Same thing here, Paul. Tire was carroded to the wheel rim. Got the bike from back east, from I believe it was Pennsylvania if memory serves. Had it shipped to me. Nice machine. Ran like a champ. But it came with two broken rear spokes, something I knew when I bought it.
The day came when I figured I'd better fix that situation. That's when I discovered how corrosive the weather back east can be on things like where tire beads seat themselves against the inside of a wheel rim.
Don't remember much in the way of alloy dust, just gummy crumbly dark gunk that acted as a kind of super cement between the wheel and the tire. Never saw anything quite like it before, and hope to never see it again..