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Corrosion of metals

Paper 1Paper 2Paper 3Paper 4Paper 5Paper 6

This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4. Practical skills related to investigating rusting are tested in Paper 5 and Paper 6.

The Conditions for Rusting
Rusting is the specific corrosion of iron and steel. It is not simply 'iron reacting with air'; it requires two specific substances to be present simultaneously. If either substance is missing, rusting cannot occur.
Required SubstanceSourceRole in Rusting
Water (or moisture)Liquid water or water vapour in the airActs as a solvent/electrolyte allowing ions to move.
OxygenFrom the air (approx. 21% O_2)Reactant that combines with iron.
Why both are needed: Iron atoms must lose electrons to form ions (Fe^{2+}). This electron loss (oxidation) can only happen if there is a pathway for the electrons to move and if oxygen is available to accept them. Water provides this conductive pathway. Without water, the reaction stops even if oxygen is present. Without oxygen, the electrons have nowhere to go, so the reaction stops even if water is present.
Rust
Rust is the name given to the reddish-brown flaky substance formed when iron corrodes. Its precise chemical name is hydrated iron(III) oxide.

Note: You must include both 'hydrated' and 'iron(III)' to be chemically accurate. Simply saying 'iron oxide' or 'iron rust' is often insufficient for full marks in theory papers.

Experimental Proof of Conditions

To prove that both water and oxygen are required, we use three test tubes with an iron nail in each:

  1. Tube A (Control): Nail in air + water. Result: Rusts. Both conditions met.
  2. Tube B (No Oxygen): Nail boiled in water (to remove dissolved oxygen), then covered with a layer of oil to prevent air from re-entering. Result: No rust. Water is present, but oxygen is excluded.
  3. Tube C (No Water): Nail placed in a test tube with a drying agent (like anhydrous calcium chloride or silica gel) and stoppered. Result: No rust. Oxygen is present, but water vapour is absorbed by the drying agent.
⚠︎ Misidentifying Rusting Requirements
The Error: Students often state that 'air' is required for rusting. While air contains oxygen, this answer is vague and can be marked incorrect if the examiner requires specific substances.

The Correction: Always specify oxygen AND water. Do not just say 'air' or 'moisture'. The markscheme looks for the specific chemical reactants.

Answering 'State the Conditions'
When to use: When asked to 'state' or 'name' the conditions for rusting.

Why examiners accept this: The question tests recall of specific reactants. Vague terms like 'humidity' or 'atmosphere' are often rejected because they do not explicitly identify the chemical species involved.

Correct Usage: Write: 'Water and oxygen.' Do not write 'Air and water' unless you specify that it is the oxygen in the air.

Past Paper Style Questions
Q:
State the two substances required for iron to rust.
A:
Water and oxygen.
Q:
Name the chemical compound formed when iron rusts.
A:
Hydrated iron(III) oxide.
Barrier Methods of Prevention
Barrier methods work by physically covering the iron surface. This prevents the oxygen and water from reaching the metal surface, thereby stopping the rusting reaction described above.
MethodHow it works
PaintingCreates a solid layer that blocks air and water.
Greasing / OilingCreates a hydrophobic (water-repelling) film that excludes moisture.
Coating with plasticEncases the metal in a non-reactive polymer layer.
Why this prevents rusting: By excluding oxygen or water, the necessary conditions for the electrochemical reaction are removed. Without contact with these reactants, iron atoms cannot lose electrons to form ions.
Barrier Method
A barrier method is any technique that prevents rusting by creating a physical layer between the iron surface and the environment (air/water). Common examples include painting, greasing, and plastic coating.
⚠︎ Vague Prevention Descriptions
The Error: Writing 'covering the iron' or 'putting something on it' as a method.

The Correction: Be specific. Use terms like painting, greasing, or coating with plastic. Examiners look for specific named methods, not general descriptions of action.

Describing How Barrier Methods Work
When to use: When asked to 'describe' or 'explain' how painting/greasing prevents rusting.

Why examiners accept this: The key is linking the physical barrier to the exclusion of reactants. You must mention that it stops oxygen and/or water from reaching the iron.

Correct Usage: Write: 'The paint acts as a barrier that prevents oxygen and water from reaching the iron surface.'

Galvanising and Sacrificial Protection

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

  1. As a Barrier Method: The zinc layer physically coats the iron, preventing oxygen and water from reaching the iron surface (just like paint).
  2. As Sacrificial Protection: Unlike paint, if the zinc coating is scratched or damaged, exposing the iron underneath, the iron is still protected. This is because zinc is more reactive than iron.
Sacrificial Protection Explained (in terms of electron loss):

Zinc is higher in the reactivity series than iron. This means zinc loses electrons more readily than iron does.

  • When exposed to water and oxygen, zinc oxidises (loses electrons): Zn \rightarrow Zn^{2+} + 2e^-
  • These electrons flow to the exposed iron.
  • Because the iron is receiving electrons from the zinc, it does not need to lose its own electrons to form Fe^{2+} ions.
  • Since rusting requires iron to lose electrons (oxidise), the iron remains unoxidised and does not rust.

In this process, zinc is 'sacrificed' (it corrodes instead of the iron) until the zinc layer is completely used up.

Sacrificial Protection
Sacrificial protection is a method of preventing corrosion where a more reactive metal (like zinc) is connected to the iron. The more reactive metal oxidises (loses electrons) in preference to the iron, thereby 'sacrificing' itself to protect the iron from rusting.
Galvanising vs. Painting

Scenario: A steel gate is coated with zinc (galvanised). The coating gets scratched, exposing the steel.

  • Painted Gate: If paint is scratched, oxygen and water reach the iron. Rusting begins immediately at the scratch.
  • Galvanised Gate: If zinc is scratched, the exposed zinc reacts with the air/water instead of the iron. The zinc corrodes, but the steel remains intact. The zinc continues to protect the iron until it is fully consumed.
⚠︎ Confusing Reactivity and Protection
The Error: Thinking that zinc protects iron because it is 'stronger' or 'harder', or incorrectly stating that zinc prevents rusting by being 'less reactive'.

The Correction: Zinc protects iron because it is more reactive. It must be more reactive to lose electrons preferentially. Do not say zinc 'blocks' the reaction chemically; it actively participates by oxidising instead.

Explaining Sacrificial Protection for Full Marks
When to use: When asked to 'explain' why zinc protects iron, especially if scratched.

Why examiners accept this: You must address two points: 1) Relative reactivity (Zn > Fe), and 2) Electron transfer (Zn loses electrons). Simply saying 'zinc is more reactive' is often only worth 1 mark. You need the mechanism.

Correct Usage: Write: 'Zinc is more reactive than iron. Therefore, zinc loses electrons (is oxidised) preferentially to iron. This prevents the iron from losing electrons and forming Fe^{2+} ions.'

Past Paper Style Questions
Q:
Name the process of coating iron with zinc.
A:
Galvanising.
Q:
Explain why a scratched galvanised nail does not rust at the scratch.
A:
Zinc is more reactive than iron. Zinc loses electrons (oxidises) instead of iron. The iron does not lose electrons to form ions, so it does not rust.
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