Discussion
Consider the following statements about corrosion.
1. A scratch in galvanised steel does not rust because zinc is anodic to iron 2. An oxide of smaller volume than the metal consumed is protective 3. Crevice corrosion arises from oxygen depletion in the gap
Answer: A
Correct answer: 1 and 3 only are correct
Statements 1 and 3 are correct: zinc is anodic to iron and protects a scratch sacrificially, and crevice corrosion is a differential aeration effect driven by oxygen depletion in the gap. Statement 2 is inverted - an oxide of smaller volume than the metal it replaced cannot cover the surface and cracks, as magnesium’s does. A protective oxide needs a Pilling–Bedworth ratio slightly greater than one.
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