Q.What is corrosion? Explain the electrochemical theory of rusting of iron.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Galvanic Corrosion
Galvanic Corrosion: From Intuition to Precision
Imagine you have two different metals — say, a copper pipe and an iron nail — and you connect them with a wire, then dip both into a bucket of salt water. If you come back a few hours later, the iron nail will be badly rusted, while the copper pipe will look almost untouched. Why?
The answer is galvanic corrosion. It is the accelerated corrosion of one metal when it is in electrical contact with a different metal in the presence of an electrolyte (like water with dissolved salts).
The Intuition: A "Battery" That Eats Metal
Think of a simple battery: you have two different metals (electrodes) and a chemical solution (electrolyte). One metal wants to give away electrons (it gets eaten away), and the other wants to accept them (it stays protected). That is exactly what happens in galvanic corrosion.
- The more reactive metal (the one that "wants" to corrode) becomes the anode. It loses electrons and dissolves into the electrolyte — that is the corrosion you see.
- The less reactive metal becomes the cathode. It does not corrode; instead, it accepts electrons from the anode, often causing the electrolyte near it to become alkaline or to produce hydrogen gas.
The key point: the two metals do not need to be physically touching. They just need electrical contact (through a wire or direct contact) and a continuous electrolyte (water, soil, concrete, etc.) to complete the circuit.
The Precise Statement
Galvanic corrosion is the electrochemical process in which a more active metal (the anode) corrodes preferentially when electrically coupled to a less active metal (the cathode) in the presence of an electrolyte. The driving force is the difference in their electrode potentials.
The Galvanic Series: The "Who Eats Whom" Chart
Not all metal pairs corrode equally. The galvanic series ranks metals and alloys by their tendency to corrode in seawater (a common electrolyte). The more negative (active) a metal is, the more likely it is to be the anode and corrode.
Here is a simplified version of the series (from most active/anodic to most noble/cathodic):
| Metal / Alloy | Relative Activity |
|---|---|
| Magnesium | Most active (anodic) |
| Zinc | |
| Aluminium | |
| Cadmium | |
| Mild steel / Iron | |
| Stainless steel (active) | |
| Tin | |
| Lead | |
| Copper | |
| Nickel | |
| Stainless steel (passive) | |
| Silver | |
| Titanium | |
| Gold / Platinum | Most noble (cathodic) |
A common mistake: students think the larger metal always corrodes. In reality, it is the more active metal that corrodes, regardless of size. However, the area ratio matters enormously — a small anode coupled to a large cathode corrodes very fast (like a tiny iron rivet holding a huge copper plate).
The Three Conditions for Galvanic Corrosion
For galvanic corrosion to occur, all three must be present:
- Two dissimilar metals (or the same metal in different environments, e.g., a steel pipe in soil vs. in air).
- Electrical contact between them (direct physical contact or through a wire).
- An electrolyte bridging them (water, moisture, soil, concrete, etc.).
Remove any one, and galvanic corrosion stops.
Real-World Examples
- The Statue of Liberty: The copper skin was originally separated from the iron framework by asbestos cloth. When the cloth degraded, the iron (anode) corroded rapidly because it was coupled to the huge copper (cathode) surface. …
Why this formula?
Galvanic Corrosion: Why the Key Formulas Hold
Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte. The key formula that governs this is the mixed potential theory, which leads to the galvanic current and corrosion rate expressions.
Let's build the reasoning step-by-step.
1. The Core Idea: Two Electrodes, One Circuit
When metals M₁ (more active, e.g., zinc) and M₂ (more noble, e.g., copper) are connected:
- M₁ acts as the anode — it oxidizes (corrodes):
M1→M1n++ne−
- M₂ acts as the cathode — it reduces something (e.g., oxygen or H⁺):
O2+2H2O+4e−→4OH−(in neutral/alkaline)
or
2H++2e−→H2(in acidic)
The two metals are electrically connected (via a wire or direct contact), and the electrolyte completes the circuit. Electrons flow from M₁ to M₂.
2. The Mixed Potential: Why It Exists
Each metal, when alone in the electrolyte, has its own open-circuit potential (OCP) — the equilibrium potential for its half-reaction. For M₁, it's Ecorr,1; for M₂, it's Ecorr,2.
