Why does hot rolled steel have mill scale?

Hot roll steel has a oxidised mil scale on the surface of the steel as it forms in the upper heat ranges from mixing with oxygen at a steel mill. Upon slow cooling, scale oxides will form a scale mainly of a type of iron oxide(s) usually of 3 distinct layers. This mil scale has a ability to be somewhat porous and can retain some moisture and is more noble in galvanic charges and at some times will potentially cause the underlying steel to corrode. The scale is not uniform in nature and can be loose in some areas and tighter in other areas and can pose a problem for people that want to paint or coat over it. The scale should be removed prior to any painting or TIG welding. Scale can be removed with various acid pickling type applications, flame treating, and mechanical blast or abrasion methods and then the freshly exposed cleaned surface of the reactive steel surface should be then protected from forming a fast acting blush rust.