CIE iGCSE Co-ordinated Sciences-C9.5 Corrosion of metals- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C9.5 Corrosion of metals – Study Notes
CIE iGCSE Co-ordinated Sciences-C9.5 Corrosion of metals – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- State the conditions required for the rusting of iron (presence of oxygen and water)
- State some common barrier methods, including painting, greasing and coating with plastic
- Describe how barrier methods prevent rusting by excluding oxygen and water
Supplement
- Describe the use of zinc in galvanising steel as an example of a barrier method and sacrificial protection
- Explain sacrificial protection in terms of the reactivity series and in terms of electron loss
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Conditions Required for the Rusting of Iron
Rusting is the corrosion of iron, where iron reacts with oxygen and water to form hydrated iron(III) oxide, commonly known as rust.
Key Conditions for Rusting:
- Oxygen: Rusting cannot occur without the presence of oxygen from the air.
- Water: Rusting also requires water (moisture). Dry iron does not rust.
Note: The presence of salts (e.g., sodium chloride) or acids can speed up rusting because they increase the conductivity of water, allowing faster electron transfer.
Example
Explain experiment with iron nails in different conditions.
▶️Answer/Explanation
Nail A in dry air → No rust (oxygen present but no water).
Nail B in boiled water covered with oil → No rust (water present but no oxygen).
Nail C in water exposed to air → Rust forms (both oxygen and water present).
Example
Explain effect of salt water on rusting.
▶️Answer/Explanation
An iron nail placed in salty water rusts faster than in pure water. This is because dissolved salts increase the conductivity of water, accelerating the rusting process.
How Barrier Methods Prevent Rusting
Barrier methods are physical coatings applied to the surface of iron or steel to prevent rusting. These coatings work by stopping oxygen and water from coming into direct contact with the iron surface, which are both essential for rusting to occur.
Since rusting requires both oxygen and water, any method that excludes these from reaching the iron surface will prevent rust from forming. If the coating is scratched and bare iron is exposed, rusting can begin at the exposed areas.
Common Barrier Methods to Prevent Rusting
- Painting: A layer of paint prevents oxygen and moisture from reaching the iron surface. Commonly used for gates, bridges, and outdoor structures.
- Greasing or Oiling: Grease or oil is applied to moving metal parts (e.g., machinery, bicycle chains) to prevent water and air contact while also reducing friction.
- Plastic Coating: A thin, durable plastic layer is applied to metal objects such as wire fences or paperclips, providing a waterproof and airproof shield.
Example
Explain how painting iron gates prevents rusting.
▶️Answer/Explanation
When iron gates are painted, the paint acts as a barrier preventing air and water from reaching the metal. As long as the paint is intact, rusting does not occur. However, if the paint cracks, rusting starts at the exposed spots.
Example
Why is plastic coating done on paperclips?
▶️Answer/Explanation
Paperclips are often covered with a thin plastic coating. This layer blocks moisture and air, so the metal inside does not rust, even if exposed to damp conditions.
Galvanising — Protection of Steel Using Zinc
Galvanising is the process of coating iron or steel with a thin layer of zinc to protect it from rusting and corrosion.
This method combines two forms of protection: 
- Barrier protection — prevents oxygen and water from reaching the metal surface.
- Sacrificial protection — zinc, being more reactive, corrodes instead of the iron.
Barrier Method of Protection:
- The thin coating of zinc acts as a physical barrier that prevents oxygen and water from reaching the surface of the steel.
- As long as the zinc layer is intact, the iron underneath cannot rust.
Explanation:
- Rusting requires both oxygen and water.
- The zinc coating blocks these substances from reaching the steel surface.
Sacrificial Protection:
- If the zinc layer is scratched or damaged, exposing the steel, the zinc still protects it.
- This happens because zinc is more reactive than iron (higher in the reactivity series).
- Zinc acts as a sacrificial metal — it oxidises (corrodes) instead of the iron.
Reactions (in terms of electron loss):
\(\mathrm{Zn → Zn^{2+} + 2e^-}\)
\(\mathrm{Fe + 2e^- → Fe}\) (iron remains unoxidised)
Explanation:
- Zinc loses electrons more readily than iron — it is oxidised first.
- The electrons released flow to the iron, preventing it from oxidising (rusting).
- This protects even the exposed parts of the steel.
Key Points about Galvanising:
- Provides two layers of defence:
- Barrier protection: blocks air and water.
- Sacrificial protection: zinc corrodes instead of iron if exposed.
- Used to protect steel structures such as fences, car bodies, and ship hulls
Example :
Explain how zinc protects steel in galvanised iron pipes, even if the zinc coating is damaged.
▶️ Answer/Explanation
Step 1: The zinc coating acts as a barrier, preventing oxygen and water from reaching the iron surface.
Step 2: If the coating is scratched, zinc still provides sacrificial protection because it is more reactive than iron.
Step 3: Zinc oxidises to form Zn²⁺ ions (\(\mathrm{Zn → Zn^{2+} + 2e^-}\)) and releases electrons.
Step 4: The electrons flow to the iron, preventing its oxidation (rusting).
Final Answer: Zinc protects steel by both blocking air and water (barrier method) and by corroding in place of iron (sacrificial protection), since zinc is more reactive and loses electrons more easily.
Sacrificial Protection of Metals
Sacrificial protection is a method used to prevent rusting and corrosion of iron or steel.
- This is achieved by attaching a more reactive metal (such as zinc or magnesium) to the surface of the iron.
- The more reactive metal acts as a sacrificial metal because it corrodes (oxidises) instead of the iron.
Explanation in Terms of the Reactivity Series:
- Metals higher in the reactivity series are more easily oxidised — they lose electrons more readily.
- When a more reactive metal (e.g. zinc) is connected to a less reactive metal (e.g. iron), the more reactive metal becomes the sacrificial anode.
- It corrodes (oxidises) instead of the iron, protecting the iron from rusting.
Example: Protecting Iron with Zinc:
- Iron (Fe) is less reactive than zinc (Zn).
- When zinc is attached to iron (e.g. in galvanised steel or ship hulls), zinc reacts first with oxygen and water.
- This prevents the iron from reacting, even if the zinc coating is scratched or damaged.
\(\mathrm{Zn → Zn^{2+} + 2e^-}\)
Zinc loses electrons (oxidised) and acts as the sacrificial metal.
Result: The iron remains protected because the zinc is higher in the reactivity series and preferentially oxidises.
Explanation in Terms of Electron Loss:
Corrosion of metals involves oxidation — the loss of electrons.
- In sacrificial protection:
- The more reactive metal (zinc) loses electrons more easily → oxidises first.
- The electrons released flow to the less reactive metal (iron).
- This keeps the iron supplied with electrons, preventing it from oxidising (rusting).
Key Points:
- The more reactive metal (e.g. Zn or Mg) oxidises instead of iron.
- It acts as a sacrificial anode.
- The iron remains protected as long as the sacrificial metal is present.
Common examples:
- Galvanising: coating iron with zinc.
- Ship hulls and pipelines: magnesium or zinc blocks attached as sacrificial anodes.
Example :
Explain, in terms of the reactivity series and electron transfer, how zinc protects iron from rusting.
▶️ Answer/Explanation
Step 1: Zinc is more reactive than iron, meaning it loses electrons more readily.
Step 2: When zinc and iron are connected, zinc is oxidised to Zn²⁺ and releases electrons.
Step 3: The electrons flow to the iron, preventing it from oxidising (forming Fe²⁺).
Final Answer: Zinc acts as a sacrificial metal because it is higher in the reactivity series and loses electrons more easily, protecting iron from rusting by preventing its oxidation.