How does iron rust in neutral water by the oxygen absorption mechanism?

In one line

In a neutral or slightly alkaline medium containing dissolved oxygen, iron is oxidised at anodic areas while oxygen is reduced to OH⁻ at cathodic areas, and the products combine to form rust.

Explained simply

Think of a scratched iron gate after rain. The water drop has dissolved oxygen from air. The exposed iron at the scratch gives up electrons and dissolves (anode). Elsewhere on the surface, oxygen and water grab those electrons and form hydroxide ions (cathode). When Fe²⁺ and OH⁻ meet, they form a precipitate that further oxidises to reddish-brown rust.

How it works

1. Conditions: neutral or mildly alkaline electrolyte (water, salt solution) with dissolved oxygen.
2. Commonly, a small area where the oxide film is broken becomes the anode, and the large surrounding filmed surface acts as the cathode (small anode, large cathode, so attack is intense locally).
3. Anode: Fe → Fe²⁺ + 2e⁻
4. Cathode: ½O₂ + H₂O + 2e⁻ → 2OH⁻
5. Fe²⁺ and OH⁻ diffuse towards each other; since Fe²⁺ ions are smaller and move faster, they meet nearer the cathode: Fe²⁺ + 2OH⁻ → Fe(OH)₂↓
6. With excess oxygen: 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃, which dehydrates to rust, Fe₂O₃·xH₂O (yellow-brown/reddish-brown).
7. With limited oxygen, black magnetite Fe₃O₄·6H₂O can form instead.

Key formula

Overall (first step): \( \mathrm{2Fe + O_2 + 2H_2O \rightarrow 2Fe(OH)_2} \)

Then: \( \mathrm{4Fe(OH)_2 + O_2 + 2H_2O \rightarrow 4Fe(OH)_3 \rightarrow Fe_2O_3\cdot xH_2O} \)

Quick example

Rust often appears as a ring a little away from a scratch on a car body or ship hull, while pitting deepens at the scratch itself. This is exactly the "anode at the break, product near the cathode" picture above. Sea water speeds it up because NaCl increases conductivity of the electrolyte.

Common mistakes

• Writing H₂ evolution at the cathode in neutral oxygenated water. The cathode reaction here is oxygen reduction.
• Saying rust forms at the anode. The precipitate forms in between, closer to the cathode, while metal is lost at the anode.
• Writing rust as Fe₃O₄; ordinary rust is hydrated Fe₂O₃.

How MU asks it

"Explain the mechanism of rusting of iron by oxygen absorption" (5 marks) or "Explain wet corrosion in neutral medium with a diagram" (5–6 marks). For full marks: conditions, labelled diagram (oxide film, crack as anode, cathode, flow of e⁻, rust deposit), all reactions including the final rust formula, and why the small-anode/large-cathode arrangement causes pitting.