Yes, in time it will but the residue is slower to rust SS than regular steel. The rule is to always clean a black powder firearm on the day it was shot.” ... The residue from fired black powder contains salts that are hydroscopic, so it's something of a double whammy when moisture is present.

Is black powder substitute corrosive?

Like many other black powder substitutes, it is made to be a volumetric substitute of black powder. It is dispensed in "black powder powder measures" for muzzleloading applications. Blackhorn 209 is essentially non-corrosive, low-fouling, very consistent in gas generation, but non-hygroscopic.

Does black powder cause rust?

The muzzle and bore were so rusted they resembled an iron pipe than had been in the ground for years — pitted, caked, eroded and far past salvage. ... Black powder, Pyrodex and other propellants create rust if left unattended, lots of it, and muzzleloaders need to be taken care of quickly.