Not a corrosion engineer here, but enough EE to pretend I know what I'm talking about, even though I don't really understand this device:
This electrical gadget-doodad thing is supposed to hook up to the battery to provide electrons for corrosion resistance, or some claim like that. This just strikes me as moving the corrosion to the inside of your battery. I don't know if that'* actually the case or not, but if so, that'* arguably worse, depending on how much actual rust prevention it gives you -vs- how many batteries it eats. Besides I think this happens naturally, I've had my positive battery cables corrode in every car I've owned, and I think that'* caused by the same reaction.
Yes, your hot water heater definitely has a sacrificial anode in it, and yes, they are typically zinc, magnesium, or aluminum. When it corrodes away, your tank rusts, leaks, and then you need a new hot water heater. According to my professional plumber buddy, it'* not worth the time or expense to replace the anode (although you can buy them for under $100), because by the time they go, you probably want to get a new water heater anyway. Although I can think of some situtations where the cheaper choice would be more desireable.
The other problem I have conceptually with this device is that it'* supposedly using the battery as a "source of electrons" to protect the body. But wait, isn't the body already physically connected to the anode of the battery? How then is *any* other electrical connection to the body possible? All connections to the body would be referenced to the negative pole of the car battery. I don't get it.
A lot of people keep saying that tires are insulators. To some extent they are not. They have a lot of carbon in them, and this is used to earth-ground a car so that a car does not build up a giant static electricity charge on the body and shock you. (Ignoring my wife'* old 1972 AMC Gremlin that would zap anyone who touched it.)
Also, there'* a fair number of reactive metals other than steel used in cars. A lot of things are made of cast aluminum, and these are known to corrode and pit as they act precicely as a sacrificial anode. There'* even a company that sells a magnesium sacrificial anode built into a radiator cap to do the same thing, for under $25. Check it out here:
These guys sell it and claim that electrolytic corrosion is a leading cause of mechanical breakdowns. (Despite my having never *EVER* heard of a mechanical breakdown caused by this.)
I'd personally be very skeptical that anything like this really works as an effective corrosion preventative. We definitely need Myth Busters!
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.