Preventing rusting by galvanising
5 marks · HardThe diagram shows an iron pipe exposed outdoors. The owner wants to protect it from rusting using galvanising.
Describe the process of galvanising as a method of preventing rusting.
Explain why the pipe does not rust even if the zinc coating is damaged in places.
State one other method, apart from galvanising, that could be used to prevent the pipe from rusting, and explain how it works.
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.
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(a) Iron is coated with a layer of zinc to protect it from air and water.
(b) Zinc is more reactive than iron, so it is oxidised in preference to iron, preventing rusting even if scratched (sacrificial protection).
(c) Painting, oiling, greasing, or plastic coating; these create a barrier to keep water and oxygen away from the iron surface.
Common mistakes: Missing sacrificial explanation for zinc; omission of both oxygen and water for rusting; confusing barrier and sacrificial methods.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that iron is coated with zinc.
B2 (a) · 1 mark · Describes galvanising as a barrier to air and water.
B3 (b) · 1 mark · Explains zinc is more reactive so it corrodes before iron (sacrificial protection).
B4 (b) · 1 mark · States iron is protected even when zinc coating is damaged.
B5 (c) · 1 mark · Names another reasonable barrier method and explains it prevents water/oxygen contact with iron.
Worked solution
a) Galvanising involves coating the iron pipe with a layer of zinc. The zinc coating acts as a barrier and prevents oxygen and water from reaching the iron surface.
b) If the zinc coating is scratched, zinc still protects the iron because it is more reactive than iron and will corrode (oxidise) first. This is called sacrificial protection.
c) Another method is painting the pipe. Paint acts as a barrier, preventing water and oxygen from reaching the iron.