When connected, the system cannot stay at two different potentials. The entire metal couple must reach a single potential — the mixed potential Emix.
- Emix lies between Ecorr,1 and Ecorr,2.
- At Emix, the total anodic current from M₁ equals the total cathodic current from M₂ (charge conservation):
Ianode=Icathode
This is the fundamental equation of galvanic corrosion.
3. Deriving the Galvanic Current
Assume each electrode follows Butler-Volmer kinetics (for activation-controlled reactions). For the anode (M₁), the anodic current density ia at potential E is:
ia=i0,1exp(RTαaF(E−E0,1))
For the cathode (M₂), the cathodic current density ic is:
ic=i0,2exp(−RTαcF(E−E0,2))
Where:
- i0,1,i0,2 = exchange current densities
- αa,αc = transfer coefficients (typically ~0.5)
- F = Faraday constant
- R = gas constant
- T = temperature
- E0,1,E0,2 = standard reduction potentials
At the mixed potential Emix:
Igalvanic=A1⋅ia(Emix)=A2⋅ic(Emix)
Where A1 and A2 are the surface areas of the anode and cathode.
Why this holds: The net current from the anode must exactly balance the net current consumed at the cathode — otherwise, charge would accumulate, which is impossible in a steady-state circuit.
4. The Corrosion Rate Formula
The corrosion rate (mass loss per time) of the anode is given by Faraday's law:
Corrosion rate=n⋅F⋅ρIgalvanic⋅M
Where:
- M = molar mass of the anode metal
- n = number of electrons transferred per atom
- ρ = density of the metal
- F = Faraday constant (96,485 C/mol)
Why this holds: Each mole of metal oxidized releases n moles of electrons. The total charge passed Q=Igalvanic⋅t corresponds to moles of metal lost:
moles lost=nFQ=nFIgalvanic⋅t
Multiply by M/ρ to get volume or thickness loss.
5. The Area Effect: Why It Matters
From the mixed potential equation:
A1⋅ia(Emix)=A2⋅ic(Emix)
If the cathode area A2 is large relative to the anode area A1, then ia(Emix) must be large to balance the current. This means:
- Small anode + large cathode → severe galvanic corrosion (high current density on the anode). …
Corrosion of iron proceeds as a tiny electrochemical cell set up on the metal's own surface, with iron oxidised at one spot and dissolved atmospheric oxygen reduced at another. …
Rusting is a galvanic-cell process on the iron surface itself: Fe is oxidised to Fe²⁺ at anodic points, dissolved O2 is reduced to water at cathodic points, and Fe²⁺ is later oxidised further and hydrated to form rust.
Corrosion is the spontaneous process by which a metal is slowly deteriorated/converted into an undesirable compound (usually an oxide, sulphide, carbonate, etc.) by reaction with atmospheric gases (O2, CO2, moisture) or other environmental substances, on the surface of the metal. Rusting of iron is the most common example.
Electrochemical theory of rusting: A piece of iron with an impure or strained/high-energy surface behaves like a tiny short-circuited galvanic cell:
- At the anodic region (a point of higher energy on the iron surface, e.g. an impurity or a strained spot): iron is oxidised to Fe²⁺ ions, releasing electrons. Fe(s)→Fe2+(aq)+2e−
- The released electrons flow through the metal to a nearby cathodic region (a lower-energy spot), where in the presence of H⁺ ions (from dissolved CO2 forming carbonic acid in moisture) dissolved atmospheric oxygen is reduced to water: O2(g)+4H+(aq)+4e−→2H2O(l)
- The overall cell reaction is therefore: …
- CBSE 2026Set ANNUAL1 markMCQQ.Formula of rust is :(a) FeO.xH2O(b) Fe3O4.xH2O(c) Fe2O3.xH2O(d) None of the above
›Reveal solutionSolution
Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O, formed by the electrochemical corrosion of iron.
Corrosion of iron is an electrochemical process. Moist air acts as an electrolytic medium; at one spot on the iron surface (anodic region) iron is oxidised:
Fe(s)→Fe2+(aq)+2e−
At another spot (cathodic region), dissolved oxygen is reduced in the presence of H⁺ (from H₂CO₃ formed by dissolved CO₂):
O2(g)+4H+(aq)+4e−→2H2O(l)
…
- CBSE 2025Set D1 markMCQQ.Rust is(a) Powdered iron(b) Ferrous oxide(c) Ferric oxide(d) Hydrated ferric oxide
›Reveal solutionSolution
Rust is hydrated ferric oxide, Fe2O3·xH2O, formed by electrochemical corrosion of iron in moist air.
