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What is rusting?

Corrosion is the loss of metallic properties of a metal due to oxidation and is accompanied by the formation of unwanted products. Copper, iron and aluminum metals all corrode over time loosing strength, lustre and electrical conductivity.

The rusting of iron

Rusting is the corrosion of iron and readily occurs in the alloy steel. The formation of a reddish brown flakes which loosely adheres to the iron is called rust. 

Steel is an alloy made of iron and carbon.  The carbon atoms in steel greatly increase the strength of the metal. They prevent the iron atoms in the crystal lattice from slipping over one another.

Steel is widely used in the manufacture of cars, white goods and the construction industry because it is mucj stronger than iron.

The carbon atoms in steel however, greatly decrease the ability of iron to resist corrosion.

In the presence of oxygen and water a series of internal galvanic cells or batteries are created. The carbon impurities become the site of reduction.

Reduction half equation: 4e- + 2H2O(l) + O2(g) ==> 4OH-(aq)

The nail is most easily oxidized at points of stress. ie the tip or the head. At these points the crystal lattice is distorted and the iron atoms are easily oxidized.

Oxidation half equation: 2Fe(s) 2Fe2+(aq) + 4e-

The overall or net equation is

2Fe(s) + 2H2O(l) + O2(g) 2Fe2+(aq) + 4OH-(aq)

Fe2+(aq) and OH-(aq) ions migrate through the water by diffusion. Refer to the above diagram. When they meet they combine to produce the precipitate, iron(II) hydroxide, Fe(OH)2, which is further oxidized to iron (III) hydroxide, Fe(OH)3, and finally dehydrated to produce rust.

The chemistry of the reaction resulting in the formation of rust can be summarized as follows.

The chemical equations for rust formation

  1. 2Fe(s) + 2H2O(l) + O2(g) 2Fe2+(aq) + 4OH-(aq)
  2. Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)
  3. Fe(OH)2(s)  =O2=> Fe(OH)3(s)
  4. Fe(OH)3(s) =dehydrates=>  Fe2O3.nH2O(s) or rust

The chemical formula for rust is Fe2O3.nH2O

The overall chemical equation for the formation of rust is

Iron + water + oxygen rust

4 Fe(s) + 6 H2O(l) + 3 O2(g) 4 Fe(OH)3(s)

Iron(III) hydroxide, Fe(OH)3 then dehydrates to produce Fe2O3.nH2O(s) or rust