9.5 Corrosion of Metals Revision Notes

1. Overview

Corrosion is the process by which metals are gradually destroyed by chemical reactions with substances in their environment. While many metals corrode, the term rusting refers specifically to the corrosion of iron and its alloy, steel, which leads to significant structural damage in buildings, bridges, and vehicles if not prevented.

Key Definitions

• Corrosion: The gradual destruction of a metal due to its reaction with substances in the environment (such as oxygen and water).
• Rusting: The specific name for the corrosion of iron and steel.
• Hydrated Iron(III) Oxide: The chemical name for rust, a flaky orange-brown solid.
• Barrier Method: A method of rust prevention that creates a physical layer to stop oxygen and water from reaching the metal surface.
• Sacrificial Protection: A method where a more reactive metal is attached to iron; the more reactive metal reacts and corrodes instead of the iron.
• Galvanising: A specific type of protection where iron is coated with a layer of zinc.

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Core Content

Conditions Required for Rusting

For iron or steel to rust, two substances must be present:

1. Oxygen (from the air)
2. Water (moisture)

If either of these is missing, rusting will not occur. Salt or acid can act as catalysts to speed up the process, but they are not the primary requirements.

The Barrier Method

Barrier methods work by providing a physical coating that excludes oxygen and water from the surface of the iron.

• Painting: Used for large, stationary objects like bridges and car bodies.
• Greasing/Oiling: Used for moving parts where a solid coating like paint would crack, such as bicycle chains or engine parts.
• Plastic Coating: Used for items like garden fences and dish racks.
• Electroplating: Coating the iron with a thin layer of a different metal (like chromium or tin) that does not corrode.

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Extended Content (Extended Curriculum Only)

Galvanising

Galvanising is the process of coating iron or steel with a layer of zinc. It is a unique method because it works in two ways:

1. As a barrier: The zinc layer prevents water and oxygen from reaching the iron.
2. As sacrificial protection: If the zinc coating is scratched, the iron is still protected because zinc is more reactive than iron.

Sacrificial Protection and the Reactivity Series

In sacrificial protection, a metal higher up the reactivity series (more reactive) is used to protect iron.

• Commonly used metals: Zinc (Zn) or Magnesium (Mg).
• These metals have a higher tendency to lose electrons and form ions than iron does.

Electron Loss and Redox

Rusting involves the oxidation of iron atoms into iron ions by the loss of electrons:

• $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$

When a more reactive metal like zinc is present, it "sacrifices" itself by losing its electrons more readily than the iron:

• $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

Because the zinc supplies the electrons, the iron atoms remain in their metallic state and do not oxidise (rust).

Worked Example: Question: Why are blocks of magnesium attached to the hulls of steel ships? Answer: Magnesium is more reactive than the iron in steel. The magnesium loses electrons more easily than iron to form $Mg^{2+}$ ions. Therefore, the magnesium corrodes "sacrificially," preventing the iron from rusting.

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Key Equations

Formation of Rust (Word Equation): iron + oxygen + water → hydrated iron(III) oxide

Formation of Rust (Symbol Equation): $4Fe(s) + 3O_2(g) + xH_2O(l) \rightarrow 2Fe_2O_3 \cdot xH_2O(s)$ (Note: 'x' represents a variable number of water molecules)

Oxidation of Zinc (Sacrificial Protection): $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

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Common Mistakes to Avoid

• ❌ Wrong: Saying that "corrosion" only happens to iron.
• ✓ Right: Corrosion is a general term for all metals; "rusting" is specific only to iron and steel.
• ❌ Wrong: Thinking that galvanising only works if the coating is intact.
• ✓ Right: Galvanising protects iron even when scratched because the zinc acts sacrificially.
• ❌ Wrong: Listing "air" as a requirement for rusting.
• ✓ Right: Be specific and say "oxygen" (air contains other gases like nitrogen that do not cause rusting).

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Exam Tips

• Command Words: If asked to "Describe an experiment," remember to mention the variables you are controlling (e.g., using boiled water to remove oxygen, or using a drying agent like calcium chloride).
• The "Hydrated" Keyword: When naming the product of rusting, always use the full name: hydrated iron(III) oxide. Just "iron oxide" is often not enough for the mark.
• Reactivity Reasoning: When explaining sacrificial protection, always state that the sacrificial metal is more reactive than iron and therefore loses electrons more easily.
• Real-world Context: Be prepared to suggest a specific protection method for a specific object. For example, use grease for a bike chain (moving parts) but paint for a bridge (large, stationary).