Iron oxide generated on the surface of the steel produced by the reaction of oxygen with the metal at high temperatures. Depending on the temperature at which oxidation occurs, the scale may be composed of ferric oxide or ferrous oxide.
It is called the primary scale the one coming from the semiproduct before being rolled and, secondary to that generated on the product already hot rolled.
To avoid damage to the steel surface in subsequent forming processes, both the primary and secondary scales must be removed by shot blasting, chemical or mechanical pickling.