Seen rusted tin, I have. Maybe that thin film of rust prevention had worn off? Maybe after being exposed to the weather for years and years, and years, it just gets tired...
Yeah. In the attic of my first house I found a chest, maybe an ancient luggage chest with a wooden frame and a tin outer skin. It was pretty beat up, so we put it on the front porch, where it sat for years. It's oxide formed rapidly into to greyish-white powder and scale. It brushed right off and the original tine gloss was back. I only did this to a small part of it. Actually there are no longer any real tin roofs. They are now just painted steel or aluminum. The steel ones will rust.