Here are some of your options:
1- Boiled Water In Sealed Container
Boiling water removes the dissolved air in it and if you put an iron object in such water, it will not rust (6 grade experiment). But you will have to seal that boiled water container in order to protect it from dissolving air again (which would rust the iron).
2- Stainless Steel (Alloy)
This requires a carefully prepared alloy of Iron, Chromium, Nickel and Carbon. For best results, Molybdenum is also required, but since it is so rare, it be very hard for your people to have it and use it for preparing the perfect quality of the alloy.
If you clean your iron objects and then carefully seal them with melted wax, they would remain rust-free. As sumelic mentioned in a comment, peat would work likewise. Wax would be a better choice than peat due to easy availability and it is much easier to remove too.