In some cases, copper roofing, flashings, or lightning protection components, though separate from an aluminum or steel roofing system, may have run-off water that will travel across the steel or aluminum roof system.
Care should always be taken to prevent contact between dissimilar metals. Copper, being a more noble metal than steel or aluminum, will cause corrosion of steel or aluminum if it comes in contact with them.
Copper run-off should not come in contact with bare steel or aluminum. While Kynar/Hylar finishes provide excellent protection from copper run-off for these metals, the Kynar/Hylar finish might become streaked by the run-off. This would not necessarily cause permanent damage to the finish but it could become unsightly, particularly with lighter colors. Copper-runoff, on the other hand, might cause permanent damage to non-Kynar/Hylar finishes so that contact should be avoided. Whenever possible, explore options for removing the copper altogether or channeling the copper run-off safely away from the steel or aluminum and off of the roof.