I also have had corrosion problems with Pyrodex.
I was new to shooting BP and got back from the range and figured I'd wait before cleaning my cheap-o brass framed 1851 Navy (.44 cal Pietta). Did not get around to it until the following weekend. That thing had rusted like crazy!
I've not seen anything like this with Goex black powder, even when I've waited a couple of days before cleaning.