Corrosion of iron in the presence of moisture and oxygen is an electrochemical process. Iron is oxidised (anode): Fe → Fe2+ + 2e-; oxygen is reduced at cathodic sites. The Fe2+ ions are further oxidised by atmospheric oxygen to Fe3+, which combines with water to form the brown, flaky deposit of hydrated ferri …
- CBSE 2025Set ANNUAL1 markMCQQ.Galvanisation is applying a coating of:(a) Cr(b) Cu(c) Zn(d) Pb
›Reveal solutionSolution
Galvanisation is the coating of iron/steel with a protective layer of zinc.
Galvanisation is an anti-corrosion technique in which a thin coating of zinc (Zn) metal is applied to iron or steel articles (e.g. by hot-dip galvanising) to protect them from rusting. Zinc, being more reactive (more easily oxidised) than iron, acts as a sacrificial anode: it corrodes prefer …
- CBSE 2024Set ANNUAL1 markQ.Write down the electrochemical reactions that takes place in the corrosion of iron.
›Reveal solutionSolution
Rusting is an electrochemical process: iron is oxidised at anodic spots, oxygen is reduced at cathodic spots, and the resulting Fe²⁺ is later air-oxidised to hydrated Fe₂O₃.
A droplet of water on an iron surface behaves like a tiny galvanic cell, with different regions of the same iron object acting as anode and cathode (driven by impurities/stress/oxygen-concentration differences).
At the anode (oxidation):
Fe(s)→Fe2+(aq)+2e−
At the cathode (reduction), in the presence of dissolved H+/O2:
O2(g)+4H+(aq)+4e−→2H2O(l)
…
- CBSE 2024Set ANNUAL1 markMCQQ.Rust is a mixture of -(a) FeO and Fe(OH)3(b) FeO and Fe(OH)2(c) Fe2O3 and Fe(OH)3(d) Fe3O4 and Fe(OH)3
›Reveal solutionSolution
Rusting is an electrochemical (corrosion) process that oxidises iron to the +3 state, giving a hydrated mixture of ferric oxide (Fe2O3⋅xH2O) and ferric hydroxide, Fe(OH)3 — option (c).
Corrosion of iron (rusting) is an electrochemical process. At an anodic site iron is oxidised:
Fe→Fe2++2e−
and atmospheric oxygen dissolved in the surface moisture is reduced at a cathodic site:
O2+4H++4e−→2H2O
…
- CBSE 2022Set ANNUAL1 markMCQQ.Rust is a mixture of(a) FeO and Fe(OH)3(b) FeO and Fe(OH)2(c) Fe2O3 and Fe(OH)3(d) Fe3O4 and Fe(OH)2
›Reveal solutionSolution
Rusting of iron is an electrochemical corrosion process in which iron is oxidized by atmospheric O2 and moisture to a hydrated Fe2O3/Fe(OH)3 mixture.
In the presence of moisture and dissolved O₂/CO₂, iron acts as the anode of a tiny electrochemical cell: Fe → Fe²⁺ + 2e⁻ at anodic spots; O₂ + 4H⁺ + 4e⁻ → 2H₂O (or O₂ + 2H₂O + 4e⁻ → 4OH⁻) at cathodic spots. The Fe²⁺ formed is further oxidized by dissolved oxygen to …
- CBSE 2021Set A1 markMCQQ.The best way to protect iron from rusting is(a) making iron cathode(b) putting it in saline water(c) both (A) and (B)(d) none of these
›Reveal solutionSolution
Rusting is oxidation of iron; making iron the cathode (cathodic protection) stops it being oxidised, so it is the best protection.
Rusting is an electrochemical process in which iron is oxidised (Fe → Fe2+ + 2e-) at anodic sites. To stop it, we must prevent iron from acting as the anode.
