Large metal object in a cold, air free environment

Factual Questions
1

This is a question about how long a large metal object buried in a cold, air free environment would last. I was watching this video on YouTube about pulling an assault gun (a Stug III to be precise) from a bog in Russia. While I was watching it, I was thinking about ghost ships in some of the great lakes or the Black/Baltic sea, etc. There are wooden ships that, in the right environment have lasted intact for hundreds, even thousands of years. If this Stug III hadn’t been found, how long could it have lasted in that bog intact? How long, potentially, could any large metal object like a tank or other complex machine last in the right environment? Say at the bottom of the Black or Baltic sea or some other very low oxygen and cold environment?

(might not be any way to answer this GQ style, so Mods feel free to move this to IMHO or whatever, as you see fit)

2

Here are some photos of the Bismarck taken in 2001 (sixty years after its sinking).

3

Also pH can affect corrosion. In an alkali environment, iron corrodes to form a dark black waterpoof oxide, that slows corrosion.

In an acidic environment, the corrosion forms “rust” , the orange crud that allows water and oxygen through, and grows (absorbs water to form a hydrate) and therefore you see steel corroding away fast in an acidic environment.