To oxidize iron you need oxygen, and water.
Things which improve electrical conductivity of water improve its ability to cause rust: usually that means ions dissolved in the water. Salt (sodium chloride) is the one we encounter most. Salt added to roads to help melt ice can facilitate rust. Salt spray / salt fog in ocean areas can facilitate rust. Other dissolved ions in salt fog might also play a role.
People who live near the ocean know this.
The reason is that the process of rusting involves electrons moving
around, and electrons move more easily in salt-water than they do in
clean water. A simple test of this is to see how easily current flows
in clean water (it does not), and then add salt to the water (then
current does flow easily)... Water is the enabler of fast
oxidation of iron so freshwater will also cause rust. However, salt
water is a very good conductor (lots of dissociated ions) and so there
are a number of electrolysis reactions that tremendously accelerate
corrosion in salt water. For example if you have iron in contact with
salt water and also in contact with another metal such as aluminum
(also in contact with the water) you effectively get a battery which
drives very fast corrosion processes.
Here is a great free full text scholarly overview of marine atmospheric effects that increase corrosion. I did not know about the SO₂ one!
Marine Atmospheric Corrosion of Carbon Steel: A Review
In 1973 Barton noted that the mechanism governing the effects of
chloride ions (Cl⁻) in AC had not been completely explained, and that
the higher corrosion rate of steel in marine atmospheres could also be
due to other causes, such as: (a) the hygroscopic nature of Cl⁻
species (sodium chloride (NaCl), calcium chloride (CaCl₂), magnesium
chloride (MgCl₂)), which promotes the electrochemical corrosion
process by favouring the formation of electrolytes at relatively low
relative humidity (RH); and (b) the solubility of the corrosion
SO₂ is neat because I think it is not acting as electrolyte to increase conductivity, but as an oxygen donor.
SO₂ gives rise to the formation and propagation of sulfate “nests”,
according to reactions, which start to appear at isolated points
on the surface but whose number increases until all the surface is
coated with a rust film... SO₂ is much more aggressive to steel when
its concentration exceeds 0.1 mg·m⁻³, a level that is easily reached
in many towns, especially in winter. Fortunately, the SO₂
concentration in urban air has decreased greatly in recent years due
to efforts to reduce pollution
But here is an end run around objections about the harshness of such atmospheres on your humans, or the fact that people do use iron in environments like this:
Worldbuild up some super fierce iron-oxidizing microbes.
These things exist in the world. In your world they can fix their own nitrogen and carbon dioxide, and for energy they oxidize metallic iron. It is a baby step away from creatures that really exist as you will read in the link. These micro-organisms will not be interested in humans, only iron. You would need to keep your iron under oil to keep it from being colonized and rusting away.