Metals and Non-metals · Lesson 12 of 13
Corrosion And Prevention Of Corrosion
“Rust, green copper and black silver show why metals need coatings, alloys and a little protection.”
• Explain corrosion using iron, silver and copper as examples. • Identify the conditions necessary for rusting of iron. • Explain common methods used to prevent corrosion. • Define galvanisation and alloying. • Compare brass, bronze, solder, amalgam and stainless steel using their compositions or purposes.
Exposure to the environment can slowly change and damage metal surfaces. Corrosion is a chemical process, and the conditions under which it occurs can be investigated experimentally.
Corrosion
Corrosion is the gradual chemical attack of a metal by substances in its surroundings.
Silver
Silver articles become black after exposure to air because a coating of silver sulphide forms.
Copper
Copper reacts slowly with moist carbon dioxide and develops a green coating of basic copper carbonate.
Iron
Iron exposed to moist air for a long time develops a brown, flaky coating called rust.
Conditions Required For Rusting
A three-test-tube investigation separates the roles of air and water. One tube contains both air and water. Another contains boiled water covered with oil so dissolved air is excluded. A third contains dry air with calcium chloride removing moisture.
Rust forms only where both air and water are available. Water without air does not produce rust, and dry air without moisture does not produce rust. Both are required under the conditions of the investigation.
Prevention Of Corrosion
Methods include painting, oiling, greasing, galvanising, chrome plating, anodising and alloying. Most work by separating the metal from the surroundings or by changing the material or surface so corrosion becomes more difficult.
Galvanisation
Galvanisation is the protection of iron or steel by coating it with a thin layer of zinc.
The zinc coating protects the underlying iron. The chapter notes that a galvanised article remains protected even if the zinc layer is damaged.
Alloying
An Alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal.
Pure iron is soft and stretches easily when hot. Adding a small amount of carbon makes it hard and strong. Iron mixed with nickel and chromium forms stainless steel, which is hard and resists rusting.
Important Alloys
| Alloy | Composition Or Description | Important Point |
|---|---|---|
| Brass | Copper + zinc | Different properties from pure copper |
| Bronze | Copper + tin | Different properties from pure copper |
| Solder | Lead + tin | Low melting point; joins electrical wires |
| Amalgam | Alloy containing mercury | Defined by mercury content |
| Stainless steel | Iron with nickel and chromium | Hard and rust resistant |
Gold Jewellery
Pure gold is very soft, so it is alloyed with silver or copper to make it harder for jewellery.
Why Alloy Properties Differ
Alloys can have different melting points and electrical conductivities from pure metals. Alloying is useful precisely because it allows properties to be adjusted.
The chapter highlights the ancient iron pillar near the Qutub Minar as an example of remarkable long-term resistance to rusting.
Quiz
What two conditions are required for rusting of iron?
What is the green coating on copper?
What is galvanisation?
What is an amalgam?
Which alloy has a low melting point and is used to join electrical wires?
Practice Problems
- Explain the three-test-tube investigation and how it proves that both air and water are needed for rusting.
- Describe corrosion of silver, copper and iron.
- Explain how painting, oiling and greasing reduce corrosion.
- Define galvanisation and explain its purpose.
- Define alloy and compare brass, bronze, solder, amalgam and stainless steel.
- Explain why pure iron is modified by alloying and why stainless steel resists rusting better.
- Explain why pure gold is alloyed for jewellery.
Key Takeaways
• Corrosion chemically damages metal surfaces over time. • Silver blackens, copper develops a green coating and iron forms brown flaky rust. • Iron rusting requires both air and water under the conditions investigated. • Painting, oiling, greasing, galvanising, chrome plating, anodising and alloying can reduce corrosion. • Alloying changes metal properties and produces useful materials such as stainless steel, brass, bronze and solder.