Merlin the wizard has a problem.
Sure, it was a piece of cake to move forward in time to the 21st century, procure a sword that was made with the highest 21st century standards, go back in time, and stick it in a pile of cement. The sword's handle, of course, is imbued with DNA detectors and personality divining spells that will retract the perpendicular spikes at the tip of the blade, allowing he whom the wizard deems worthy to slide the blade out of the rock.
So Merlin looks at his handy work, smiling with satisfaction, but as he turns to go a thought pops up -
How can he expect the sword to last through the ages? Sure, 21st century grade steel is tough, but will it survive centuries in the British outdoors?
Merlin turns back in alarm, sighs and starts working on making the sword last.
What should he do? He's a wizard and uses magic, sure, but I'd like to minimize the handwaving, so pointing out specific changes that Merlin would make to the environment will be appreciated. Also, he's no engineer and would rather minimize the use of technology for this, but he does have limited access to 21st century tech (mostly stuff that he can buy/pay someone to make for him, but he can't bring people back in time to help).
Also - For how long can he get the sword to last (how long in the past can Merlin go to start the sword-in-the-rock legend? The longer the better, as ancient legends go)
Ye olde EDIT the 1st: to avoid being too story based I'll rephrase: Set in our world, wishing to minimize handwaving, what minimal changes to the environment around the sword (weather, terrain, maybe society, etc.) would prevent the sword from corroding or breaking, for as long as possible? Said changes should be explained in as scientific a way as possible, but I'm not looking for diamond-hard science, just a few paragraphs to explain to the readers how the sword lasted for as long as it did.
Ye olde EDIT the 2nd: The blade will be steel or some steel alloy. The sword will be used in combat and needs to be stronger than other swords. The idea is to have something close-to-but-somewhat-better than middle ages swords. A lot of answers go for an all powerful material, but if I do that, it might as well be Adamantium that cuts through walls and shoots fireballs. Minimimalization is what I'm looking for (treks in time not withstanding).