Wild Hobby Bobby
Pyrodex is extremely corrosive in my experience, but blackpowder is not. It will attract moisture and rust will form, but it does not seem to eat the metal anywhere near as quickly as Pyrodex does.
I am sure black powder is corrosive because it has sulfur in it. Moisture and sulfur create sulfuric acid.
But I agree with you that it is probably being cosumed and blown out for the most part. When the weather warms a little I will be duplexing some loads in my conical shootin TC renagade.
Wayles