Ask A Scientist Chemistry Archive

name         Joyce C.
status       student
age          15

Question -   Why does the presence of ionic substance speed up the
rusting process?
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As a guess, perhaps it makes it easier for charge to move from
one place to another. Electrons carry charge through metals,
but ions carry charge through solutions.

Robert Q. Topper
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This question has a pretty complicated answer. Ions play several roles.
Since corrosion is an electrochemical process, it requires electrical
conductivity. Pure water is a poor conductor, so ions increase the
conductivity dramatically. Some ions change the pH of the solution (e.g.
NH4(+1)) and that has a significant effect on corrosion, other ions have a
catalytic effect. The classical example is Cl(-1). The web site below gives a
rather complete discussion of corrosion of iron and other metals.
http://www.roscoemoss.com/tech_manuals/fmcf/fmcf.pdf

Vince Calder
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Joyce,

In order for iron to oxidize and form iron oxide (rust), iron atoms must
surrender electrons to oxygen atoms. In the presence of dry oxygen, dry iron
rusts very slowly. However, water contaminated with salt (or any other water
soluble ionic compound) facilitates rusting because the conductive solution
allows electrons to freely flow through it. Thus enabled by the solution,
electrons easily move from iron to oxygen and rusting occurs readily.

By the way, a system of dry iron in the presence of dry oxygen and dry 
(soluble)
ionic compounds shows very little reaction. Nevertheless, if a tiny trace of
water enters the system, the water dissolves the ions, the resulting
solution enables free flow of electrons, and rusting really takes off.

Regards,
ProfHoff 516
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