Corrosion is the gradual degradation of metals by reaction with the environment. Iron rusting is the most common example. CBSE Class 10 and 12 cover it; NEET tests prevention methods.
Core Concepts
Rusting of iron
Iron reacts with oxygen and water to form hydrated iron oxide (). The process is electrochemical — some regions act as anodes (Fe oxidised) and others as cathodes (O2 reduced).
The electrochemical mechanism in detail:
At microscopic anodic spots on the iron surface:
Iron atoms lose electrons and dissolve into the moisture film as Fe2+ ions.
At nearby cathodic spots:
(in acidic conditions) or
(in neutral/alkaline conditions). The electrons released at the anode flow to the cathode through the metal.
Fe2+ combines with OH-:
Fe(OH)2 is further oxidised by atmospheric O2:
Fe(OH)3 dehydrates to form rust:
The electrochemical mechanism explains why rusting is faster at scratches, dents, and stress points — these regions have higher energy and preferentially act as anodes (where iron dissolves). This is why cars rust first at scratches and door edges.
Conditions needed
Moisture and oxygen are both required. Rusting does not occur in completely dry air or in air-free water. Salt accelerates rusting because it increases the solution’s conductivity.
Experimental proof:
| Condition | Setup | Result |
|---|---|---|
| Dry air only | Iron nail in anhydrous CaCl2 (desiccant) jar | No rust |
| Air-free water | Iron nail in boiled water sealed with oil | No rust |
| Moist air | Iron nail exposed to air and moisture | Rust forms |
| Salt water | Iron nail in NaCl solution | Rust forms faster |
The salt water experiment shows that dissolved ions increase the conductivity of the moisture film, allowing electrons to flow more easily between anodic and cathodic regions. This is why coastal areas and road-salted regions see much more corrosion.
Prevention methods
Painting, oiling, greasing — physical barrier. Galvanisation — coating with zinc. Tin plating — coating with tin. Electroplating — coating with chromium or nickel. Alloying — stainless steel with chromium and nickel. Cathodic protection with sacrificial anodes.
| Method | Principle | Protection If Scratched? | Example |
|---|---|---|---|
| Painting/oiling | Physical barrier | No — iron exposed at scratch | Bridge painting, machinery oiling |
| Galvanisation | Zinc coating + sacrificial anode | Yes — zinc corrodes instead | Roofing sheets, water pipes |
| Tin plating | Tin coating (physical barrier) | No — iron corrodes faster at scratch | Food cans (tin cans) |
| Electroplating (Cr, Ni) | Attractive + resistant coating | No — iron exposed at scratch | Bumpers, bathroom fittings |
| Stainless steel | Chromium forms Cr2O3 layer | Yes — Cr2O3 reforms | Cutlery, surgical instruments |
| Cathodic protection | Sacrificial anode (Zn or Mg) | Yes — anode corrodes instead | Ship hulls, underground pipes |
Galvanisation
Iron is coated with zinc. Even if the coating is scratched, zinc acts as a sacrificial anode — it corrodes preferentially, protecting the iron. Widely used for roofing and pipes.
Why zinc protects even when scratched: Zinc is more reactive than iron (higher in the reactivity series, more negative electrode potential). When both metals are exposed, zinc preferentially oxidises:
The electrons flow to the iron, keeping it in a reduced state. Iron acts as the cathode and is protected. This is called cathodic protection — the iron is forced to be the cathode (where reduction occurs, not oxidation).
Tin plating vs galvanisation
This comparison is a favourite NEET question. Tin is LESS reactive than iron; zinc is MORE reactive.
If tin plating is scratched:
- Tin is the cathode (less reactive), iron is the anode
- Iron corrodes preferentially at the scratch
- Corrosion is actually accelerated at the exposed spot
- The intact tin surrounding the scratch creates a large cathode/small anode ratio, intensifying corrosion
If galvanisation is scratched:
- Zinc is the anode (more reactive), iron is the cathode
- Zinc corrodes preferentially
- Iron is protected even at the scratch
This is the single most important comparison in this chapter. “Why does galvanised iron not rust at a scratch but tinned iron does?” Know the electrochemical reasoning — which metal is the anode and which is the cathode.
Other metals
Aluminium forms a thin protective oxide layer that prevents further corrosion — why aluminium does not rust visibly. Copper forms a green patina (basic copper carbonate) over time.
Aluminium passivation: Al2O3 forms almost instantly on fresh aluminium surfaces. This layer is only 5-10 nm thick but is extremely tough, adherent, and impervious to oxygen and water. Anodising makes this layer thicker (up to 25 micrometres) for enhanced protection.
Copper patina: Over years, copper reacts with CO2, H2O, and SO2 in the atmosphere:
The green patina on the Statue of Liberty is basic copper carbonate. Unlike rust, the patina is protective — it slows further corrosion.
Worked Examples
Unlike zinc, tin is less active than iron. If the tin coating is scratched, iron acts as the anode and corrodes faster than it would without any coating.
