There are 2 key things to know
1, Rusting is where oxygen binds to iron and forms iron oxide.
So once iron rusts, there is oxygen, just not in air; it's in the iron oxide.
So given this, if you take away all of the oxygen in the air, the is 0 atm. And the only thing iron oxide can do is decompose, then the pressure will only go up (very slightly though.)
(0.000000000001atm is greater than 0atm)