Preventing rusting by galvanising
3 marks · NormalExplain why covering iron with a plastic coat is less effective at preventing rust than galvanising, especially if the plastic is damaged.
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
If plastic coating is damaged, iron is exposed to water and air and rusts. In galvanising, even if the zinc layer is scratched, the zinc corrodes first, protecting the iron due to sacrificial protection.
Also accept: Plastic just acts as barrier; when broken, iron can rust; zinc protects by being more reactive.
Common mistakes: Just states plastic is a barrier.; Does not explain sacrificial action of zinc.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Mentions that plastic is a barrier and if damaged, iron is exposed and will rust.
M2 · 1 mark · States that in galvanising, the zinc protects the iron even if coating is scratched (sacrificial protection).
A1 · 1 mark · Clearly links the above points in a logical comparison.
Worked solution
A plastic coating only acts as a barrier to keep water and air away from iron. If the plastic is scratched, the exposed iron rusts quickly. In contrast, with galvanising, zinc is more reactive than iron, so if the zinc layer is scratched, it will corrode instead of iron. Thus, galvanising protects iron even if the coating is damaged.