Rusting and Prevention Exam Questions

• Water / moisture [1]
• Oxygen / air [1]

Examiner note: Both must be present simultaneously. Iron in dry air does not rust. Iron in boiled (deoxygenated) water does not rust. Only when both water AND oxygen are present does rusting occur. This is the foundation of all rust prevention methods.

• Tube A: rusts — both water and oxygen are present [1]
• Tube B: does not rust — the boiled water has no dissolved oxygen, and the oil prevents oxygen from reaching the water [1]
• Tube C: does not rust — there is no water present (the desiccant keeps the air dry) [1]
• Tube D: rusts — and faster than A, because salt water is a better electrolyte and speeds up the electrochemical rusting process [1]

Examiner note: This is a classic controlled experiment. Tubes B and C are the controls showing that removing either water or oxygen prevents rust. Tube D shows that salt accelerates rusting — important for coastal environments and road salt.

Any four from:

• Painting [1]
• Oiling / greasing [1]
• Galvanising (coating with zinc) [1]
• Electroplating with a less reactive metal (e.g. chromium, tin) [1]
• Plastic coating [1]
• Using stainless steel (alloying with chromium and nickel) [1]
• Sacrificial protection (attaching blocks of a more reactive metal such as zinc or magnesium) [1]

Examiner note: All methods work by either creating a barrier (keeping water and oxygen away from the iron) or by sacrificial protection (a more reactive metal corrodes instead of the iron). Name specific methods, not just “cover it up.”

• The zinc coating acts as a physical barrier, preventing water and oxygen from reaching the iron surface [1]
• Even if the zinc coating is scratched and the iron is exposed, the zinc still protects the iron by sacrificial protection [1]
• Zinc is more reactive than iron, so it is oxidised/corrodes preferentially instead of the iron [1]

Examiner note: Galvanising provides dual protection — barrier AND sacrificial. This is why it is more effective than tin plating: if tin plating is scratched, the iron rusts faster because tin is less reactive. If zinc coating is scratched, the zinc still protects the iron.

• Blocks of a more reactive metal (such as zinc or magnesium) are attached to the steel pipeline [1]
• The more reactive metal is oxidised / corrodes instead of the iron in the steel [1]
• The more reactive metal acts as the negative electrode / anode in an electrochemical cell, donating electrons to the iron and preventing it from being oxidised [1]

Examiner note: Magnesium is commonly used for underground and underwater pipes because it is more reactive than zinc. The blocks must be replaced periodically as they corrode away. Ships’ hulls also use zinc blocks for the same reason.

• 4Fe + 3O2 + 2xH2O → 2Fe2O3.xH2O [1] (simplified: iron + oxygen + water → hydrated iron(III) oxide)
• The product is hydrated iron(III) oxide, commonly called rust [1]
• Rust is a brown, flaky solid that does not protect the underlying iron, allowing rusting to continue [1]

Examiner note: The “x” in the formula represents a variable amount of water — rust is not a precise compound. Unlike aluminium oxide (which forms a tight, protective layer), rust is porous and flaky, so it keeps exposing fresh iron to air and water.

• Aluminium reacts with oxygen in the air to form a thin layer of aluminium oxide (Al2O3) on its surface [1]
• This oxide layer is tough, unreactive, and strongly adhered to the surface [1]
• It acts as a barrier preventing further reaction of the aluminium underneath with water and oxygen [1]

Examiner note: Iron oxide (rust) is porous and flaky, so it does not protect the metal. Aluminium oxide is dense and adherent, so it does protect. This is why aluminium appears unreactive despite being high in the reactivity series.