Galvanic sacrificial anode attached to the hull of a ship; here, the sacrificial anode shows corrosion but the metal it is attached to does not. The anode, a piece of a more electrochemically "active" metal, is attached to the vulnerable metal surface where it is exposed to an electrolyte; the potential of the vulnerable surface is polarized to be more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction with the protected surface is removed. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced, but the cathodic material is protected.
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