Brainlag PlusWhat rusting needs and how to stop it, alloys and gold carats, glass and ceramics, the two kinds of polymer, and composites.
Rusting and how to stop it
Corrosion is the destruction of a material by reactions with substances around it.
Iron rusts only when both oxygen and water are present; salt speeds it up.
iron + oxygen + water → hydrated iron(III) oxide (rust)
- A nail in boiled water under a layer of oil does not rust: boiling removes dissolved air, and the oil stops more dissolving.
- A nail in a sealed tube with anhydrous calcium chloride does not rust: the drying agent removes water vapour.
- Barriers (paint, oil, grease, plastic, electroplating) keep oxygen and water out.
- Sacrificial protection: a more reactive metal (zinc, magnesium) is attached or coated on. It is oxidised instead of the iron, even through a scratch. Galvanising is a zinc coating.
A tin coating is only a barrier: tin is less reactive than iron, so once scratched the iron rusts.
The rest of this lesson and its 9 practice questions are in Plus.
Screen 1 of 4, free to read.
- 9 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards