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I'm working on an idea similar to the one presented in the Old Guard; basically an immortal character is trapped in an iron casing from which they cannot escape, that is sent at the bottom of the sea. The casing is subject to corrosion and eventually becomes weak enough to be destroyed.

Question: if we're talking about an iron box that's roughly a centimeter thick, how long will it take until the character can punch through (assuming they are a normal human apart from being immortal)?

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    $\begingroup$ Do you have any specifics about the iron used for box/cage? Is it a pure iron, antique hammered iron, wrought iron, cast iron, or maybe even stainless steel? $\endgroup$
    – Alexander
    Commented Mar 19, 2021 at 21:07
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    $\begingroup$ does their immortality let them expend energy without eating? Do they regenerate? They might be able to shorten the duration by constantly scraping, or cause metal fatigue by slightly flexing the metal in and out repetitively for years. Constant scraping might also accelerate rust by creating irregularities and removing pieces of rust. What kind of iron, made with what level of technology? $\endgroup$
    – DWKraus
    Commented Mar 19, 2021 at 21:22
  • $\begingroup$ Also a difference if the box is airtight. The difference in air pressure inside the box and water pressure outside might cause it to warp, which would further weaken it. If that is enough to dislocate a hinge, the prisoner might be out of there in 6 months; just open the door and swim out $\endgroup$
    – nzaman
    Commented Mar 20, 2021 at 4:30
  • $\begingroup$ How does "immortal" cover eating or breathing, please? Why would anyone capable of encasing the prisoner in an iron bax, stop at making it a mere centimetre thin? $\endgroup$
    – Robbie Goodwin
    Commented Mar 20, 2021 at 16:00
  • $\begingroup$ After a few decades alone in an iron box on the ocean floor, will your otherwise human immortal actually have any sentience left? Or will they have gone insane or catatonic, or whatever happens when on such a situation with no way to know how long it lasts, maybe forever. $\endgroup$
    – Stilez
    Commented Mar 23, 2021 at 22:37

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The corrosion rate of unprotected iron in aerated warm seawater is ~0.1mm/y, in very deep water (abyssal plain) it will be much slower (2-8 times).

in 50 years it will be half a centimeter thick, a fit human bracing their body against the walls and using their legs might be able break that with repeated attempts. If the box is not water tight it will only take ~25 years (corrodes from the inside and outside).

punching through it will be much harder humans can't generate much force underwater, the box will likely start to leak before it it is weak enough to punch through, so 75-100 years if the box starts water tight.

https://www.nma.gov.au/__data/assets/pdf_file/0007/346039/NMA_metals_s1_p6_corrosion_cathodic_protection.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4064120/

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