Dstorm,
The corrosiveness from BP is caused by salt residues. In particular potassium salts (same as with corrosive priming) from the saltpeter (which itself is a salt) and perhaps from acid compounds formed from the sulfur and nitrates. All of these are water soluble. I've seen tests of the residue, but I don't remember the composition offhand.
The residue itself is mostly just carbon and gunk. It can hold on to water too if enough is present, but cleaning with water removes salts and most loose residue. Most BP substitutes are as corrosive as BP and in some cases (Pyrodex) even more so. They are based on the same chemistry, just modified a bit. I know 777 isn't corrosive, but it's not typical of most BP subs.
Gary,
Elephant is some of the dirtiest powder I've ever tried. It fouls so bad that my smaller caliber guns required almost constant swabbing to remain shootable. It was really noticeable during cleaning also. It took a lot more work than Goex. It wasn't as powerful and it was less consistent. I don't know if I had a bad batch or what, but I've heard a lot of other folks grumble about it in the past.
This is what led to my comment about the powder quality of the day. I've always been confused about that, as there has been speculation that some powders of the day were more powerful than our modern equivelants. I wish I could remember the artical, but they were comparing the sight settings on some period guns and there's no way modern powders could duplicate the ballistics using the same charge weights. Other speculation has it that their powders were less powerful and more dirty, due to lower quality raw materials. I guess it's not much different than today, where we have cleaner and dirtier powders too.