[OAC_Sparky]

Actually, it's called "galvanic action".

Put on your science caps.

The reason why iron rusts is (IIRC) because iron (Fe) gives up electrons and ioniozes into Fe+ which then combines with oxygen (air) and hydroxide ions (OH-) in water (liquid or airborn) to create Fe(OH)^3 (rust). Take this and immerse it into a salt solution (an electrolyte), and it speeds the process.

This is "galvanic action", which is basically the same principle that batteries run on. This electron movement creates an electron flow from negative to positive (or conventional current flow from positive to negative).

The premise behind rust-inhibiting devices is to continually bombard the iron with electrons so that iron stays un-ionized (Fe) so it can't combine with the oxygen and hydrozide ions so rust doesn't form.

This is the kicker. In order for the electrons to flow, it needs to create a circuit; just like flicking a light switch turning off a light, you are breaking the circuit path stopping electron flow.

Well, in order for the electrons to flow in rust inhibitors, it has to create a path. But, where? Every sharp edge of steel in a car provides a jump-off point for free electrons. That's why once rust starts it accelerates because the "roughness" of rust is like tiny little peaks that leak electrons faster. So no matter what, there will be spots where these iron ions exist to form rust.

So ... what to do? Well, just like in high school, if you want to get laid, you ask out the "easy girl", the one that gives it up more readily than the others. Chemically, this is called a "sacrificial anode". What you do is introduce something in the circuit that gives up its electrons more readily than the iron. This can be magnesium, aluminum, or various other metals. That way, when it comes to oxidation it eats up the sacrificial anode (key word "sacrificed") which is like the easy girl and leaves the iron (the tease) alone.

Now, in order for the sacrificial anode to work best, it's always better to dip it into an electrolyte like say, salt water. THAT'S why it's extensively used in marine applications, because the ship (or boat) is always in contact with water. The sacrificial anodes need to be periodically replaced as they rot out. In many ships/boats you don't have to even electrify the hull because the galvanic reaction creates its own electricity which charges the hull.

In a car, though, you aren't constantly in contact with water, there is no sacrificial anode in the system that you replace. So while you may get a very small real benefit, the placebo effect is much greater.