Reactivity Series

7 marks · Hard

Iron water tanks and pipelines are often protected from rusting using different methods. One way is to coat the iron with a layer of zinc (galvanising). Another way is to attach a block of a more reactive metal to the iron (sacrificial protection).

(a)

Explain how galvanising protects iron from rusting, and why this method is more effective than simply painting the iron.

[3 marks]

(b)

A buried iron pipeline is protected by attaching solid blocks of a more reactive metal. Using the result of part (a), explain why these 'sacrificial anode' blocks must be replaced regularly. In your answer, explain why the metal used for sacrificial protection must be more reactive than iron.

[4 marks]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) — · must show Correct link that zinc is more reactive than iron or corrodes first, Zinc provides a physical barrier (mention of barrier), If barrier is damaged, zinc still protects iron by sacrificial action, unlike paint/coating where rusting starts if damaged.

(b) — · must show Sacrificial anodes react and are consumed over time, so must be replaced., Explanation that sacrificial metal must be more reactive than iron to oxidise in preference to iron (lose electrons first)., Link to previous part: protection continues even if coating is damaged because the more reactive metal corrodes in place of iron., Clear statement that if less reactive, the iron would rust instead.

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · States that galvanising protects iron by coating it with a layer of zinc (barrier to air and water).

M2 (a) · 1 mark · Explains that if zinc coating is scratched, zinc (being more reactive) still protects the iron by corroding in its place (sacrificial action).

A1 (a) · 1 mark · Explains why this is more effective than paint (if paint is scratched, iron rusts rapidly at exposed areas; zinc continues to protect the iron even if damaged).

M3 (b) · 1 mark · States that sacrificial anode blocks are gradually used up (corrode/oxidise/disappear) and must be replaced.

M4 (b) · 1 mark · Explains that the sacrificial metal must be more reactive than iron to oxidise in preference to iron (reacts/corrodes first).

A2 (b) · 1 mark · Explains the need for the metal to be more reactive (if a less reactive metal is used, protection will not occur and iron will rust instead).

M5 (b) · 1 mark · Uses the connection from (a) to explain how this protection works even if the protective block is eroded or damaged (linking sacrificial to galvanising principle). (follow-through)

Worked solution

a) Galvanising protects iron by coating it with a layer of zinc. This zinc acts as a barrier, keeping water and oxygen away from the iron so rust cannot form. If the zinc layer is damaged, the iron is still protected because zinc is more reactive than iron. The zinc corrodes in preference to the iron (sacrificially), preventing rusting even where the coating is scratched. In contrast, if paint is scratched, the iron underneath rusts quickly.

b) The solid blocks attached to the iron pipeline for sacrificial protection are made from a metal that is more reactive than iron, such as zinc or magnesium. These blocks react (are oxidised) in preference to the iron, providing continuous protection even if some parts of the pipeline are exposed to air or water. Over time, the blocks are gradually used up (corroded away), so they must be replaced regularly to maintain protection. If a metal less reactive than iron were used, it would not oxidise first and the iron would rust instead. This method works on the same principle as galvanising, where the more reactive metal sacrifices itself to protect the iron.