Corrosion

When materials deteriorate by chemical processes, the materials are said to corrode. Corrosion is commonly discussed in reference to metals, which corrode electrochemically. This kind of corrosion is a two-step process that requires three things: a metallic surface, an electrolyte, and oxygen. During the corrosion process, a metal atom at the surface dissolves into an aqueous solution, leaving the metal with excess negatively charged ions. These resultant ions are removed by a suitable electron acceptor. Corrosion can be thought of as the spontaneous return of metals to their ores through the process of oxidation.

The conductive properties of metal enable the oxidation and reduction steps that occur during corrosion to take place at separate sites on the metal's surface. The conductivity allows electrons to flow from the anodic to cathodic regions of the metal.

In a corrosion system, the metal being corroded acts as the anode of a short-circuited electrochemical cell:

$Fe (s) \rightarrow Fe^{2+} (aq) + 2 e^-$

The free electrons reduce the electron acceptor, resulting in any of the following cathodic steps:

$O_2 + 2 H_2O + 4e^- \rightarrow 4 OH^- $

$H^+ + e^- \rightarrow \frac{1}{2} H_2 (g)$

$M^{2+} + 2 e^- \rightarrow M (s)$

In this cathodic step, M is a metal.

How susceptible a particular metal is to corrosion can be determined by its reduction potential. The higher a metal's reduction potential, the less likely it is to be oxidized.

Corrosion is a common nuisance with real impact. We see its effects in rusted out car frames, the bursting of water mains and the failure of bridges.

Corrosion is a nuisance

This photo of the Nandu River Iron Bridge in Hainan, China displays evidence of damage by corrosion.