What does "scale" refer to in the context of coating and corrosion?

In the context of coating and corrosion, "scale" specifically refers to a thick layer of corrosion that can develop on steel and other metals when they are exposed to environmental conditions that promote oxidation. This condition occurs during corrosion processes, often leading to the deterioration of the substrate metal beneath.

Scale is typically composed of oxides, hydroxides, and other corrosion products that form on the surface, which can hinder the adhesion of paints and coatings. Proper surface preparation is crucial for effective painting or coating application, as scale must be removed to ensure that coatings can adhere properly and protect the underlying metal from further corrosion.

The other options describe different phenomena or principles in relating to coatings but do not accurately capture the specific meaning of "scale" in the corrosion context.