What is it?
This defect usually appears on dried and chemically treated galvanized coils. It appears as friction marks that are present up to one inch of the outer-diameter, and are repeated at distances that match the circumference of the coil. This defect is also known as friction marks or black marks. The defect usually occurs when the galvanized surfaces are compressed during shipment, or when there is a reoccurring pressure point between the two zinc coated surfaces that creates a black friction mark on both the top and bottom surfaces.