Ask A Scientist© General Science Archive

name         David
status       other
age          50s

Question -   While driving in the winter in Kentucky, an auto gets it
underside covered with salt and other snow melting corrosive solutions.
Question, after washing under the auto what can be sprayed on the bottom
of the auto to neutralize these solutions? No matter how well the car is
washed corrosive solutions are traped in the cracks and crevices? Will
any of the following mixed with water do the job: borax, vinigar, baking
soda? What would you recommend for the best rust prevention. We get tons
of salt and calicum scattered on our roads each winter.

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David,

You have raised an issue as nettlesome as death and taxes -- all inescapable.
Simply put, there is no practical way to prevent salt from encouraging your
car to rust out. Your idea to was the underside of the car as best you can is
a good idea even if you can't rinse out all the nooks and crannies. Also
beware that some caw washes use recycled water. Imagine the damage that can do
when your well-intentioned efforts actually make things worse. If you use a
car wash, ask the operator if they use recycled water.

None of the things you list will neutralize the salt or its effects. In fact,
all of them would be almost as bad as salt. Actually, salt (both sodium and
calcium chloride) simply makes the water better able to conduct an electric
current. On order for iron to oxidize and form iron oxide (rust), it must
surrender electrons to oxygen. Dry iron in the presence of oxygen oxidizes
very slowly. However, if salty water is present, electrons can flow through
the solution from the iron to the oxygen and rusting can really take off.

Unlike salt water, a  non-ionic de-icer -- like the ethylene glycol antifreeze
in your radiator -- would not encourage rusting because the glycol is a
non-conductor. However, economic constraints and other factors make such a
de-icer impractical for use on roads.

I have always wondered why auto manufacturers don't use sacrificial
(magnesium) anodes like those attached to the steel hulls of some ships.
Magnesium oxidizes more easily than iron. When the two metals are in
electrical contact, any oxidation that occurs happens on the magnesium rather
than on the steel.

I have a satellite dish that is bolted to a concrete pier. I have successfully
prevented the metalwork from rusting by electrically connecting the structure
to a buried piece of magnesium salvaged from an automobile engine. The system
has been in place for four years and there is no trace of rust on it anywhere.

I hope you find at least some of my response to be helpful.

Regards,
ProfHoff
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