rusting

Rusting is the corrosion of iron ($\mathrm{Fe}$) into rust ($\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot \mathrm{H}{}_2\mathrm{O}$). When a piece of iron comes into contact with a water droplet, $\mathrm{Fe}$ oxidizes into $\mathrm{Fe}{}^{2+}$. When ferrous comes into contact with oxygen, rust is formed. When we interpret rusting as a galvanic cell, the water droplet is the anode, and the outside air is the cathode where ferrous will end up and form rust deposits after travelling through the droplet.

Here’s the chemical reaction that takes place (with a simplified cathode):

$$\begin{array}{rlrl}\text{Anode:}& & 2\mathrm{Fe}(\mathrm{s})& \to 2\mathrm{Fe}{}^{2+}(\mathrm{aq})+4\mathrm{e}{}^{-}\\ \text{Cathode:}& & \mathrm{O}{}_2(\mathrm{g})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})+4\mathrm{e}{}^{-}& \to 4\mathrm{OH}{}^{-}(\mathrm{aq})\end{array}$$

Rusting has a cell potential of $0.841\mathrm{V}$.