Definition - What does Mill Coating mean?
A mill coating is the coating condition of a metal product (such as steel pipe) applied at the mill where it was produced. It may refer to the mill scale produced from a milling process such as weld slag. Or it may be an additional protective coating applied by the manufacturer. In either case, the role of the mill coating is to protect the steel from the environment and detrimental effects such as corrosion.
Temporary mill coatings may interfere with the final coating system for a particular installation. In such cases, the mill coating must be removed before applying the final coating. Abrasive blasting may be required to remove the mill coating.
Corrosionpedia explains Mill Coating
When plates, sheets and other materials made of steel are produced, they produce a mill scale consisting of iron oxides that layer on the top surface, protecting the underlying steel from corrosion. Coatings cannot be applied over this mill coating, so the mill coating must be removed before further protective processes are performed.
Other mill coatings include varnishes, lacquers, paints and adhesives; these coatings typically need to be removed before subsequent protective processes such as galvanizing are performed. Welding near mill coatings can create soot when the varnish is carbonized, which adds to the difficult mill coating cleaning process.
Failure to remove and clean the mill coating may make future coatings fail and make the steel prone to corrosion.