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Name:  Andras
Status: student
Grade: 9-12
Location:N/A
Country: N/A
Date: 8/28/2005


Question:
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 solution.


Replies:
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.

3. Oxygen.

In addition,

4. pH,

5. Salt (usually NaCl), and

6. Temperature.

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.

Vince Calder



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