I believe it's post-WS and pre-CW.
Bucky's in a cave in an old Hydra base (possibly former Nazi?) near the Black Sea. Vague memories have brought him there. Rumlow shows up with a Strike team, so Bucky hides in a small submersible which turns out to be an alien craft that's linked to his arm. He ends up living out of it.
I'm sure it's on AO3, but I was also sure I'd bookmarked it. :(
Any help will be appreciated.