Electronic rust inhibitor

That's one of the problems with these - they don't actually create a charge on anything, positive or negative.

I know somebody who actually tested a number of these units at the University of Akron and they just didn't do anything at all.

Most of these consist of a connection to the ground terminal of the battery (or other chassis ground), then they periodically and repeatedly charge a small capacitor and try to discharge it into a couple of graphite pucks that they glue to the painted metal of your car body.

You get very little charge on the car body at all, and it never extends more than a fraction of a mm beyond the pucks. Some of these devices didn't produce any charge at all, and simply blinked an LED so you'd think it was doing something.

My 92 Corolla Wagon has never had its gas tank replaced.