I have never heard of unfired powder in a clean vessel, ie:
clean loaded chambers, to absorb moisture or cause pitting.

Now, if the gun was EVER fired with Pyrodex & not cleaned
over and over again over the course of several days - then THAT 'residual' fouling
in the pores in the steel, even
microscopic ones, will indeed cause pitting & dissolving
of the iron molecules due to it's acidic nature.

BP itself in granular from in not to the best of my knowledge hygroscopic.

If the revolver spent those two years in an extremely humid environment, then perhaps this could happen.

When I lived down the coast of BC, I had no such trouble with any black powder guns.

BP fouling, as well as Fake Black Powder fouling is very

hygroscopic. The phony powders that contain perclorates are

especially so and VERY dangerous to your bore or chambers

over time to come. Many guys who bought the Stainless Rugers

thought they could get away without cleaning them and thus

ruined their guns.