I have asked a question similar to this one, but I am writing a post apocalyptic story that takes place 10,000 to 20,000 years in the future. However, I found out that regular metals decompose in a 1,000 years. I have tried searching online, but I cannot find a good answer. Do you have an answer to how long stainless steel will last?
Metals don't decompose, they oxidize
In other words it doesn't matter so much as what it is made of but more of where it is left.
Drop a Buick in a tar pit for a million years, clean her up, change the battery and she'll run just fine. (assuming the heat doesn't melt the plastics ~ )
Machines don't like water and air
From comments :
..but you see that is precisely my point, it is the environment that determines the longevity of a material. More precisely, it is the composition of the material with respect to the active chemistry of the environment in which it is left that determines a material longevity
can be further refined to:
"it is the composition of the material with respect to the active chemistry and energy of the environment in which it is left that determines a material longevity"
added to ambiguously encompass other important factors.
The chief way to break down a metal is to oxidise it. Water, heat, and strain help to speed the oxidation. I don't know if stainless steel would last 10ka on a hypothetical hillside, but if you just need to maintain a metal object there are easy story solutions -- get the metal away from oxygen and water.
Obvious methods would be sitting in anoxic lake sediment, or sealing inside a stalactite or amber. Doesn't even need to be stainless steel.