Protective coatings
What would you suggest to prevent iron corroding? You are likely to suggest keeping the essential factors for rusting away from the iron. Indeed, if iron does not come into contact with oxygen and water, rusting is prevented.
The following methods can be used for this.
Paint, grease or oil
Coating iron with paint, grease or oil. This stops the iron touching oxygen and water (moisture).
Tin
Coating iron with tin metal. This too stops the iron touching oxygen and water (moisture).
In both cases the coated layer acts as a protective barrier.
| Coating | Used on |
|---|---|
| Paint | Gates, bridges, cars, window grilles |
| Grease or oil | Moving machine parts, bicycle chains, tools |
| Tin | Food cans (tin plate) |
Why a scratch matters
A barrier only works while it is whole. Paint that chips, or grease that is wiped off, lets air and water back in, so these coatings have to be kept up. Tin needs special care. It is below iron in the activity series. If a tin can is dented or scratched through to the iron, the iron and tin touch in the damp air and form a cell — and, as in the iron-copper cell of 12.1, the more reactive iron becomes the anode and is eaten away. A scratched tin can rusts faster at the scratch than bare iron would. The next page tests this effect directly.
Dented cans
Do not buy food cans that are badly dented or rusted along a seam: the tin layer there may be broken and the iron beneath corroding.
Keep oxygen and water out
Paint, grease, oil and tin protect iron by forming a barrier between it and oxygen and water — as long as the barrier stays unbroken.