Rusting as a Redox Reaction

When metals are exposed to their environment, they undergo corrosion. For example, after some time, a shiny aluminium pot will lose its shine, silverware will tarnish and an iron structure will rust.

Corrosion of metal is a redox reaction in which a metal is oxidised naturally to its ions, resulting in partial or complete destruction of the metal. During corrosion, the metal atoms lose electrons to form positive ions.

M ---> Mⁿ⁺ + n^e-

Rusting is the corrosion of iron. It is the most common corrosion of metal around. For iron to rust, oxygen and water must be present. Rusting is a redox reaction whereby oxygen acts as the oxidising agent and iron act as the reducing agent.

The surface of iron at the middle of the water drerves as the anode, the electrode at which oxidation occurs. The iron atom here lose electrons to form iron(ii) ions.

Oxidation half equation :

Fe(s) --> Fe^{2+(aq) + 2e-}

The electrons flow to the edge of the water droplet, where the is plenty of dissolved oxygen. The iron surface there serves as the cathode, The electrode at which reduction occurs. oxygen gains the electrons and is reduced to hydroxide ions.

Reduction half-equation:

O₂(g) + 2H₂O(l) + 4e^- --> 4OH^-(aq)

The iron(ii) ions produced combine with the hydroxide ions to form iron(ii) hydroxide.

Fe²⁺(aq) + 2OH^-(aq) --> Fe(OH)₂(s)

Thus, the overall redox reaction is as follow:

At the anode: 2Fe(s) --> 2Fe²⁺(aq) + 4e^-
At the cathode: O₂(g) + 2H₂O(l) + 4e^- --> 4OH^-(aq)
Overall equation: 2Fe(s) +O₂(g) + 2H₂O --> 2Fe(OH)₂(s)

The iron(ii) hydroxide is then further oxidised by oxygen to form hydrated iron(iii) oxide, Fe₂O₃.xH₂O whereby x varies. The hydrates come in various shades of brown and ornage and together make up what In the presence of acids and salts, rusting occurs faster. These substances increase hte electrical conductivity of water, making water a better electrolyte. is commonly know as rust.For example:

(a) iron structures such as bridges, fences and cars at coastal areas rust faster due to the presence of salts in the coastal breeze.

(b) iron structures in industrial areas rust quickly as these areas have air polluted with acidic gases such as sulphur dioxide and nitrogen oxides.