Is corrosion an electrochemical process?

1 Answer
Sep 5, 2014

It doesn't have to be an electrochemical process, but it occurs much faster when it is.

A piece of iron rusts quickly if you leave it outside in the rain.

It rusts even more quickly in the presence of salt water. A water droplet in contact with the metal then makes a great voltaic cell.

The iron surface inside the droplet acts as the anode for the process:

Fe(s) →Fe²⁺(aq) + 2e⁻

The electrons can move through the iron to the edge of the droplet, where they can reduce the atmospheric oxygen:

O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)

The OH⁻ ions can move within the drop toward the Fe²⁺ ions. When they meet, they form a precipitate of iron(II) hydroxide.

Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)

The precipitate reacts rapidly with oxygen to form rust.

4Fe(OH)₂(s) + O₂(g) → 2Fe₂O₃•H₂O(s) + 2H₂O(l)