Corrosion and Salt Concentration
My experiment is to find out in what conditions does
rusting occurs the fastest. Why does 7 grams of dissolved salt in water
make a mild steel nail rust faster than a higher concentration of salt?
For example, 7 grams of salt solution forms more rust than a 21gram salt
Good Experiment!! Corrosion is a complex electrochemical reaction, that is
it requires the transfer of electrons from one electrode to another. In
the case of corrosion it may be different parts of the same piece of
metal. Corrosion requires 3 necessary components, and 3 additional factors
in most cases:
1. Two metals, or compositional non-uniformity in the same
piece of metal (almost unavoidable in practice).
2. Water (either liquid or even moisture is adequate, to provide a medium
for the transfer of electrons.
5. Salt (usually NaCl), and
In your experiment, the metal, the temperature, the pH, and
the water are pretty much fixed. What is varying, and is probably the
dominant variable is the solubility of oxygen in the water. The solubility
of oxygen in water decreases rapidly with increasing salt concentration.
See the web site: http://www.corrosion-doctors.org/Seawater/Oxygen.htm for
example. Remove one of the first three factors and corrosion will be
reduced significantly. I suspect that the decreased solubility of oxygen
with increasing salt concentration is the reason for the reduced rate of
corrosion you observe. The low amount of oxygen in deep ocean water is the
reason that ships remain more or less intact despite the obvious presence
of water and salt.
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Update: June 2012