A sample of some wall coping. I replaced the rusted original tin with copper when I replaced the slate roof. Somebody had shingled it 6 years ago and used 3-tab shingles to do the cricket. ( The cricket is the area behind the coping where the water drains to and the cricket diverts the water to either side of the wall.) I used leaded copper for the cricket. Acid rain and slate seem to have no effect on leaded copper. I've seen 16 ounce copper rot out in as little as 9 years. The lead seems to neutralize harmful things.
Albert's Roofing Repairs | Slate and Window Flashing Co., Rich., Va.