Preventing rusting by galvanising
2 marks · HardA steel beam is coated with zinc to prevent rusting. Explain why salty water increases the rate of rusting if the zinc coating is damaged.
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Also accept: Salt increases electrical conductivity, so rusting is faster; zinc still protects iron but corrodes more quickly in salty water.
Common mistakes: Not mentioning increased conductivity or corrosion rate; Just saying it rusts faster without explanation
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Marking points
B1 · 1 mark · States salt increases the rate of rusting by increasing electrical conductivity (more ions present).
B2 · 1 mark · Explains zinc still protects iron but is used up faster in salty water.
Worked solution
Salty water contains ions that make it a better conductor, which speeds up the corrosion reactions. As a result, zinc will corrode more quickly, so the protection for the iron is reduced more rapidly.