What factors would be working to damage or erode them and how long could they conceivably last? Would making them airtight help in any way, or would the contents last longer in a vacuum (assuming you could vent the atmosphere without letting water in)?
closed as primarily opinion-based by sphennings, L.Dutch♦, Vylix, JBH, Amadeus Aug 26 '17 at 22:48
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Europa oceans are water with still largely unknown amount of solutes.
Bottom is, at least, as deep as Marianne fault, possibly ten times more (depends on model, scientists didn't make up their mind, yet).
This is a very unfriendly environment for most likely metals. Plastic and glass/ceramic might fare a bit better.
Keeping airtight anything sizable for extended periods of time (> 1 year) would be unfeasible with current materials. Starfaring races may have something better, but I cannot know how much.
Your best bet is to have the spaceship melt some of the ice at the bottom (either leaking heat or mechanically); soon after ice would form again making a preserving cocoon. In this conditions it's likely it could be preserved for long time (centuries or more) almost intact.
I think - We need to know what oxidants are in Europa's ocean. NASA/JPL are speculating that oxidants are formed from radiation from Jupiter hitting the surface ice. I believe ice floats particularly well in a saline environment, which Europa's ocean may be. Anyway, oxidants created by radiation from Jupiter may cycle deep into the ocean where they could react with your civilizations and spaceship.
Another possible source of oxidants, I believe but am not certain, could come any cracks (fissures) in the seafloor. Same link as above, mentions this.
Of course, fissures introduces another possible source of energy for your scenario. If you haven't seen it, look here:
How long to degrade? I'd use Earth as a basis since the chemistry of Europa's ocean is thought to be similar (see above). You could incorporate biological mechanisms since you re world building, and these could accelerate degradation. Like, marine fungi that are metal starved, and so colonize the space ship rapidly.
My daughter suggests carbon fibre materials to avoid degradation.