To chemists, "oxidation" has come to mean any reaction in which a substance loses electrons. The opposite (gaining electrons) is called reduction. Oxidation and reduction always happen together in pairs: one substance gives up electrons, another accepts them. These are called redox reactions, and they power batteries, photography and most of biology.

Try this Put three steel nails in three glasses. Cover one with tap water. Cover the second with cooking oil. Leave the third in the air, dry. Wait three days. The nail in tap water will rust noticeably (iron + oxygen from dissolved air + water). The oil-covered nail will not rust (oil blocks oxygen and water). The dry nail will hardly rust at all (no water). You have proved that all three ingredients (iron, oxygen, water) are needed for rust.
Deeper dive: how the Statue of Liberty stays green

The Statue of Liberty in New York harbour is made of 31 tonnes of copper sheet, shaped over an iron skeleton designed by Gustave Eiffel (the same engineer behind the Eiffel Tower). When it was unveiled in 1886, the statue was the dull orange-pink colour of fresh copper.

Within 30 years it had turned the famous greenish-blue colour we know today. The colour comes from a thin layer of copper compounds that gradually formed on the surface as the copper reacted with oxygen, water and salts from the sea air. The exact compounds include:

  • Copper carbonate (from CO2 in the air)
  • Copper sulfate (from sulfur compounds in pollution)
  • Copper chloride (from salt spray)

This greenish coating is called the patina. Crucially, it actually protects the copper underneath from further damage, like a chemical paint. So although the statue looks rusty in a sense, the patina has been preserving the metal for over 130 years.

Other famous green statues around the world (the Bronze Buddha of Kamakura in Japan, the Charles V monument in Madrid, many roof finials on European churches) have the same chemistry behind them. Engineers have learned to deliberately produce patinas on new copper roofs to give the same green protective layer from day one.

For more, see combustion and acids reacting with metals.