Ask A Scientist Chemistry Archive

name         Akshen J.
status       student
age          16

Question -   How does the combination of salt and sun eat away car paint, and does the heat from the sun act as a catalyst? Thank you!
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The combined degradation caused by SALT, SUN (along with WATER, TEMPERATURE, and OXYGEN) is 
called "weathering". The collective action of these four components is very complicated, and 
is a topic of ongoing research and development in every industry that puts "some--thing" 
outside. Each of these factors affect the interfaces: atmosphere/coating(s), 
atmosphere/substrate, coating(s)/substrate. Oxygen, light (especially ultraviolet), and water 
cause chemical reactions in the paint that destroy the chemical bonds holding the protective 
coating together. Oxygen, water, and salt cause oxidation of the steel components of an 
automobile. Temperature cycling causes a differential expansion and contraction of the substrate 
and the coating, behaving like a thermal "chisel" delaminating the substrate and coating.
The general strategy of corrosion prevention is to minimize one or more of these five factors 
(and/or) their combined interactions. Inhibiting even one of these factors can have a dramatic 
effect. The whole problem is far too complicated to go into much detail but one example is: In 
the desert southwest you find many more "vintage" autos than in the northeast and upper midwest 
because the factors of salt, and to some extent water, have been removed. Oxygen and high 
temperature, in the absence of the other factors,
are not so important.

Vince Calder
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Ashken,

You have addressed a popular myth. "the combination of salt and sun (does not) eat away (intact) 
car paint."

Modern automobile paints are very resistant to such effects and they are certainly not damaged 
by salt. Automotive paints are a kind of very tough plastic. You can demonstrate their 
resistance to salt by piling some dampened salt on a piece of plastic and then observing it over 
a period of months. You will see that the salt did not damage the plastic.

Have you ever seen the TV infomercial for a "miracle car polish" wherein hydrochloric acid is 
poured on the hood of the car and then wiped off showing no damage to the paint? The commercial 
is a con-job inasmuch as the acid would not have harmed the paint even in the absence of the 
"miracle polish." Car paints are themselves are not attacked by hydrochloric acid.

Car paints are, however, very sensitive to damage by the sun. The energy of sunlight is 
sufficient to fade the pigments and degrade the polymer structure of the paint. In time, if 
the car's finish is not protected, it is surface will become roughened. If the roughening is 
allowed to penetrate all they way through to the underlying metal, a rusting disaster is the 
result. Thus rock chips, scratches, and minor dings that penetrate the paint are the start of 
rusting catastrophes.

Then, the effects of salt become apparent. Salt dissolved in water is an excellent conductor of 
electricity. In order for the underlying steel (iron) to rust, electrons must flow from the iron 
atoms in the steel to oxygen from the air. Iron plus oxygen makes iron oxide (rust). When this 
process is facilitated by an electrically conductive salt solution, rusting occurs rapidly.

And so it is, that one must keep the car's finish clean and covered with an overcoating of a 
good quality car wax (preserver) that is non-abrasive. Remember, we do not want to allow the 
surface to be come roughened.

Regards,
ProfHoff 648
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