They might find it fun to lick, but other than that I see no real danger (unless it oxidizes and they lick the rust). Is this stamped metal or milled metal? Might have to scrub the heck out of it if it's stamped maybe electroplate to prevent corrosion (at least in my experience, metal sheets are kinda dirty and have some coating on it from factory).
If you can make the outer surface rough (maybe acid treat) you'll probably find that they like to sit on top of it more than just hiding in it, my gecko uses his black plastic hide like that, but he has a hard time staying on top because it's kinda slippery.
-Lemur 6