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Cathodic Protection: Sacrificial Anode Principle

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Cathodic protection connects a metal needing protection to a more easily oxidized metal, so the second corrodes preferentially as sacrificial anode, the protected metal staying cathode.

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Zinc blocks on a ship's steel hull corrode in the steel's place, since zinc has a more negative reduction potential.

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A sacrificial anode must oxidize more easily than what it protects, or it wastes uselessly.