Alloys, Corrosion and its prevention

What Are Alloys?

• Alloys are mixtures of two or more elements, where at least one is a metal.
• They are made to improve properties such as strength, hardness, corrosion resistance, and malleability.
• Pure metals have layers of atoms that slide over each other easily, making them soft and malleable.
• When mixed with other elements (becomes an alloy), these layers become distorted, so the metal becomes harder as these layers can no longer slide past each other easily.

high carbon steel                            low-carbon steel                          stainless steel

Steel’s strength, durability, and versatility make it a key material in everyday life.

• Pure gold (24 carat) is too soft for jewellery.
• Mixed with other metals (e.g. copper, silver, zinc) to increase hardness.
• The carat system shows the fraction of pure gold in the alloy.

• Corrosion is the gradual destruction of a material (usually reactive metals) by reacting with substances in the environment.
• Rusting is a type of corrosion that only happens to iron or steel.
• Rusting requires both air (oxygen) and water:

Iron + oxygen + water → hydrated iron(III) oxide (rust)

• Rust flakes off, exposing fresh iron, so corrosion continues until the metal is destroyed.

Aluminium also reacts with oxygen, but its oxide forms a protective layer that prevents further corrosion.

Rusting Experiments

The experiment shown in the image helps to prove the conditions needed for iron to rust.

A drying agent is a substance that absorbs water to keep other substances dry, also called a desiccant (similar to small silica gel bags found when buying new bags/trainers to keep our moisture and keep them smelling fresh).

Preventing Corrosion

1. Creating a Barrier

• Painting or plastic coating prevents air and water from reaching the metal.
• Oiling or greasing protects moving parts.
• Electroplating uses electrolysis to coat a metal (e.g. iron) with a thin layer of another metal (e.g. nickel, chromium).

2. Sacrificial Protection

• A more reactive metal (like zinc or magnesium) is attached to the metal being protected (there are normal methods and special methods like galvanising).
• Normal sacrificial protection attaches blocks of a more reactive metal (like magnesium or zinc) to the iron surface - these blocks corrode instead of the iron but do not form a continuous barrier.

3. Mixed Methods Protection

• Galvanising combines both physical barriers and sacrificial protection. By coating the entire surface of iron or steel with a relatively thicker layer (compared to electroplating) of zinc specifically
• The thick coating of zinc acts as a physical barrier to keep out air and water and also provides sacrificial protection by reacting first if the coating is scratched.

Practice Questions