Electrochemical Mechanism of Corrosion

Rust formation requires both oxygen AND water together — neither alone is sufficient — and because it is fundamentally an electrochemical redox process, it requires a spatially separated anode and cathode, both located on the very same piece of metal.

Consider a droplet of water sitting on an iron surface. The region of metal directly enclosed and covered by the water droplet is starved of dissolved oxygen (oxygen diffuses into water only slowly), so this low-oxygen region becomes the ANODE, where iron itself is oxidised: $\text{Fe(s)} \rightarrow \text{Fe}^{2+}\text{(aq)} + 2e^{-}$, with $E^{o} = 0.44$ V (as an oxidation potential). The surrounding region beyond the droplet, exposed to much more atmospheric oxygen, becomes the CATHODE, where dissolved oxygen is reduced, aided by H⁺ ions supplied from atmospheric CO₂ dissolving in water to form carbonic acid: $\text{O}_2\text{(g)} + 4\text{H}^{+}\text{(aq)} + 4e^{-} \rightarrow 2\text{H}_2\text{O(l)}$, with $E^{o} = 1.23$ V. Electrons released at the anodic site travel through the iron metal itself to reach the cathodic site, and the electrical circuit is completed by ions migrating through the water droplet.

Combining the two half-reactions gives the overall redox reaction driving corrosion: $2\text{Fe(s)} + \text{O}_2\text{(g)} + 4\text{H}^{+}\text{(aq)} \rightarrow 2\text{Fe}^{2+}\text{(aq)} + 2\text{H}_2\text{O(l)}$, with an overall $E^{o} = 0.44 + 1.23 = 1.67$ V. This clearly positive emf confirms that iron corrosion is genuinely spontaneous under ordinary atmospheric conditions.

The Fe²⁺ ions produced at the anode do not stop there — they are further oxidised to Fe³⁺: $4\text{Fe}^{2+}\text{(aq)} + \text{O}_2\text{(g)} + 4\text{H}^{+}\text{(aq)} \rightarrow 4\text{Fe}^{3+}\text{(aq)} + 2\text{H}_2\text{O(l)}$, and the resulting Fe³⁺ then reacts further with water to precipitate as the familiar reddish-brown hydrated rust: $2\text{Fe}^{3+}\text{(aq)} + 4\text{H}_2\text{O(l)} \rightarrow \text{Fe}_2\text{O}_3\cdot\text{H}_2\text{O(s)} + 6\text{H}^{+}\text{(aq)}$. …