CDS Chemistry · Metals and Non-Metals
Corrosion and Its Prevention
What iron, silver and copper turn into when they corrode, and the ways corrosion is stopped, anodising included.
Why this matters
Five CDS questions, and three of them ask the same thing: why silver turns black. The answer is silver sulphide every time. The one HARD question is on anodising.
Concept 1 of 2: What each metal forms when it corrodes
Definition
The three to know:
- Iron rusts: with oxygen and water it forms reddish-brown hydrated iron(III) oxide, Fe₂O₃·xH₂O. Both are needed.
- Silver tarnishes black: it reacts with sulphur (hydrogen sulphide) in air to form silver sulphide, Ag₂S. It is not an oxide.
- Copper turns green: with moist CO₂ it forms basic copper carbonate, CuCO₃·Cu(OH)₂.
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Metals and Non-Metals · Corrosion and Its Prevention
Silver does not form an oxide
Concept 2 of 2: Preventing corrosion, and anodising
Definition
The methods:
- Painting, oiling, greasing: a barrier against air and water.
- Galvanising: coating iron with zinc. Zinc is more reactive, so it corrodes first and protects the iron even when scratched.
- Tinning, electroplating (chromium, nickel): a coat of a less reactive metal.
- Alloying: stainless steel resists rust.
- Anodising: the clean aluminium article is made the anode in electrolysis. Oxygen is released at the anode and thickens the protective oxide layer; hydrogen is released at the cathode. Aluminium, titanium and magnesium can be anodised; it is used in the aircraft industry.
| Method | How it protects |
|---|---|
| Painting, oiling | Keeps air and moisture away |
| Galvanising | Zinc coat corrodes first, protecting iron |
| Electroplating, tinning | Coat of a less reactive metal |
| Anodising | Aluminium as the anode; O₂ at the anode thickens the oxide filmQ CDS 2023 (I), HARD: 'oxygen gas is evolved at the cathode' was the false statement. |
Practice this concept2 quick reps
The same idea in a real exam question:
Example 2 · Metals and Non-Metals · Corrosion and Its Prevention
In anodising, oxygen forms at the anode
Galvanising uses zinc, not tin
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)
Preventing corrosion, and anodising4 rows
| Method | How it protects |
|---|---|
| Painting, oiling | Keeps air and moisture away |
| Galvanising | Zinc coat corrodes first, protecting iron |
| Electroplating, tinning | Coat of a less reactive metal |
| Anodising | Aluminium as the anode; O₂ at the anode thickens the oxide filmQ CDS 2023 (I), HARD: 'oxygen gas is evolved at the cathode' was the false statement. |
Watch out for (3)
- Silver does not form an oxide→ What each metal forms when it corrodes
- In anodising, oxygen forms at the anode→ Preventing corrosion, and anodising
- Galvanising uses zinc, not tin→ Preventing corrosion, and anodising
Test yourself on Metals and Non-Metals
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.