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Iron's oxide scale is only weakly protective because it is

  • A.porous and cracks on cooling
  • B.volatile at ordinary temperatures
  • C.of smaller volume than the iron consumed
  • D.soluble in the surrounding atmosphere

Answer: A

The three oxides formed have different volumes and expansion coefficients, so the scale is stratified and spalls away as the temperature changes. Fresh metal is exposed at every cycle, which is why mill scale is removed rather than relied upon.

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