I've actually been saving this ask for awhile because I wanted to look at it more indepth, and at the time- when it was sent, I had only just started writing on this blog so I didn't feel like I knew enough to actually answer it. But what we can speculate about the arm ( it's widely accepted that the arm is either pure vibranium or is plated with it ) ; my Bucky's arm, in particular, is pure vibranium, which means it can take a lot of hits with minimal damage. Vibranium as far as I can tell is made to withstand rust, kind of like stainless steel in that regard. Especially in the environment that Bucky worked, he wouldn't have been fitted with an arm that could corrode. Mechanical damage, on the other hand. Devices that can disrupt/short out circuits, because the plates move and shift like muscle, can get wedged into the cracks and cause a lot of damage. The arm would then have to pried open and the wiring fixed. It can obviously withstand bullets. As we see in TWS, he gets shot at several times, using his arm as a shield and they simply deflect, so most manufactured weapons wouldn't do much to it. The only thing that truly seems to do any damage to it, is the shield, and probably other advanced technologies by Stark Industries or HYDRA. And obviously super powers by other heroes, considering Steve managed to injure it on the Hellicarrier. So it's not infallible, and can be susceptible to injury, but - like any military grade - advanced technology - it is made to endure and withstand most issues; i.e. fire, bullets, rust, etc., most high powered weaponry wouldn't so much as a scratch it. And would be near impossible to damage unless you had a super power or weapon strong enough to do the job. Even with Steve's super human abilities, he still had to work at it.
