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The Pilling-Bedworth rule states that an oxide film is protective when the volume of oxide formed is

  • A.exactly equal to the volume of metal consumed
  • B.independent of the volume of metal consumed
  • C.greater than the volume of metal consumed
  • D.less than the volume of metal consumed

Answer: C

An oxide of larger volume covers the surface completely and remains under compression, whereas one of smaller volume cannot cover what it replaced and leaves a porous layer. Magnesium and the alkali metals fail the rule, which is why their films offer no protection.

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