Iron weapons and armors, denoted by the "Ferrous" tag, are made of iron and are subject to rust and spells that damage iron equipment. Rust occurs naturally whenever iron is underwater, but may also be caused by spells like Rust Mist.
Rusted weapons may break in combat, and will deal reduce slash and piercing damage. Rusted Armor will have their protection values reduced by -10.
Rust can be "repaired" by having the affected unit hang out in a province with enough resources to replace the weapon. So a theoretical unit with a 1000 resource cost would have all the rust cleared away from their ferrous items within one turn in a theoretical 1000 resource province. Likewise, it would take 2 turns in a 500 resource province, etc.