- (A) Making iron the CATHODE — cathodic protection (e.g. connecting a more active sacrificial metal like Zn or Mg, or an impressed current) — forces iron to be reduced/protected rather than oxidised. This is the most effective method. …
- CBSE 2020Set 56/1/11 markQ.Out of zinc and tin, whose coating is better to protect iron objects?
›Reveal solutionSolution
The key idea is Galvanic Corrosion — when two metals are in contact in an electrolyte, the more active (anodic) metal corrodes preferentially, protecting the less active (cathodic) metal. For iron, zinc coating is better because zinc is more reactive than iron and acts as a sacrificial anode, whereas tin is less reactive and only protects as long as the coating is intact.
Why This Question Matters
This isn't just a memorisation problem — it's about understanding how corrosion protection actually works in the real world. Iron objects rust because iron readily gives up electrons to oxygen and moisture. When you coat iron with another metal, you're changing the electrochemical behaviour of the surface. The question asks: which coating — zinc or tin — gives better long-term protection?
The answer depends on the electrochemical series and what happens when the coating gets scratched.
Step-by-Step Reasoning
1. Recall the reactivity order of the three metals
In the electrochemical series (from most reactive to least):
Zn>Fe>Sn
- Zinc is more reactive (more anodic) than iron.
- Tin is less reactive (more cathodic) than iron.
Standard reduction potentials (at 25°C):
Zn2++2e−Fe2++2e−Sn2++2e−→ZnE∘=−0.76 V→FeE∘=−0.44 V→SnE∘=−0.14 V
More negative E∘ means greater tendency to lose electrons (oxidise).
2. Understand what happens when the coating is intact
When the coating is perfect (no scratches), both zinc and tin protect iron by simply acting as a physical barrier — they keep oxygen and moisture away from the iron surface. In this ideal scenario, both work equally well.
But coatings never stay perfect forever. Scratches happen.
3. Analyse the scratched coating scenario — this is the crux
When a scratch exposes the underlying iron to the environment, you now have two different metals in electrical contact through the electrolyte (moist air, water). This creates a galvanic cell.
NoteIn a galvanic cell, the more reactive metal becomes the anode and corrodes (loses electrons). The less reactive metal becomes the cathode and is protected (gains electrons).
Case 1: Zinc coating (scratched)
- Zinc is more reactive than iron → Zinc becomes the anode.
- Iron becomes the cathode.
- Result: Zinc corrodes instead of iron. Even at the scratch site, the iron is protected because electrons flow from zinc to iron. This is called sacrificial protection or cathodic protection.
Case 2: Tin coating (scratched)
- Tin is less reactive than iron → Iron becomes the anode.
- Tin becomes the cathode.
- Result: Iron corrodes at the scratch site, and the corrosion actually accelerates because the tin coating acts as a large cathode, driving the anodic reaction on the exposed iron. This is worse than having no coating at all! …
- CBSE 2019Set ANNUAL1 markQ.What is corrosion?
›Reveal solutionSolution
Corrosion is a naturally occurring, spontaneous electrochemical process by which a metal is gradually converted into its oxide/other compound, deteriorating the metal — the most familiar example being iron rusting.
…
- CBSE 2018Set ANNUAL1 markMCQQ.Match the following. Group A item: Corrosion. Choose its correct match from Group B: (A) Lanthanides and Actinides (B) Used in treatment of cancer (C) Purple of Cassius (D) Barbituric acid (E) Hydrated ferric oxide
›Reveal solutionSolution
Corrosion of iron (rusting) produces hydrated ferric oxide.
Corrosion is an electrochemical process in which a metal is oxidised by atmospheric oxygen and moisture, forming a coating of its oxide/hydroxide on the surface. In the specific case of iron, this proce …
- CBSE 2017Set ANNUAL1 markQ.Zinc protects iron from corrosion better than tin. Give reason.
›Reveal solutionSolution
Zinc coats iron sacrificially (self-corrodes to protect the exposed metal), whereas a scratched tin coating instead accelerates iron's corrosion by making iron the anode.
Zinc lies above iron in the reactivity/electrochemical series (more negative reduction potential), so in the electrochemical corrosion cell formed at a scratch, zinc acts as the anode and is preferentially oxidised, while iron is protected as the cathode — this continues to protect iron even after the coating is locally broken (galvanisation, "sacrificial protection").
…
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