Question
Why does iron rust, and what is the electrochemical mechanism behind corrosion? Also explain two methods to prevent it.
This appeared in NEET 2024 and is a consistent 1-mark question in CBSE Class 12 boards. Easy marks if you understand the mechanism — students lose them by memorising without understanding.
Solution — Step by Step
Iron in moist air doesn’t corrode uniformly — it behaves like a galvanic cell. Some regions act as the anode (where iron dissolves) and others act as the cathode (where oxygen is reduced). This difference arises due to impurities, stress points, or composition variation in the metal.
At the anode, iron loses electrons:
This is oxidation. The iron atoms here literally dissolve into the solution as ions. This is why rust forms at weak spots — that’s where the anode is.
The electrons released at the anode travel through the metal to the cathodic region. There, dissolved oxygen (from moist air) is reduced:
In neutral or slightly acidic conditions, the overall cathodic reaction consumes both and .
The ions migrate through the electrolyte (moisture) toward the cathodic region, where they get further oxidised:
The actual rust you see is hydrated ferric oxide, — that familiar reddish-brown coating.
Two high-weightage methods:
Cathodic protection (sacrificial anode): Attach a more active metal like zinc or magnesium to iron. Zinc preferentially acts as the anode and corrodes instead — iron is protected. This is used in ship hulls and underground pipelines.
Galvanisation: Coat iron with a thin layer of zinc. Even if the zinc coating scratches, zinc still acts as the sacrificial anode due to its higher reactivity.
Why This Works
Corrosion is essentially an involuntary galvanic cell. The driving force is the difference in electrode potential between different regions of the iron surface. The more active region (lower reduction potential) becomes the anode and oxidises — exactly like the negative terminal of a battery.
The role of moisture is as the electrolyte. Dry iron doesn’t rust because there’s no ionic conductor to complete the circuit. Dissolved or in rainwater makes it acidic and a better electrolyte — that’s why corrosion is faster in industrial areas with air pollution.
Saline water (near the sea) corrodes iron even faster because is a much better electrolyte than pure water. This is why cars in coastal cities rust noticeably faster.
Alternative Method
Alloying is another prevention approach — not always listed as electrochemical, but conceptually important.
In stainless steel, chromium (12-18%) forms a thin, adherent layer of on the surface. This passive oxide layer physically blocks oxygen and moisture from reaching the iron underneath. Unlike rust (), this chromium oxide layer is dense, non-porous, and self-healing.
For board exams and NEET, the three prevention methods in order of importance: (1) galvanisation/sacrificial anode — always electrochemical, (2) painting/oiling — physical barrier, (3) alloying — compositional change. The question will usually specify which category it wants.
Common Mistake
Students write that rust forms at the anode. It doesn’t. Iron dissolves at the anode as , but rust () actually deposits near the cathodic region, where the ions migrate and get oxidised further. This is why rust appears away from the actual corroded pit. In NEET 2024, this distinction was directly tested — the question asked where the anodic reaction occurs, and many students confused the site of dissolution with the site of rust deposition.