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I'm writing a fantasy story where an iron key was thrown down a deep well. The key needs to rust away completely in order to reform in another place. So how long would it take for an 8 cm ornate wrought iron key to rust away completely in the bottom of a well that still has water? Edit: This is taking into account that the magic in the key would want to make it rust about four times faster in order to appear in a different place sooner. The cross section of the key is 44 mm.

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    $\begingroup$ As log or as quick as your story needs. Adjusting the water, the salinity, the ph, the manufacture and composition of the key itself, feasibly make for any time period required. $\endgroup$ Jul 10, 2022 at 14:03
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    $\begingroup$ How thick is the iron? How well crafted the key? What quality is the iron? How much surface area does the key have? Basically, you're asking a straight up chemistry question which really isn't a good fit for this forum. I'm voting to close on the basis that this is not a worldbuilding question. $\endgroup$
    – elemtilas
    Jul 10, 2022 at 16:03
  • $\begingroup$ @elemtilas I'm sorry if I offended you. The question was asked here because it was essential for me to know the timeframe of the dissolution of an iron key in water in order to create a significant part of the story timeline on which I am working. I was just asking for an approximate measurement, because then magic would be added to the mix $\endgroup$ Jul 11, 2022 at 10:24
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    $\begingroup$ Length doesn't matter too much, cross sectional area does. that is how thick is it? $\endgroup$ Jul 11, 2022 at 19:39
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    $\begingroup$ You didn't offend anyone, least of all me! I understand that you need to know the answer, but please also understand how SE works! Wait! Stop the presses! Dude. Adding magic to the mix is something you should be telling us in your question, not in a comment! If your world has magic that would affect the rate of rusting, that becomes a question of worldbuilding! I shall happily retract my close vote if you edit your question accordingly. $\endgroup$
    – elemtilas
    Jul 12, 2022 at 3:31

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The answer very much depends on the chemistry of your well and key. The nature of the water in the well is also important. A high oxygen content would accelerate the corrosion process as could many other mineral impurities that might be found in small quantities in drinking water.

Another factor is the way the key was made. If the intention was to make the key susceptible to corrosion by introducing flaw, laminations or micropores of some sort then disintegration could be rapid.

I suggest the key could be gone in a few years to a few decades

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    $\begingroup$ Don't forget the pH of the water. $\endgroup$
    – L.Dutch
    Jul 10, 2022 at 7:06
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    $\begingroup$ Looking around some, it seems that 1-2 mm per year is certainly possible. If the water is soft or Ph less than 4 (acidic) the corrosion rates are higher. A lot has to to do with how the corrosion starts and how much, oxygen is present. Even microbes can help cause corrosion. Of the flip side in some conditions a protective film can build up that limits the corrosion. $\endgroup$
    – UVphoton
    Jul 10, 2022 at 14:49
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The key is eaten by an eel.

watch eated by eel

https://paperspast.natlib.govt.nz/periodicals/NZT19070411.2.45

Your well has a resident eel. Eels can live in wells, sometimes for decades. Or longer.

https://www.smithsonianmag.com/smart-news/rip-worlds-oldest-eel-180952306/

In 1962, Tomas Kjellman's family bought a cottage in southern Sweden, knowing very well that the property was still occupied by another long-term resident. A eel nearly a century old lived in the property's well, The Local reports. Young Tomas and his family took to their slippery new friend, dubbing him Åle ("eel" in Swedish) and often introducing him to family friends that visited.

The key would last a very long time in the pure water at the bottom of a well. But not so long in the belly of an eel. Plus an ancient eel trapped in a well seems more fitting for a fantasy, because it is fantastic.

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I have seen of half inch iron (bolts, rebar) lasting decades on coastal beaches. That is iron exposed to wave action, and tides. Some parts were rusted away others not, very non uniform. But still, more then a decade to several decades in what most would consider ideal place to rust away iron.

Many factors to consider:

  • Has the key been rust protected in some form(ie beeswax near last quench)
  • What is the cross sectional area, the larger this is the longer it will take to rust.
  • Depends on how it was made, source materials. The iron pillar of Delhi is over 1500 years old.
  • What pH is the well, unless dug in the 'wrong' spot will have pH close to 7 so won't have big affect. If in very hard water the iron itself can start to dissolve.
  • What is ambient temperature, warmer implies faster reactions
  • Moving water or still? Moving water would help move rust and expose it to more oxygen then still water.
  • Is the key full submerged or fully in air? a fully submerge piece of iron will last longer then one fully exposed to air. both will last longer then one that is periodically submerged.
  • How rusted must if be to be rusted away? Chunks of hard rust could easy outlast any still metallic iron.

If the water in the well is somewhat soft/alkaline and the key always submerged, still water. I would expect the Key to last many decades to centuries.

If the key is in very hard water well(eg rotten egg smell of H2S). Alternatively above and below water over course of the year. water flowing to some degree. Then perhaps less then a decade or two.

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