What is it?
Similar to white rust on galvanized material, this galvanneal defect is not continuous, and is random throughout the coil. It also includes a mirror finish that is close to the mill edge. This defect looks black due to the iron content in the galvanneal coating, which is absent in a galvanized coating. It is also known as condensation rust or black stain. As with White Rust, the cause for this defect is exposure to moisture during transit.