NDA Chemistry · Metals and Non-Metals
Corrosion and Its Prevention
Why metals are eaten away by air and moisture — iron rusting, copper turning green — and how galvanization and sacrificial protection stop it.
Why this matters
A steady pocket — about 5 PYQs, one of the chapter's denser subtopics. The bank tests which metal corrodes fastest, what the green coat on copper is, and the chemistry behind galvanization (why zinc protects iron even when scratched). Most of it is one-line recall once you know zinc is the sacrificial protector.
Concept 1 of 2
Corrosion of common metals
Intuition
Corrosion is the slow eating-away of a metal by air, moisture and gases. Iron rusts fastest of the common metals, forming reddish-brown hydrated iron oxide; copper develops a green coat of basic copper carbonate. Knowing the product and colour for each metal answers most questions here.
Definition
What forms on each metal:
- Iron corrodes (rusts) rapidly in moist air to give hydrated iron(III) oxide (Fe₂O₃·xH₂O), a reddish-brown flaky layer that does not protect the metal beneath.
- Copper in moist air slowly gains a green coat of basic copper carbonate (a malachite-like patina), not rust.
- Aluminium forms a thin, protective oxide layer that stops further corrosion (it does not flake off like rust).
- Silver tarnishes black (silver sulphide) in air containing sulphur compounds.
| Metal | Corrosion product | Colour / behaviour |
|---|---|---|
| Iron | Hydrated iron(III) oxide (rust) | Reddish-brown; flakes off — corrodes rapidly Of the common metals, iron corrodes the fastest — its rust flakes away and exposes fresh metal. |
| Copper | Basic copper carbonate | Green coat (patina) in moist air The green coat on old copper is basic copper carbonate, NOT copper oxide. |
| Aluminium | Aluminium oxide | Thin, protective layer — stops further attack |
| Silver | Silver sulphide | Black tarnish in sulphur-containing air |
Iron rusts (reddish-brown) fastest; copper goes green (basic carbonate); aluminium self-protects.
Practice this conceptself-check · 4 quick reps
Try it yourself
An old copper statue has turned green. Name the compound responsible and state whether this is the same process as iron rusting.
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Which common metal corrodes most rapidly?
- 2.What is the green coat on copper?
- 3.What is rust chemically?
- 4.Why does aluminium resist corrosion?
From the bank · past-year question
Which one of the following elements corrodes rapidly ?
[Q91 · Apr · 2017]
Copper goes green, iron goes brown
The green coat belongs to copper (basic copper carbonate); the reddish-brown flaky layer is iron rust. Don't swap the colours or the products.
Concept 2 of 2
Preventing corrosion — galvanization and sacrificial protection
Intuition
The standard way to protect iron is to coat it with a more reactive metal, zinc — this is galvanization. Zinc works even when the coating is scratched because it is more electropositive than iron, so it corrodes first and is sacrificed to save the iron underneath. The bank tests both the method name and the reason.
Definition
How protection works:
- Galvanization = coating iron or steel with a thin layer of zinc to stop rusting.
- Zinc protects iron because zinc is more electropositive (more reactive) than iron — it acts as a sacrificial anode, corroding in place of the iron even if the layer is broken.
- Other methods: painting, oiling/greasing, electroplating with tin or chromium, and alloying (e.g. stainless steel).
- The general rule: connect or coat the object with a metal higher in the reactivity series so that metal corrodes first.
| Method | What is applied | Why it works |
|---|---|---|
| Galvanization | Thin layer of zinc | Zinc is more reactive — sacrifices itself to protect iron Zinc protects iron even when scratched because it is more electropositive than iron (sacrificial protection). |
| Painting / oiling | Paint or grease film | Keeps air and moisture off the metal |
| Electroplating | Tin or chromium layer | Inert coating barrier |
| Alloying | Mix with Cr, Ni (stainless steel) | Forms a corrosion-resistant alloy |
Galvanization (zinc coat) is the headline method; zinc works as a sacrificial anode because it is more reactive than iron.
Practice this conceptself-check · 4 quick reps
Try it yourself
A galvanized iron sheet gets a deep scratch exposing the iron. Does the iron now rust at the scratch? Explain.
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.What metal is coated on iron during galvanization?
- 2.Why does zinc protect iron?
- 3.Name one corrosion-prevention method besides galvanization.
- 4.Galvanization protects iron by coating it with a thin layer of which metal?
From the bank · past-year question
Zinc is used to protect iron from corrosion because zinc is
[Q116 · Sep · 2017]
Zinc protects because it is MORE reactive
Zinc does not protect iron by being cheaper or a good conductor — it protects because it is more electropositive (more reactive) than iron and is sacrificed first. That is the answer the bank wants.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Corrosion of common metals4 rows
| Metal | Corrosion product | Colour / behaviour |
|---|---|---|
| Iron | Hydrated iron(III) oxide (rust) | Reddish-brown; flakes off — corrodes rapidly Of the common metals, iron corrodes the fastest — its rust flakes away and exposes fresh metal. |
| Copper | Basic copper carbonate | Green coat (patina) in moist air The green coat on old copper is basic copper carbonate, NOT copper oxide. |
| Aluminium | Aluminium oxide | Thin, protective layer — stops further attack |
| Silver | Silver sulphide | Black tarnish in sulphur-containing air |
Iron rusts (reddish-brown) fastest; copper goes green (basic carbonate); aluminium self-protects.
Preventing corrosion — galvanization and sacrificial protection4 rows
| Method | What is applied | Why it works |
|---|---|---|
| Galvanization | Thin layer of zinc | Zinc is more reactive — sacrifices itself to protect iron Zinc protects iron even when scratched because it is more electropositive than iron (sacrificial protection). |
| Painting / oiling | Paint or grease film | Keeps air and moisture off the metal |
| Electroplating | Tin or chromium layer | Inert coating barrier |
| Alloying | Mix with Cr, Ni (stainless steel) | Forms a corrosion-resistant alloy |
Galvanization (zinc coat) is the headline method; zinc works as a sacrificial anode because it is more reactive than iron.
Watch out for (2)
- Copper goes green, iron goes brown→ Corrosion of common metals
- Zinc protects because it is MORE reactive→ Preventing corrosion — galvanization and sacrificial protection
Mastery check — 3 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
On exposure to moist air, copper gains a green coat on its surface due to formation of which one of the following compounds?
[Q101 · Apr · 2019]
Galvanization is a method of protecting iron from rusting by coating with a thin layer of
[Q56 · Sep · 2023]
To protect steel and iron from rusting, a thin layer of which one of the following metals is applied?
[Q124 · Sep · 2021]
Drill every past-year question on this subtopic
5 questions from the bank — paginated, with cart and Word-export support.