Zinc or magnesium blocks are attached to steel hulls. They corrode in place of the hull because they are more electropositive. Replaced periodically.
Stainless steel contains at least 10.5% chromium. Chromium reacts with oxygen to form a thin, invisible Cr2O3 layer on the surface. This layer is self-healing — if scratched, it reforms immediately from the chromium in the alloy. As long as the chromium content is above 10.5%, the passive layer protects the steel.
If a steel bridge loses 0.1 mm of thickness per year to corrosion, and the steel is 10 mm thick, the bridge has about 100 years before structural failure — assuming uniform corrosion. But corrosion is rarely uniform; pitting corrosion can create holes much faster. This is why regular inspection and repainting is essential.
Common Mistakes
Saying rusting happens in dry air. It needs moisture.
Confusing tin plating and galvanisation. Tin is less reactive than iron (fails if scratched); zinc is more reactive (protects even if scratched).
Writing that aluminium does not corrode. It corrodes but forms a protective oxide layer.
Saying painting provides electrochemical protection. Painting is a physical barrier only — it has no electrochemical component. Once scratched, the exposed iron corrodes normally.
Confusing cathodic protection with coating. Cathodic protection uses a more reactive metal (sacrificial anode) that corrodes instead of the protected metal. It works even without coating.
Exam Weightage and Revision
Corrosion carries 1-2 NEET questions per year and 2-3 marks in CBSE Class 10 and 12 boards. The most tested concepts: conditions for rusting, galvanisation vs tin plating, and cathodic protection. JEE may include an electrochemical cell potential calculation for a corrosion cell.
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Conditions for rusting | Every year | Easy |
| Galvanisation principle | Most years | Easy |
| Tin vs zinc comparison | Every 2 years | Medium |
| Cathodic protection | Occasional | Medium |
| Prevention methods list | Most years | Easy |
The two guaranteed marks: (1) “Name two conditions required for rusting” — moisture and oxygen. (2) “Why is galvanised iron preferred?” — zinc is a sacrificial anode, protects iron even when scratched.
Practice Questions
Q1. Iron nails are placed in four test tubes: (a) dry air (b) boiled water sealed with oil (c) moist air (d) salt water. In which tubes will rusting occur?
(a) No rusting — no moisture. (b) No rusting — no oxygen (water is boiled to remove dissolved O2, oil layer prevents fresh O2 from entering). (c) Rusting occurs — both moisture and oxygen are present. (d) Rusting occurs — and faster than (c) because dissolved NaCl increases the conductivity of the moisture film, facilitating the electrochemical process.
Q2. Why are underground pipelines often connected to zinc or magnesium blocks?
This is cathodic protection using sacrificial anodes. The zinc/magnesium blocks are more reactive than the steel pipe. They oxidise preferentially (), and the electrons flow to the steel pipe, keeping it as the cathode (where reduction occurs). The pipe is thus protected from corrosion. The zinc/magnesium blocks are periodically replaced as they dissolve.
Q3. A food can is made of tin-plated iron. Why is it safe as long as the tin layer is intact but dangerous if the can is dented?
Intact tin layer provides a physical barrier — food does not contact iron. If dented or scratched, iron is exposed. Since iron is more reactive than tin, iron becomes the anode and corrodes. The dissolved iron (Fe2+ ions) contaminate the food, and the corrosion is actually accelerated because of the large tin cathode area relative to small exposed iron anode area. This is why dented cans should be discarded.
Q4. Why does silver tarnish (turn black) but not corrode like iron?
Silver reacts with H2S (hydrogen sulphide) in air to form Ag2S (silver sulphide), which is black. . This is tarnishing, not rusting. The Ag2S layer is thin and does not continue to grow because it is impermeable (unlike rust, which is porous and flaky, allowing continued corrosion underneath). Silver is also very low in the reactivity series, so it does not corrode in the way iron does.
FAQs
How much does corrosion cost the economy?
Corrosion costs about 3-4% of GDP globally. For India, this translates to hundreds of billions of rupees annually. Replacing corroded bridges, pipelines, vehicles, and industrial equipment is a massive ongoing expense. Prevention (painting, galvanising, cathodic protection) is far cheaper than replacement.
Can stainless steel rust?
In normal conditions, no. But in very aggressive environments (concentrated chloride solutions, high temperatures, low pH), even stainless steel can corrode — this is called pitting corrosion. The Cr2O3 passive layer breaks down locally, and corrosion proceeds at that point. Marine environments and chemical plants require special high-alloy stainless steels.
Is rusting reversible?
Not naturally. Rust (Fe2O3) is thermodynamically stable and does not spontaneously convert back to iron. However, rust can be reduced to iron by heating with carbon (essentially a mini blast furnace reaction) — but this is impractical for corroded objects. Prevention is always better than cure.
Three prevention methods to memorise — galvanisation, painting, cathodic protection.
Corrosion is slow-motion chemistry with huge economic impact. Preventing it saves more money than most people realise.