Re: How does electronic rust protection work on automobiles?

While I have no direct experience with one of these systems I can comment
on some of the claims and explanations offered by the companies that make
and sell them.  First, as I’m sure you know, rusting is a very complex
process involving several different iron oxide and hydroxide compounds
formed in a variety of ways under a variety of conditions.  These reactions
do all however involve the loss of electrons from the iron as it is
oxidized.  What these systems all try to do is make a source of electrons
available in excess of those from the Iron alone.  They claim to do this by
causing the metallic parts of the car body to become the negative plate of
a charged capacitor.  This capacitor is then charged periodically as the
negative charge drains away to the air and ground around the car.

Because of the physics involved two things should be noted.  First, based
on the pictures I saw, only a fairly small capacitance is produced, which
mean only a very small charge is stored at any given time.  This is a good
thing since you really don’t want a large charge on a plate the size of a car
body for safety reasons.  Second, as at least one site correctly reports,
this results in a very uneven distribution of this small amount of charge
over the body of the car with a concentration of electrons at the sharper
edges. At least one explanation offered also gets into some strange
supposed alternating layers of opposite charges somehow progressing out
through the surroundings that is almost certainly incorrect. As it adds
nothing of any real importance to the argument physically or chemically I
believe you can safely ignore it in trying to understand what’s going on.

Given the nature of the chemistry involved all this would appear to only
slow the rate of rusting not stop or certainly not reverse it to any real
degree.  I can see no good physical or chemical reason to discount their
claim to slow rusting.  In fact a careful look at the literature on a
couple of sites reveals they actually claim no more than this.  Because of
the large number of variables involved however I know of no way to tell
other than by experiment how much the rate of rusting might be slowed for a
particular application. A person would already have to have a pretty good
idea how quickly their vehicle rusted and in exactly what spots, or better
have two “identical’ cars each driven and washed etc. the same way over a
season or two to tell if the one with or without had noticeably different
results.  Some of the customer testimonials out there claim to have done
some comparisons possibly like this with favorable results in industrial
applications. Asked another way perhaps, “Given the cost of these things
how does their price compare to keeping the car washed?” or maybe better, 
“Do they give significant improvement above that already achieved by
regular washing?”  I didn’t see enough data out there to really say.  So in
short, yes it’s possible these devices work to some degree as claimed. It’s
just hard to tell how well.