Can air contain water vapour or droplets of water? Can gunpowder hydrate itself (hygroscopic)? If so, can air enter through the lid of the container?
The amount of air in a container comes into question first off.
As for powder being hydroscopic, I've seen powder immersed in water, dried out and used.
When powder starts to break down to the point rust is appearing on the container lids and clumping that isn't water.