A well known question is how to prevent metalwork against the elements. Iron does turn into rust fast. This is also the case with metal arms and armour. Therefore the Roman army did protect parts of the weaponry. Swords and pugiones get their own scabbards, where a shield get a leather cover ‘tegimen’.
Helmets need the same kind of protection. Two different kinds are know. In Deurne, Netherlands, a helmet was found in a drawsting bag. An more elaborate find has been uncovered in Vindonissa (Windisch), Switzerland. There a helmet was found with organic remains: a felt cap on the inside and what is identified as a tailor made leather cover on the outside. Not much is know about the exact construction.
source:
E. Deschler-Erb, R. Fellmann Brugli, & T. Kahlau 2004: ‘Ein “Fellhelm” aus Vindonissa’, Jahresbericht der Gesellschaft Pro Vindonissa 2004, 3-12