The chemistry of corrosion

The rusting of iron can be summarised by the equation

4 Fe_(s) + 3 O_2(g) + H₂O_(l)  ⇾  2 Fe₂O₃•xH₂O_(s)

In reality, the process takes place via a series of half reactions that take place in the iron itself and the water droplet.

1. Iron metal is oxidised to iron(II) ions in the centre of the water droplet, where the dissolved oxygen concentration is lower.  The Fe²⁺ ions pass into solution.

Fe_(s) ⇾ Fe²⁺_(aq) + 2e^–      E^⦵ = – 0.44V

2. In regions of the water droplet with a higher concentration of dissolved oxygen (the edges), oxygen is reduced to hydroxide ions by electrons that flow from the oxidation of iron through the metal. 

O_2(g) + 2H₂O_(l) + 4e^– ⇾ 4OH^–_(aq)       E^⦵ = +0.40V

3. Fe²⁺ and OH^– ions react to form Fe(OH)₂, a green precipitate, away from the surface of the iron so there is no protective oxide layer formed. 

4. Fe²⁺_(aq) ions are oxidised to Fe³⁺_(aq), which then react with OH^– ions and water to form the insoluble hydrated oxide known as rust. 

Sea water has a greater conductivity because of the high concentration of dissolved ions and so iron rusts faster in the sea than in rain water. 

Preventing corrosion

• oil / paint / grease will all help prevent water and oxygen from reaching the surface of the iron
• galvanising is a process in which the iron surface is coated in zinc which forms an impermeable oxide layer

Zn²⁺_(aq) + 2e^– ⇌ Zn_(s) E^⦵ = – 0.76V

Fe²⁺_(aq) + 2e^– ⇌ Fe_(s) E^⦵ = – 0.44V

O_2(g) + 2H₂O_(l) + 2e^– ⇌ 4OH^–_(aq) E^⦵= + 0.40V

If scratched, the iron (Fe) is oxidised to Fe²⁺ by O_2(g) / H₂O_(l) / OH^–_(aq) half cell which has the more positive E^⦵. The Fe²⁺ is then reduced back to Fe by the Zn²⁺_(aq) / Zn_(s) half cell which has the less positive E^⦵ of the two and so forms an oxidation half cell.

• cathodic protection is where the iron is attached by a wire to a block of a more easily oxidised metal such as magnesium

Mg²⁺_(aq) + 2e^– ⇌ Mg_(s) E^⦵ = – 2.37V

Fe²⁺_(aq) + 2e^– ⇌ Fe_(s) E^⦵ = – 0.44V

The magnesium is a sacrificial anode, reacting with in preference to iron with O_2(g) / H₂O_(l), and it needs to be replaced periodically.

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