Some base metals become relatively resistant to corrosion by formation of a dense surface film of oxide. The protective oxide films are mostly very thin, however, so that they can only resist very aggressive chemicals for a limited time. The stability of such metals can be increased by an artificial strengthening of the oxide film, a process known as passification.
Two different procedures are used in this experiment:
1. Passification of the metal by a brief action of concentrated nitric acid on its surface
2. Anodic oxidation of the metal, carried out by hanging it as anode in an electrolysis bath containing dilute sulphuric acid, whereby the nascent oxygen which is evolved strengthens the oxide film. The Eloxal procedure for treating the surface of aluminium has made this procedure particularly well known.
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- Principle of corrosion protection
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