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I have conducted an experiment where I test the affect of $\ce{NaCl}$ concentration on corrosion rate of iron. I did this in an environment were all of the iron was submerged in water and the water was not aerated. The results I got are shown in the graph below.

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Are there any research papers that could support or neglect my data? Can anyone explain this relationship? Is 1M just an outlier?

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    $\begingroup$ Run one more experiment, this time at 0.9 or maybe at 1.2. One data point can be a meaningful trend, or it can be an outlier, you can never tell. Well, if the experiment actually takes years to run, then things become tricky. $\endgroup$
    – Ivan Neretin
    Commented Feb 8, 2016 at 14:52
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    $\begingroup$ > water was not aerated || in this case the corrosion is attributed solely to dissolved oxygen. Solubility of oxygen decreases with salt concentraion AFAIK, while chlorine ions promote corrosion of iron. $\endgroup$
    – permeakra
    Commented Feb 8, 2016 at 17:20
  • $\begingroup$ Are you completely sure that that point does not correspond to 0 mol/L? It would fit better there, if still a bit low. And it is an obvious point to include in the trials. If so, the interpretation would be that with a very small amount of salt the corrosion increases a lot, and with higher quantities, the corrosion still increases but not so fast. $\endgroup$
    – Raoul Kessels
    Commented Feb 6, 2018 at 17:16
  • $\begingroup$ 100% sure that the point corresponds to 1 mol/L, as couple of trials were conducted $\endgroup$
    – kyczawon
    Commented Feb 10, 2018 at 15:27

3 Answers 3

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The involvement of water accounts for the fact that rusting occurs much more rapidly in moist conditions as compared to a dry environment such as a desert. Many other factors affect the rate of corrosion. For example the presence of salt greatly enhances the rusting of metals. This is due to the fact that the dissolved salt increases the conductivity of the aqueous solution formed at the surface of the metal and enhances the rate of electrochemical corrosion. This is one reason why iron or steel tend to corrode much more quickly when exposed to salt (such as that used to melt snow or ice on roads) or moist salty air near the ocean.

Water is the enabler of fast oxidation of iron so freshwater will also cause rust. However, salt water is a very good conductor (lots of dissociated ions) and so there are a number of electrolysis reactions that tremendously accelerate corrosion in salt water. For example if you have iron in contact with salt water and also in contact with another metal such as aluminum (also in contact with the water) you effectively get a battery which drives very fast corrosion processes. This effect can be reversed by using a metal (like zinc) which causes the current to be reversed and in effect the zinc corrodes rapidly, protecting the iron. This is the principle of a 'Cal-rod' which is used to slow the rusting of hot water tanks

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Corrosion of iron requires oxygen which is a non-polar molecule. If you add salt to water you increase the mixture's polarity making it more difficult for oxygen to dissolve in it. This may contribute to the lack of corrosion in your sample. I can tell you for certain though that this is NOT an outlier. This observation is very common that corrosion is inhibited at higher salt concentrations.

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The comments and other answers suggest that there is more than one variable affecting corrosion. Concentration of salt, aeration, buildup of corrosion product (stifling), oxygen solubility, stirring/liquid mobility...

A possible conclusion could be "Over the range from 0.2 to 0.8, the corrosion rate is affected proportionately by NaCl concentration..." and do another set of experiments over another range.

Note that the slope of corrosion rate does not intersect zero at zero concentration, indicating that the iron corrodes even without NaCl.

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