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We did an experiment in school where we put a steel nail in a solution consisting of:

  1. 1 g agar;
  2. 100 ml water;
  3. 100 ml indicator solution (which gets blue whenever $\ce{Fe^2+}$ is created and red when $\ce{OH-}$ develops).

We let it rest for a few days and this is what happened to the nail:

photo of corroded nail in agar

My hypothesis was that the corrosion would create two ends of the nail where reduction takes place in one end and oxidation on the other end. Why isn't this the case?

My teacher gave us a hint and told us it could have something to do with both:

  1. The length of the nail;
  2. The ion concentration of solution.
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    $\begingroup$ Cool observation! The two ends of your nail are pretty much identical, chemically. If anything, the middle is different from the ends. As you have seen. Can you imagine what that might be? Think about how (in what steps) a nail is made! $\endgroup$
    – Karl
    Apr 13 '20 at 16:22
  • $\begingroup$ Is the nail surface uniform? Before putting it in the agar, did you completely remove contaminants from the nail's surface? $\endgroup$ Apr 14 '20 at 0:34
  • $\begingroup$ @Karl They're the same right? What am I missing here? $\endgroup$
    – kostberg
    Apr 14 '20 at 9:08
  • $\begingroup$ @DrMoishePippik no we did not $\endgroup$
    – kostberg
    Apr 14 '20 at 9:09
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Very good question and thought provoking too. It appears that the nail somehow knows or is it is able to decide which part will be a cathode and which part will be an anode! Why is that so? Why one end was not a cathode and the other end an anode, just the one shown in the cartoon taken from here Imaging metal corrosion

rust nail

So the fact you should keep in mind is that corrosion is superficially a random process just like cancer. Corrosion comes from corrodere "to gnaw to bits, wear away," it can start from any defect on the metal (scratch, impurity, etc.). However once it starts, just like cancer, it decides its propagation (i.e., the flow of electrons), hence the direction of cathode or an anode. It is likely that in your case the nail tip and the head had more defects than the center, so corrosion started at the tips. The pattern you are seeing is not universal or a fundamentally symmetric phenomenon.

Let us now bend a nail, create stress or defect in the center. Now the corrosion also starts in the center. Have a look at this gallery. Corrosion gallery rusty nails

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