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.

Black Rust

Black Rust