SS3.4 Why does steel rust?

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Steel rusts because it reacts with oxygen and water. This video demonstrates the chemical changes taking place during rusting. Ferroxyl indicator is a mixture of Phenolphthalein in a sodium chloride solution and potassium hexacyanoferrate(III). It produces a blue colour with Fe(III) ions and a pink colour with hydroxide ions. Ferroxyl indicator is placed in a large droplet on a piece of polished steel sheet.

The central area of the steel turns blue due to the oxidation of iron in the centre of the droplet where there is less oxygen. The outer edges turn pink where reduction of the oxygen is taking place where dissolved oxygen is plentiful.

Right click here and open a new window to view a PowerPoint presentation summarising the rusting process based on a PowerPoint from Lesley Johnson with additional photographs added.

For further information see Salters Advanced Chemistry, 2000, Activities and Assessment Pack, Second Edition, ISBN 0 435 631217