Topic 10: Alloys and corrosion

Question 1

How are alloys made?

Answer 1

Alloys are made by adding another element to a metal.

Question 2

Why are alloys often harder than pure metals?

Answer 2

In the solid state, a pure metal has a giant metallic structure. The atoms are arranged in layers. When a force is applied, the layers may slide over each other. The greater the force needed, the harder and stronger the metal.

In a pure metal, the force needed to make the layers slide over each other is small. This explains why many pure metals are soft.

In an alloy, there are atoms of different sizes. The smaller or bigger atoms distort the layers of atoms in the pure metal. This means that a greater force is required for the layers to slide over each other. The alloy is harder and stronger than the pure metal.

Question 3

LOW CARBON STEEL
1. What is one use of low carbon steel?
2. What property makes it suitable for this use?

Answer 3

1. Used in car bodies
2. It is easily shaped.

Question 4

HIGH CARBON STEEL
1. Give two uses of high carbon steel.
2. Give a property of high carbon steel that make it suitable for these uses.

Answer 4

1. Blades for cutting tools, bridges
2. It is very hard and inflexible.

Question 5

STAINLESS STEEL
1. What is added to iron to make stainless steel?
2. Give two uses of stainless steel.
3. Give a property of stainless steel that makes it suitable for these uses.

Answer 5

1. Chromium (sometimes nickel)
2. Cutlery, containers for corrosive substances.
3. It is corrosion resistant.

Question 6

BRONZE
1. What is bronze made from?
2. What are some uses of bronze?
3. Which is harder, bronze or copper?

Answer 6

1. Copper + tin
2. Medals, decorative ornaments, statues
3. Bronze

Question 7

BRASS
1. What is brass made from?
2. What is brass used for?

Answer 7

1. Copper + zinc
2. Water taps, door fittings (situations where lower friction is required, because brass is more malleable than bronze)

Question 8

GOLD ALLOYS
1. Why are other metals (e.g zinc, copper, silver) used to harden gold?
2. Give a use of gold alloys.
3. What unit is used to measure gold’s purity?

Answer 8

1. Gold is very soft.
2. Jewellery
3. Carats. 24 carat gold is pure gold. (18 carat gold means that 18/24 parts of the alloy are pure gold - 18 carat gold is 75% gold)

Question 9

ALUMINIUM ALLOYS
What property of aluminium is important in aircraft manufacture?

Answer 9

It has a low density (so will be light). However, pure aluminium is too soft for making aeroplanes so it is alloyed with small amounts of other metals to make is stronger.

Question 10

What is corrosion?

Answer 10

Corrosion is where metals react with other substances in their environment and are gradually destroyed.

Question 11

RUST
1. What is rust?
2. Can an object containing iron be completely corroded?

Answer 11

1. Rust is a soft, crumbly solid formed when iron and its alloys (steels) corrode. It is actually hydrated iron (III) oxide.

iron + oxygen + water —> hydrated iron (III) oxide

2. Even though corrosion only happens on the surface of a material, rust can result in the corrosion of all the iron in an object because it flakes off to leave more iron exposed to the air (more iron is available to rust again)

Question 12

ALUMINIUM CORROSION
1. What forms when aluminium corrodes?
2. Why aren’t objects made of aluminium completely destroyed by corrosion? (unlike what occurs in iron)

Answer 12

1. Aluminium oxide
2. Aluminium oxide forms a protective layer that sticks firmly to the aluminium below and doesn’t flake off (rust does in the case of iron), preventing any further corrosion from taking place.

Question 13

AIR + WATER needed for iron to rust…
Describe an experiment that would demonstrate this.

Answer 13

https://www.bbc.co.uk/bitesize/guides/z8kmcj6/revision/1

Question 14

Describe three ways to prevent rusting that involve coating the iron in a barrier to prevent it being exposed to oxygen and water.

Answer 14

1. Painting/coating with plastic - ideal for big/small structures alike. Can be decorative :)
2. Electroplating - uses electrolysis to reduce metal ions onto an iron electrode. Can be used to coat the iron with a layer of a different metal that won’t be corroded away.
3. Oiling/greasing - has to be used when moving parts are involved (e.g on bike chains)

Question 15

What is the sacrificial method?

Answer 15

The sacrificial method involves placing a more reactive metal (e.g zinc, magnesium) over iron to prevent it from rusting. Water and oxygen react with the sacrificial metal instead of with the iron.

Question 16

What is galvanisation?

Answer 16

If an object is galvanised, it is sprayed with a coating of zinc. The zinc layer is firstly protective, but if scratched, the zinc around the site of the scratch works as a sacrificial metal.