We have a CAPAC on our ship, cathodic protection. It does not use a sacraficial anode. It uses platinum electrodes located in 6 locations on the hull of the ship to give it a slightly positive charge. Platinum is used because it is very resistant to errosion caused by electrons leaving its surface (it isn't sacrificial). The positive charge prevents electrons from leaving the surface of the metal hull under water. When electrons leave the surface, they carry little bits of metal with them and that is what causes the corrosion. Instead, electrons from the water enter the hull of the ship. Too much of a positive charge will actually cause a build up of crud on the hull because the electrons carry crud with them onto the hull. Electrons will move into the hull anywhere there is missing paint. When current travels through water it, the water breaks up into oxygen and hydrogen. The hydrogen sometimes will form a bubble on the exposed area and can keep water off of the hull, further protecting it.

Cathodic protection works any time you have a metal suspended in an electrolyte. A steel hull in salt water, rebar in concrete and damp soil, water moving through a steel pipe, etc... But, I don't believe it will have any effect on something in the air like a car. The top side of my school's ship still rusts real bad.