Question
Explain the electrochemical mechanism of rusting of iron. Why does iron corrode faster in saltwater than in pure water? Mention two methods to prevent corrosion.
(NCERT Class 12, Chapter 3 — this is a scoring topic for boards and NEET)
Solution — Step by Step
When iron is exposed to moist air, tiny electrochemical cells form on its surface. Different regions act as anodes and cathodes due to impurities, stress points, or differences in oxygen concentration.
At the anode (iron dissolves):
Electrons released at the anode travel through the metal to the cathode region, where they reduce dissolved oxygen:
The overall cell potential is V — positive, so the reaction is spontaneous.
ions combine with to form green rust (), which is further oxidised by oxygen to form hydrated iron(III) oxide — the brown rust we see:
Salt (NaCl) increases the conductivity of water, allowing ions to migrate faster between anode and cathode regions. This increases the current flow in the electrochemical cell and speeds up the corrosion rate. Pure water has low ion concentration, so corrosion is slower.
Why This Works
Corrosion is essentially an electrochemical cell running in an uncontrolled way. Iron acts as a reactive anode, and the dissolved oxygen provides the driving force at the cathode. Both moisture (to act as electrolyte) and oxygen (to drive the cathode reaction) are essential — iron does not rust in dry air or in oxygen-free water.
Prevention methods:
- Galvanization — coating iron with zinc. Zinc is more reactive ( V) and corrodes preferentially, protecting iron even if the coating is scratched.
- Cathodic protection — connecting iron to a more reactive metal (sacrificial anode) like magnesium or zinc.
- Barrier methods — painting, oiling, or coating to prevent contact with moisture and oxygen.
Alternative Method
You can also explain corrosion using the Nernst equation. The cell potential depends on the concentration of dissolved oxygen and pH. Lower pH (acidic conditions) and higher oxygen concentration both increase the cell potential, accelerating corrosion.
For CBSE boards, make sure to write both the anode and cathode half-reactions with their standard electrode potentials. Also mention that both water and oxygen are necessary for rusting — this is a commonly asked assertion-reason question.
Common Mistake
Students often write the cathode reaction as (like in acid corrosion). In atmospheric corrosion of iron, the cathode reaction involves oxygen reduction in neutral/slightly acidic water, not hydrogen evolution. The hydrogen evolution mechanism applies only in strongly acidic environments.