Describe the conditions needed for iron to rust, explain methods of rust prevention including sacrificial protection, and relate the properties of steel and alloys to their uses

What this dot point is asking

SEAB wants you to state the conditions required for iron to rust, explain the methods used to prevent rusting (barrier methods and sacrificial protection), and relate the properties of steel and other alloys to their uses. Rusting is the most important everyday example of corrosion, and the prevention methods tie directly back to the reactivity series.

The answer

The conditions needed for rusting

Rusting is the corrosion of iron (and steel). Experiments using iron nails in different conditions show that both water and oxygen (from the air) must be present for iron to rust:

• Iron in water and air rusts.
• Iron in water with no air does not rust.
• Iron in dry air (no water) does not rust.

So removing either water or oxygen prevents rusting. Salt (and acidic conditions) speeds up rusting, which is why cars rust faster near the sea and on salted winter roads. Rust is a form of hydrated iron(III) oxide.

Barrier methods of prevention

If air and water are kept away from the iron, it cannot rust. Barrier methods coat the surface:

• Painting, oiling or greasing, and coating with plastic put a physical layer between the iron and the air and water. These are cheap but fail if the coating is scratched.
• Galvanising coats the iron with a layer of zinc, which both acts as a barrier and gives sacrificial protection (below), so it keeps working even if scratched.

Sacrificial protection

Sacrificial protection uses a more reactive metal to protect the iron. Blocks of a more reactive metal such as zinc (or magnesium) are attached to the iron object (for example a ship's hull or an underground pipe). Because the attached metal is more reactive, it loses electrons and corrodes in preference to the iron, so the iron does not rust. The reactive metal is "sacrificed" and replaced when it wears away. A less reactive metal would not work, because it would not corrode in preference to the iron.

Steel and alloys

Pure iron is soft and rusts easily, so iron is usually used as steel, an alloy of iron with a small amount of carbon. The carbon atoms disrupt the layers of iron atoms, making steel harder and stronger than pure iron. Adjusting the composition tailors the properties:

• Mild steel (low carbon) is strong and easily shaped, used for car bodies and girders.
• Stainless steel (with chromium and nickel) resists rusting, used for cutlery and surgical tools.

Other alloys, such as brass and bronze, are chosen similarly for hardness and resistance to corrosion.

Examples in context

Example 1. Galvanised roofing. Corrugated iron roofing is coated with zinc (galvanised) so it resists rusting in the rain. The zinc both blocks air and water and, if the coating is scratched, sacrificially protects the exposed iron, giving longer-lasting protection than paint alone.

Example 2. Stainless steel cutlery. Knives and forks are made of stainless steel because the chromium it contains makes it resist corrosion, so cutlery does not rust when washed. The choice of alloy for the job shows how adjusting steel's composition matches it to an everyday use.

Try this

Q1. State the two conditions necessary for iron to rust. [1 mark]

• Cue. Water and oxygen (from the air) must both be present.

Q2. Explain why zinc, rather than copper, is used to sacrificially protect an iron object. [2 marks]

• Cue. Zinc is more reactive than iron, so it corrodes in preference to the iron and protects it; copper is less reactive than iron, so it would not protect the iron.

Q3. Explain why steel is used instead of pure iron for car bodies. [2 marks]

• Cue. Steel is an alloy of iron and carbon; the carbon makes it harder and stronger than pure iron, so it is more suitable for the structure of a car.