How rusting happens and the two ways to stop it, why alloys are harder and how to work with carats and recipes, and choosing between glass, ceramics, polymers and composites using their properties.
What you will learn
- Explain barrier and sacrificial protection
- Calculate carats, alloy recipes and percentage change in mass
- Explain why alloys are harder than pure metals
- Compare glass, ceramics, polymers and composites and choose one from data
Corrosion and how to stop it
Corrosion is the destruction of a material by chemical reactions with its surroundings.
Rusting needs both oxygen and water; salt speeds it up. Rust is hydrated iron(III) oxide, and rusting is an oxidation, so a rusting nail gets heavier.
- A nail goes from 2.40 g to 2.52 g in salty water
- increase = 2.52 − 2.40 = 0.12 g
- percentage increase = 0.12 ÷ 2.40 × 100 = 5.00%
- Barriers keep oxygen and water out: paint, oil, grease, plastic, electroplating (for example with tin). Once scratched, they stop working.
- Sacrificial protection: a more reactive metal (zinc, magnesium) is oxidised instead of the iron, even through a scratch. Galvanising is a coating of zinc. Magnesium blocks on ships are slowly used up and replaced.
Aluminium is more reactive than iron but does not corrode away: a thin oxide layer forms and protects it.
The rest of this lesson and its 13 practice questions are locked.
In this lesson
- Corrosion and how to stop it
- Alloys
- Glass, ceramics, polymers and composites
- Try it: Find the carat
13 practice questions, marked as you go; about 30 minutes in all.
Key terms
Corrosion, Rusting, Barriers, Galvanising, Alloy, Gold, Clay ceramics, Reinforcement, Matrix
In this unit: Using resources
Making water potable and the water practical, life cycle assessments and recycling, phytomining and bioleaching, corrosion and materials, and the Haber process with NPK fertilisers, with every calculation worked.
- Potable water, waste water and the water practical
- Life cycle assessment and recycling
- Phytomining and bioleaching
- Corrosion, alloys, glass, ceramics, polymers and composites
- The Haber process and NPK fertilisers