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I have recently done an experiment for my chemistry EEI (Year 12) where we are testing what factors effect the rate of corrosion. I used vinegar (acetic acid) as one of the acids and Hydrochloric acid as the other.

In the one with the HCl, the results were as expected – the area exposed to the acid will corrode, the unexposed part won't (in the time frame I did).

However, for the vinegar, the results were different. The part exposed to the vinegar didn't show any rust however the part of it that wasn't exposed had a brown coating. Now I understand that the vinegar reacts with the rust, hence the reason we cant see it on the iron nail, but my question is: What is the brown coating on the top of the nail, and what is it caused from? Was there some sort of gas from the vinegar that caused it? If so, what is this gas called and how does it cause corrosion?

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    $\begingroup$ It could be simply that the moisture in the air rusted the unexposed part ... Though to confirm it you'll probably need to do more experiments..... $\endgroup$
    – Kartik
    May 16, 2017 at 8:30
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    $\begingroup$ They were in a container with a lid. So i feel like this isn't the case. Especially since it didn't do it to any of the ones. (HCl, water, NaOH). I don't have time to do more tests because the final is due on Friday. hmmmm. Thanks anyway $\endgroup$ May 16, 2017 at 23:57
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    $\begingroup$ Do you have an images? A Visual aid could make identification easier. $\endgroup$ May 21, 2017 at 13:22

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The nail is in two solutions. The lower solution is 5% acetic acid, saturated with air (20% $O_2$, 80% $N_2$). At 0.2 atmospheres, $O_2$ solubility in water is 40 mg/L x 0.2 = 8 mg/L, or about 8 parts per million.

The upper solution is a thin film on the nail (perhaps 10 microns thick - or less!) of $H_2O$ with some acetic acid (probably about 3-4%) and 8 ppm $O_2$. This film may have deposited from the lower solution by vapor deposition due to temperature differences, or by capillary action, as the meniscus crept up in a thin layer.

In the lower solution, $Fe^0$ goes into solution as $Fe^{++}$ and electrons go to $O_2$.

4$e^-$ + $O_2$ + 2 $H_2O$ --> 4 $OH^-$

The 8 ppm $O_2$ in the lower solution doesn't go very far, but the meniscus, and any film going higher up, will be continually refreshed with $O_2$, and will develop a higher pH and will precipitate iron hydroxide. Any iron dissolving in the lower solution will stay in solution because of the acidity, and the nail may not even show obvious signs of corrosion damage like pitting. The only obvious sign is the brown film of precipitated iron hydroxide.

This is an example of a differential aeration cell, where the upper solution is an oxygenated cathode, and the lower solution contains the anodic portion of the nail. http://www.corrosion-doctors.org/Corrosion-Factors-Cells/corrosion-cells-aeration.htm

We consider this corrosion in acetic acid to be an electrolytic reaction, and also the reaction of the nail with $HCl$, but the scale of cathodes and anodes in the case of $HCl$ is so small that we can't detect them. And in $HCl$ the $H_2$ is generated so fast that it bubbles off and doesn't need to be depolarized by dissolved oxygen.

Adding a picture suggesting the state of the nail:

enter image description here

https://www.ddcoatings.co.uk/2343/what-is-differential-aeration-corrosion.

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  • $\begingroup$ Help! What is the source of discontent for this answer? If there is some confusion, let me address it. $\endgroup$ Feb 9, 2021 at 14:52

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