Rust and what stops it
Two iron panels get the same scratch and the same rain. Only one grows an orange crust.
- A painted panel and a galvanised panel, both scratched.
- Rust grows at the scratch in the paint. The galvanised panel stays clean.
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The rule
Iron rusts when air and water are both present. Paint, oil and grease keep air and water away. A zinc coat, called galvanising, protects iron even when it is scratched. Mixing iron with other metals, as in stainless steel, resists rust.
Corrosion is slow damage to a metal by air, water or acid. Rust is the corrosion of iron.
Rust is iron oxide with water in it. The rust flakes off, and the iron underneath keeps rusting. Fresh iron is then exposed to air and water.
| Method | How it works |
|---|---|
| Paint, oil, grease | A barrier that keeps out air and water |
| Galvanising | A zinc coat that protects even when scratched |
| Alloying | Iron with nickel and chromium is stainless steel |
Worked example
A galvanised pipe is scratched. Zinc is more reactive than iron. So the zinc corrodes first, and the iron is protected.
A painted pipe is scratched. Air and water reach the iron through the scratch. The iron rusts.