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Recently, while cleaning a neighbour's fridge (turned off for a few weeks), I came across a cup (closed with a lid). Inside the cup was, to my olfactory horror, congealed milk, with a steel (iron) teaspoon nicely wedged into it.

The milk had separated into a blue cheese-like solid, overlain with a semi-translucent liquid.

When I took the teaspoon out of the congealed mess, there was a layer of rust on the teaspoon where it was in the liquid. But, I observed almost no rust on the parts of the teaspoon in the blue-cheese-like solid and that exposed to the air.

The illustration below is a rough schematic of where the rust occurred (the 2 layers observed are labelled A and B, and the air in the top of the container is labelled C and the diagonal line represents the teaspoon:

enter image description here

A = congealed blue-cheese-like solid = virtually no rust on this part of the teaspoon
B = semi translucent liquid = heavily rusted part of the teaspoon
C = air = virtually no rust on this part of the teaspoon

How could the milk, or the liquid separation, cause rust on the teaspoon?

A hunch I have (and I could be way off the mark), is it possible that the corrosion could be due to, at least in some significant way, due to the fact that milk is slightly acidic $^{(1)}$?

$^{(1)}$ According to the International Livestock Research Institute, they state that

Fresh milk has a pH of 6.7 and is therefore slightly acidic

Related question and answer Silver and milk (or milk products)

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    $\begingroup$ Maybe, that steel was mostly iron and milk has H2O. $\endgroup$ – anshabhi Jul 7 '15 at 11:06
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    $\begingroup$ Yes, I am aware that steel is mostly iron, and water makes up most of water - but as I said, the rust was on the teaspoon within the liquid part of the separated mess - have clarified that there was virtually no rust on the other parts of the spoon. $\endgroup$ – user15489 Jul 7 '15 at 11:11
  • $\begingroup$ Rusting requires oxygen (i.e. air) and water. The top of the spoon wasn't wet - no water. The base of the spoon was surrounded by cheese - no air. Hence only the part of the spoon at the water-air interface rusted. You can go and find this on steel pillars in the sea if you like - it rusts most quickly in the parts which are alternately wet and dry. $\endgroup$ – AndyT Jul 7 '15 at 13:50
  • $\begingroup$ @AndyT it was not just at the interface (if that were the case I would not have asked, as I am well aware how rust forms there), as I said there was a layer of rust on the teaspoon where it was in the liquid. - the part of the teaspoon immersed in the separated liquid. $\endgroup$ – user15489 Jul 7 '15 at 13:57
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    $\begingroup$ I don't have the chemistry expertise to formulate a full answer. I would guess that the spoon is actually stainless steel, and not just plain carbon steel. My understanding of how stainless steel works is that a passive film of chromium oxide is formed on the surface, which prevents further oxidization. Presumably something (perhaps lactic acid???) in the liquid part of the separated milk is able to react with the chromium oxide layer to the extent that this protective film is removed and then iron oxide is able to form. $\endgroup$ – Digital Trauma Jul 7 '15 at 17:22
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Technically, even simple water can cause rust, so nothing surprising here. However, spoilage of milk most probably produced a lot of organic acids (lactic acid and similar) which can speed up any corrosion process. Lactic fermentation is a natural fermentation process in milk, when bacteria start converting the sugar content of milk to lactic acid. It is not the only possible product, but a very common one. Link: https://en.wikipedia.org/wiki/Lactic_acid_fermentation

There are two important factors that help corrosion: First, acidic environment (low pH) itself generally help corrosion. Second, the organic acids produced in fermentation can often form metal complexes, therefore they help remove any protecting oxide layer from the surface of metal objects as well as shift the redox equilibrium towards an oxidized complex form.

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  • $\begingroup$ Thank you for this answer! I usually wait a few days before accepting an answer - in the meantime, could you please include some links, particularly for your second point in the second paragraph? $\endgroup$ – user15489 Jul 9 '15 at 0:55
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    $\begingroup$ That is a kind of tricky: all I said is pretty common knowledge in chemistry. The fact that organic acids (carboxyl acids) can form complexes with metals is a trivial statement. Lactic acid silver or other metal complexes are not particularly famous or important. The complex of silver with lactic acid is sold by Sigma, SID 24896481. $\endgroup$ – Greg Jul 14 '15 at 5:18

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