Mill scale is the bluish-black iron oxide layer formed on the surface of steel during hot rolling. This layer, while initially adherent, is prone to cracking and detachment over time, particularly when exposed to environmental factors. The practice of applying coatings directly onto this layer is a subject of considerable debate within the painting and coating industry.
The suitability of coating a surface covered with mill scale directly impacts long-term corrosion protection and the aesthetic durability of the applied finish. Historically, surface preparation techniques prioritized its removal to ensure proper adhesion of protective coatings. This is because its inherent instability compromises the bond between the steel substrate and the coating layer, potentially leading to premature coating failure and accelerated corrosion.
