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:
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.
Sacrificial Protection
Sacrificial protection is a method of preventing rusting in which a more reactive metal is placed in contact with iron or steel. The more reactive metal corrodes (oxidises) instead of the iron, thereby protecting it.
Explanation in Terms of Reactivity Series:
- Metals higher in the reactivity series (such as zinc or magnesium) lose electrons more readily than iron.
- When these reactive metals are attached to iron or steel, they preferentially oxidise and corrode, preventing the iron from rusting.
- Even if the iron surface is scratched or exposed, the more reactive metal continues to protect it by sacrificing itself.
Explanation in Terms of Electron Loss:
- The sacrificial metal (e.g., zinc) loses electrons to form positive ions.
- These electrons flow to the iron, preventing the iron atoms from losing their own electrons and forming iron ions.
- As a result, the iron does not undergo oxidation and rusting is prevented.
Example
Why are magnesium blocks attached to ships’ hulls?
▶️Answer/Explanation
In seawater, magnesium (more reactive than iron) corrodes instead of the steel hull. The magnesium gradually wears away, but the hull remains protected. The magnesium blocks must be replaced periodically.
Use of Zinc in Galvanising Steel
Galvanising is the process of coating steel or iron with a thin layer of zinc. This provides protection against rusting in two ways:
- Barrier Method: The zinc layer forms a physical barrier that prevents oxygen and water from reaching the steel surface.
- Sacrificial Protection: Even if the zinc coating is scratched and the steel is exposed, zinc is more reactive than iron (higher in the reactivity series). Zinc will corrode (oxidise) first, protecting the underlying steel.
This dual protection makes galvanising a very effective method to prevent rusting, especially for structures exposed to moisture, such as car bodies, buckets, or fences.
Example
Why is steel bucket galvanised?
▶️Answer/Explanation
The zinc coating prevents oxygen and water from reaching the iron. If scratched, zinc corrodes first instead of iron, keeping the steel bucket rust-free for longer.
Example
Why are car body parts galvanised?
▶️Answer/Explanation
Cars often use galvanised steel panels. The zinc layer protects against rusting due to rain and road salt. Even if the coating is damaged, zinc undergoes corrosion first, protecting the car’s steel body underneath.