Mill Scale

Question:

I am just wondering what mill scale is. I have heard different things on what it is, so I figure you are the person to ask. Thank you. R.D.

Answer:

Mill scale is defined by the ASM Metals Reference Book as “The heavy oxide layer that forms during the hot fabrication or heat treatment of metals.” In my experience, I have only heard this term mentioned in association with steel manufacturing. Much of the reduction of steel from billets to sheet or plate is done at the steel mill while the material is very hot. While it is hot, steel is particularly prone to oxidize which forms a “scaly” surface appearance. Hot-rolled steel will be in this form and often will go through a pickling (acid cleaning) and oiling step. This form of the steel is then referred to as Hot-Rolled Pickled and Oiled (HRPO).