Fig. 1 shows a cross-section through a galvanised steel sheet that has been scratched through to the metal. Describe why the steel does not rust at the scra...

Assessment: Design & Technology: Product Design 9252 | Paper 1 Mock 01 | Technical, designing and making principles Subject: Design & Technology: Product Design - 9252

Question 1 Report

9252-p1-surface-galvanised-scratch-1

Fig. 1 shows a cross-section through a galvanised steel sheet that has been scratched through to the metal. Describe why the steel does not rust at the scratch. [4]

Answer Details

Zinc is more reactive than iron, so where moisture is present the zinc corrodes in preference to the steel.

This is sacrificial protection: the zinc behaves as the anode and the exposed steel as the cathode, so the steel is protected even where the coating has been broken and bare metal is showing. This is the key difference from paint, which protects only by keeping moisture out and therefore fails as soon as it is scratched.

The protection continues until the surrounding zinc has been consumed, which is why a galvanised sheet keeps working long after it is first damaged.

Read lesson note on Surface Treatments And Finishes (OxfordAQA)
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