Corrosion occurs when a metal deteriorates due to chemical processes. The corrosion of a metal often occurs where the metal is weak or isolated from the air. The process of corrosion occurs in two steps, oxidation and reduction. Let's look at the corrosion process that can occur in iron (Fe).
Oxidation Step: Oxidation occurs when the Fe atoms at a weakened spot on the metal dissolve into water. This causes the Fe atoms to lose electrons and become positively charged as shown below:
Fe -> `~Fe^2^+` + `~2e^-`
The reaction shown above is an example of an oxidation reaction. The area of the metal that is oxidized is called the anode. The electrons are then free to travel to a different location on the metal.
Reduction Step: Reduction occurs when the free electrons produced during oxidation step are removed from the metal by a depolarizer. Examples of depolarizer substances include: oxygen, acids, and other metals. The example below, shows the reduction process when an acid is used as the depolarizer:
`~2H^+` + `~2e^-` -> `~H_2`
The reaction shown above is an example of a reduction reaction. The area of the metal that is reduced is called the cathode.
Notice that the oxidation and reduction steps occur at different locations on the metal. Because of this, electrons are able to flow from the anode site to the cathode site.