Moisture from humidity can corrode the casing too. And the cap. I can’t say I’d want to try shooting ww1 ammo.
“Moisture negatively affects gunpowder by absorbing heat during the burning process, slowing it down or preventing ignition entirely, which can lead to a dangerous "hang-fire" (a delayed ignition) or a complete misfire. Moisture also causes the individual powder grains to become heavier and clump together, decreasing the powder's overall effectiveness and changing its burn rate and resulting muzzle velocity. This effect is why it is crucial to keep gunpowder dry by storing it in airtight, moisture-resistant containers”