Reactivity Series
7 marks · HardIron structures are often exposed to damp air and water. Two common methods to prevent iron from rusting are galvanising and using sacrificial protection.
State the two conditions necessary for iron to rust.
Describe how galvanising protects iron from rusting. Include what metal is used and how it provides two types of protection.
Ships are often fitted with blocks of magnesium on their steel hulls. Explain how these magnesium blocks protect the iron hull from rusting.
Explain why the iron hull will eventually begin to rust if the magnesium blocks are not replaced.
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) Both oxygen (from air) and water must be present.
(b) Galvanising coats iron with zinc. The zinc acts as a barrier and, if scratched, still protects by sacrificing itself (zinc corrodes in preference to iron).
(c) Magnesium is more reactive than iron and corrodes (oxidises) in preference to iron, preventing the iron from rusting.
(d) If the magnesium is all used up (oxidised away), there is no more sacrificial protection so the iron is exposed and will rust.
Common mistakes: naming aluminium instead of magnesium or zinc; saying air instead of oxygen alone for rusting; not mentioning loss of protection when sacrificial anode is used up
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States both oxygen and water are required for rusting.
B2 (b) · 1 mark · Identifies zinc as the metal used and as a physical barrier.
B3 (b) · 1 mark · Explains that zinc continues to protect even if scratched (sacrificial protection).
B4 (c) · 1 mark · Explains magnesium is more reactive than iron and corrodes first.
B5 (c) · 1 mark · Describes that magnesium's corrosion prevents the iron hull from rusting (sacrificial protection explanation).
B6 (d) · 1 mark · States that if magnesium is used up, it can no longer protect the iron.
B7 (d) · 1 mark · Explains iron rusts if no sacrificial metal remains.
Worked solution
a) For rusting to occur, both oxygen and water must be present.
b) Galvanising protects iron by coating it with zinc. First, the zinc barrier keeps oxygen and water away from the iron. If the coating is scratched, zinc is more reactive and corrodes (oxidises) in preference to the iron, thus protecting iron even at the scratched area.
c) Magnesium blocks act as sacrificial anodes on steel hulls because magnesium is more reactive than iron. The magnesium corrodes (oxidises) instead of the iron, which prevents the iron from rusting.
d) Once the magnesium blocks have completely corroded and are used up, there is no more protective action. The iron hull is now unprotected and is exposed to rusting in the presence of oxygen and water.