BP -- after being burned -- leaves salts which readily absorb moisture/turn corrosive.
Unburned BP is only slightly prone to humidity absorption. When dry, it is compatible
with most metals. When wet, it will attacks all common metals except stainless (MSDS)
That appears to be true, that it usually doesn't absorb a whole lot of moisture.
However 2 posters mentioned that Swiss Null B can cake and clump, both in the original bottle and enough to clog a primer horn. --->>>
https://americanlongrifles.org/forum/index.php?topic=56206.0
Stoner creek posted about Swiss Null B
"Yeah I like it too. I’ve already used up a pound of it. My only qualm with it beyond the cost is that it tends to clump up a bit. Even in the original plastic bottle it clumped up. Nothing a little stir with a wood dowel wouldn’t cure quickly."
Bob McBride posted:
"Will clump and stop up a small hole in a priming horn on occasion. Nothing a good shake or vent pick won’t fix."
I Googled the question:
Is Charcoal hygroscopic?
"One of the inherent properties of
charcoal is its high moisture absorption capacity, as a
hygroscopic material. ... According to Luengo and Emmerich (1997), during the first hours after its production,
charcoal can absorb from 4% to 16% of its weight in water, decreasing with time." --->>>
https://www.google.com/search?clien...+hygroscopic&sourceid=opera&ie=UTF-8&oe=UTF-8
Some folks think that powders can absorb enough moisture to cause corrosion in the breech of guns where the powder was left loaded.
There was a test published in Muzzle Blasts where a ML loaded with a PRB was stored in a damp cellar for a year and then examined for corrosion.
And the barrel ended up rusting from where the patched ball was seated and below it.
Zonie on the MLF attributes this to the openings in the barrel not being completely closed off. --->>>
https://www.muzzleloadingforum.com/threads/leaving-guns-loaded.16766/page-2
For the most part, powder flask interiors are closed off from humid air by a gate.
As long as the gate closes then the powder would seem to be fairly well protected.