13: Corrosion of Metals and their Protection

Question 1

What is the definition of corrosion?

Answer 1

Corrosion is the deterioration of a metal caused by chemical interaction with oxygen, moisture, or other substances in the environment.

Question 2

What is rusting?

Answer 2

Rusting refers to the corrosion of iron only.

Question 3

What is the chemical name and formula of rust?

Answer 3

hydrated iron(III) oxide, Fe₂O₃ · xH₂O

Question 4

An iron nail is put inside a boiling tube containing boiled distilled water and a layer of oil on top. Explain whether it will rust or not.

Answer 4

It will not rust. Boiling the water drives out any dissolved oxygen in the water, and the layer of oil on the water surface prevents oxygen from dissolving into the water. The iron nail is exposed to only water but no oxygen, so it will not rust.

Question 5

An iron nail is put inside a boiling tube covered with anhydrous calcium chloride and cotton wool. Explain whether it will rust or not.

Answer 5

It will not rust. Anhydrous calcium chloride absorbs moisture from the air inside the tube. The iron nai is exposed to only oxygen but not water, so it will not rust.

Question 6

What are the conditions for rusting to occur?

Answer 6

Both air (oxygen) and water are needed for iron to rust.

Question 7

How do rust indicators indicate rust?

Answer 7

Rust indicator contains potassium hexacyanoferrate(III) and phenolphthalein. Potassium hexacyanoferrate(III) reacts with iron(II) ions to give a blue product, while phenolphthalein gives a pink colour in the presence of hydroxide ions. When iron rusts, iron loses electrons to oxygen and water to form iron(II) hydroxide, which then forms hydrated iron(III) oxide. Therefore, it can be detected by the colour changes in the rust indicator.

Question 8

What is the chemical equation for rusting?

Answer 8

4Fe(s) + 3O₂(g) + 2xH₂O(l) -> 2Fe₂O₃ · xH₂O(s)

Question 9

List out five factors that speed up the rusting process.

Answer 9

Presence of ionic substances, presence of acidic pollutants, higher temperature, scratching r bending, attachment to a less reactive metal.

Question 10

Why does the presence of ionic substances speed up the rusting process?

Answer 10

Ionic substances dissolved in water contain mobile ions, which increases electrical conductivity.

Question 11

Why does the presence of acidic pollutants speed up the rusting process?

Answer 11

Acidic gases form acids with moisture in air and attack the iron.

Question 12

Why does higher temperature speed up the rusting process?

Answer 12

At higher temperatures, chemical reactions become faster, so the rusting process becomes faster.

Question 13

Why does scratching or bending speed up the rusting process?

Answer 13

Scratching or bending expose a larger surface for reaction.

Question 14

List four ways which iron can be protected from rusting.

Answer 14

protective coating on surface, sacrificial protection, impresses current cathodic protection, and the use of stainless steel.

Question 15

List 5 ways how iron can be coated on the surface to protect it from rusting.

Answer 15

coating with paint, coating with oil or grease, coating with plastic, tin-plating, and electroplating

Question 16

How does coating the iron protect it from rusting?

Answer 16

The surface of an iron object is covered and air and water are kept out from the iron underneath.

Question 17

Give an example of coating iron with oil or grease and explain why other methods are not used.

Answer 17

Oiling bicycle chains: since bicycle chains are moving patters of an engine, they cannot be painted as they rub against each other and the paint will wear away. Oiling them keeps out air and water, and also help to lubricate the chains.

Question 18

List two advantages and one disadvantage of tin-plating.

Answer 18

Advantages: tin ions are less reactive than iron and do not react with air or water at room temperature. It acts as a protective layer and stops air and water from reaching the iron. In addition, tin ions are non-poisonous, therefore it can be plated onto iron to make food cans.
Disadvantage: since tin is less reactive than iron, when the surface is scratched, the iron inside rusts faster than normal due to sacrificial protection offered to tin.

Question 19

List two advantages and one disadvantage of electroplating.

Answer 19

Advantages: the plating layer can be very thin, and it is not easily chipped off.
Disadvantage: the cost of electroplating is high.

Question 20

List two advantages and one disadvantage of galvanising.

Answer 20

Advantages: The layer of zinc stops air and water from reaching the iron or steel underneath. The zinc also acts as a sacrificial metal so that the object is protected even if the zinc layer is damaged.
Disadvantage: zinc ions are poisonous.

Question 21

Which metal is used to protect ships and why?

Answer 21

Zinc, because it can protect the ship hulls from rusting by acting as a sacrificial metal.

Question 22

Why is magnesium usually not used for protecting ships?

Answer 22

Magnesium does not react with pure water at room temperature, but it reacts with sea water at room temperature due to the presence of ionic substances in sea water.

Question 23

What is impressed current cathodic protection?

Answer 23

Impressed current cathodic protection is a technique used to prevent rusting of iron by making it into the cathode of an electric circuit.

Question 24

Why is iron protected in impressed current cathodic protection?

Answer 24

The power source supplies electrons to the iron, so iron cannot lose electrons to form iron(II) ions.

Question 25

List one advantage and one disadvantage of using stainless steel.

Answer 25

Advantage: no maintenance is needed.
Disadvantage: the cost of producing stainless steel is high.

Question 26

Why is aluminium resistant to corrosion?

Answer 26

It reacts with air to form a layer of aluminium oxide. This layer is impermeable to oxygen and water, so it stops oxygen and water from reaching and reacting with the rest of the metal. Therefore, the aluminium is not corroded.

Question 27

What is aluminium anodisation?

Answer 27

Aluminium anodisation is the process of increasing the thickness of the oxide layer of aluminium by making it the anode of an electric circuit.

Question 28

List 3 advantages of aluminium anodisation.

Answer 28

1. It makes aluminium harder.
2. It makes aluminium more corrosion resistant.
3. It can absorb dyes easily.