Primarily a galvanneal defect black rust is sporadic showing up randomly throughout a coil. Galvanneal has an iron content coating which does not exist in galvanized coils. This is why this defect is specific to galvannealed coils. It can also be called black stain or condensation stain. Exposure to moisture or weather during transit is the culprit for this defect as with most stain or rust defects on steel. Surplus Metals is a buyer of coils with black rust so if you are looking to sell defect coils feel free to reach out anytime.
Residue from the corrosion of steel consisting of iron (ferrous) oxide(s). It is often referred to as “scale”. It is usually formed during annealing. The thickness of the black oxide is a function of the cooling step. This form of rust adheres more tightly to the steel surface than the more familiar loose and flaky red rust. Black rust can be smooth enough to be mistaken for the steel surface itself. See also red rust.
Here is a really in depth look at black rust from Wikipedia if you are interested in more information on steel defects we try to post as many as possible here on our site as well.