Corrosion is material degradation driven by chemical or electrochemical reactions with the environment. For many metals in wet environments, it operates as a coupled electrochemical cell on the material surface.

At anodic regions, metal dissolves by oxidation, for example

$$\mathrm{Fe\rightarrow Fe^{2+}+2e^-}.$$

The released electrons travel through the metal to cathodic regions, where a reduction reaction consumes them. In aerated neutral water, a common cathodic reaction is oxygen reduction:

$$\mathrm{O_2+2H_2O+4e^-\rightarrow4OH^-}.$$

Ionic current through the electrolyte closes the circuit. Corrosion therefore requires an anodic reaction, a cathodic reaction, electronic conduction and ionic conduction.

Thermodynamics versus rate

Electrode potentials indicate whether a corrosion reaction is thermodynamically possible, but service life depends on corrosion rate. Surface kinetics, oxygen supply, electrolyte resistance, diffusion and protective films can strongly limit current.

Faraday's law connects electrochemical charge to material loss. If dissolving one metal atom transfers $z$ electrons, a total charge $Q$ corresponds to an amount of dissolved metal

$$n_M=\frac{Q}{zF},$$

where $F$ is Faraday's constant.

Corrosion is therefore not simply 'rusting'. It is an electrochemical transport-and-reaction process whose geometry, environment and surface state determine where material is lost and how fast.