"You wonder if Sauron ever worried about his tower rusting or suffering water damage. Maybe you could make it out of snakes... what? I meant stainless steel, though you could make it out of snakes"
- Vi Hart, Doodling in Math Class: Dragon Dungeons, 18th Oct 2013
Video Link: http://www.youtube.com/watch?v=dsvLLKQCxeA
Apparently in Sindarin, the name of Sauron's Tower was "Barad-dûr" which meant "Dark Tower". Obviously this is rather confusing as JS Pachelbel's "Kanon D-dur" means "Canon in D Minor" but I digress. The question and the point here is what Sauron's Tower was actually built from.
JRR Tokien tells us that the tower took more than six hundred years to complete and was on so massive a scale, it was surreal. Okay, hardly anything to go by but we are told of Barad-dûr that "rising black, blacker and darker than the vast shades amid which it stood, the cruel pinnacles and iron crown of the topmost tower" and that Frodo Baggins perceived the immense tower as "wall upon wall, battlement upon battlement, black, immeasurably strong, mountain of iron, gate of steel, tower of adamant"
It seems then that Sauron's Tower was mainly built from Iron.
Would Sauron have worried about rust or water damage? Possibly not. Though I can think of one hazard which would have trouble him greatly and that is - fire.
I think of the The Crystal Palace which was built in 1851 to house the Great Exhibition and which Crystal Palace F.C (currently 0-0 with Arsenal as I type this) derives its name.
The Crystal Palace itself suffered its final fate by fire when on 30th November 1936 when an explosion of unknown cause, triggered the fire and by the next morning, the whole structure had been razed to the ground.
Fire of course would have been quite the problem from Sauron. Considering that he chose to build his monolithic tower Barad-dûr in Mordor, which has as its most dominant geographical landmark Mount Doom, which is itself a volcano, it leaves it open to damage by molten rock.
Iron itself although being harder to work than say bronze and having a melting point of 1538 °C (or 1811 Kelvins) is quite open to corrosion once it has been heated. Placing a tower made of iron next to a volcano is almost certainly a recipe for disaster and you'd think that someone like Sauron with that sort of intellect would have noticed just how silly it was but given that he still didn't after 600 years, perhaps all that power had just gone to his head.
As for stainless steel, it was German firm Krupp who finally produced it in 1908. Presumably Sauron wouldn't have had access to stainless steel or else he may have considered it.
Could you build a tower out of snakes though? Probably not. Snakes aren't exactly the best building material because as Medusa found out, you can't even build a decent wig out of them; let alone a massive tower that would take 600 years to construct. Virtually all of them would have died and rotting snake flesh again isn't exactly the best building material.
If I was Sauron, I would have considered building the tower from stone as that's quite fire and water resistant and many buildings like the Parthenon have outlived all sorts of buildings. Europe's great cathedrals are all stone buildings; so it seems that quite grand structures can be made from the stuff. Stone is therefore far more suited to the task.
Certainly not iron or snakes.