What is Corrosion? Definition, Types, Factors & Class 12 Chemistry Notes

Corrosion is a natural phenomenon that occurs when metal comes in contact with the atmosphere. This reaction is called a redox reaction. Now we shall check the definition of corrosion as per the NCERT: "Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal. The rusting of iron, tarnishing of silver, development of green coating on copper and bronze are some of the examples of corrosion".

Important Topics:

Corrosion involves redox reactions where the metal undergoes oxidation (anode) and another species $\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)\to {\mathrm{F}\mathrm{e}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+2{\mathrm{e}}^{-}$ (e.g., oxygen) undergoes reduction (cathode). For iron rusting:

• Anode: (oxidation).
• Cathode: ${\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{a}\mathrm{q}\right)+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ (reduction in acidic medium).
• Overall reaction: $2\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}^{+}\left(\mathrm{a}\mathrm{q}\right)\to 2{\mathrm{F}\mathrm{e}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ .
  ${\mathrm{F}\mathrm{e}}^{2+}$ ions further react with ${\mathrm{O}\mathrm{H}}^{-}$ to form rust ${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\cdot x{\mathrm{H}}_2\mathrm{O}$ .

Rusting is a process in which iron is exposed to moist air. Water droplets on the surface of iron act as miniature electrochemical cells:

• Anodic Region: Iron oxidizes, releasing electrons.
• Cathodic Region: Oxygen is reduced, consuming electrons.
• Electrolyte: Moisture with dissolved ${\mathrm{C}\mathrm{O}}_2$ or salts conducts ions.

The ${\mathrm{F}\mathrm{e}}^{2+}$  ions migrate and react with ${\mathrm{O}\mathrm{H}}^{-}$ ions (from water dissociation or ${\mathrm{O}}_2$  reduction) to form $\mathrm{F}\mathrm{e}(\mathrm{O}\mathrm{H}{)}_2$ , which oxidizes to ${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\cdot x{\mathrm{H}}_2\mathrm{O}$ (rust).

Electrolytes have an essential role in the process of corrosion. It acts as a conductive medium for electrochemical reactions. It is a substance that comprises free ions and conducts an electric current. In the corrosion process, dissolved salts, acids, or bases act as electrolytes.

Electrolytes like sodium chloride, also known as salt NaCl (from seawater) or acidic pollutants (e.g., ${\mathrm{S}\mathrm{O}}_2$ ) accelerate corrosion by enhancing ion conductivity. Dissolved salts lower the resistance of the water film, speeding up electron transfer.

Factors affecting  corrosion

• Presence of Electrolytes:
  • Electrolytes NaCl, acid or bases speeds up electrochemical process.
• Moisture (Humidity)
  • It is most important for corrosion.
  • When humidity is high it forms a layer of water on metal surface causing corrosion.
• Temperature:
  • When temperature is high, increases the chemical reaction process, leading to corrosion.
  • Corrosion is faster in humid and hot climate.
• Design of Metal Structure
  • Poor drainage or airflow leads to corrosion.
  • Design of structure with sharp corners, joints, or crevices traps water and promote corrosion.
• Nature of Metal
  • Purity
  • Reactivity and
  • Surface area 

The process of rusting of iron is like a natural galvanic cell in action. When a metal is exposed to moisture and oxygen, different part of the metal surface acts as electrodes. One part of the metal will act as an anode and the other as a cathode. So when iron and oxygen in the air react, it forms rust.

Therefore, corrosion is an electrochemical process in which the metal surface sets up tiny galvanic cells. The flow of electrons from anode to cathode drives the process of weakening the metal.

$E_{{\mathrm{F}\mathrm{e}}^{2+/\mathrm{F}\mathrm{e}}}^{\circ }=-0.76\mathrm{V},\mathrm{}{E}_{{\mathrm{C}\mathrm{u}}^{2+}/\mathrm{C}\mathrm{u}}^{\circ }=+0.34\mathrm{V}$

Iron (Fe) corrodes as the anode.

Corrosion occurs in different ways depending on conditions. The main types of corrosion are:

Uniform Corrosion: This is the most common type of corrosion. In uniform corrosion, the surface of the metal is attacked regularly, which leads to a uniform thinning of the material. The example of uniform corrosion is the rusting of iron when exposed to moist air.

Galvanic Corrosion: This corrosion occurs when two different types of metals in contact with each other are dipped in an electrolyte. The more reactive metal will act as anode and corrode faster, and the less reactive metal is protected.

Pitting Corrosion: It is type of localized corrosion. Pitting corrosion forms a small hole in the metal. Also, they are unpredictable types of corrosion because they remain hidden under the surface.