What exactly is Mill Scale?
Q. My name is Frank Rea. I am a coatings consultant, predominately for steel structures. Arguments with contractors about mill scale are common. In order to discuss this issue knowledgeably I would like to know:
What is the chemical composition of mill scale?
Does it vary by process?
Are there primary constituents that must be present for a substance to be classified as mill scale.
Consultant - Gainesville, Florida
A. Mill scale consists primarily of magnetite, Fe3O4, of characteristic blue-gray "steely" color.
An extremely thin outer film of hematite, Fe2O3, is invisible to the naked eye.
The inner portion of the magnetite contains fine metal grains and sometimes, residual black FeO (see below), which contribute to the roughness of descaled metal. Mill scale is found on all hot-rolled steel products unless processed in a protective atmosphere or descaled (e.g., for galvanizing).
At hot rolling temperatures, an inner layer of wüstite, FeO, exists between the steel and magnetite.
"At elevated temperatures, FeO constitutes about 85% of the scale thickness, Fe3O4 about 10 to 15% and Fe2O3 about 0.5 to 2%. During slow cooling [below 1040 °F] of hot-rolled coils of hot-rolled strip [and heavy shapes], most of the FeO is transformed to Fe and Fe3O4, and the latter oxide is predominant after cooling."
- --- The Making, Shaping and Treating of Steel, 10th Edn., p. 1083 (1985). ISBN 0930767004.
The alloying metals form cations within the mill scale at about the alloying proportions* due to the almost instantaneous oxidation of freshly exposed metal at high temperature. Fe3O4 contains both Fe+2 and Fe+3, and is sometimes written FeO·Fe2O3. Ni+2 substitutes for Fe+2, Al+3 and Cr+3 substitute for Fe+3. Thus, (Fe,Ni)O·(Fe,Al,Cr)2O3. Cations of other valences, e.g., Si+4 and Ti+4, are accompanied by cation vacancies to maintain overall electrical neutrality. With very slow cooling, small grains of Si- and Ti-containing oxides may precipitate within the scale.
*unlike general heating or heat treatment scale, where Al, Cr, etc. preferentially oxidize.
Steel mills descale sheet and plate by passage through scalebreaker rolls, which flexes the metal enough to fracture the scale. This allows pickling acid (HCl) to attack the more soluble (partially transformed) inner wüstite layer.
- Goleta, California
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April 20, 2008
Q. I am particularly concerned about the effects of Mill Scale if left in place and painted over. Does it have any long term negative effects on the steel or subsequent paint or coating surfaces. Secondly if left in place and painted how can in be removed in the field.Gary Kopp
- Barrie, Ontario, Canada
A. In response to the question on whether or not mill scale on hot rolled steel is detrimental, I would submit that is very much so as it is anodic to the steel. Meaning that once moisture enter the picture the steel corrode (rust) and not the mill scale. Which is why the most protective coating products designed for immersion and other tough environments require an SSPC SP-5 white metal degree of cleaning as this process removes all mill scale.Burt Olhiser
- Middletown, California
February 16, 2009
A. When you are applying a coating to a steel surfaces in order to produce sufficient adhesion against corrosion it is necessary to remove all contaminants off the surface. Mill scale is porous and poorly adherent scale of magnetite, it will only give limited short term protection to steel against corrosion. If you are coating any steel surface against corrosion it is necessary to blast clean (with garnet or similar) to a finish of Sa2.5 blast standard before coating.
- Adelaide, South Australia, Australia
July 14, 2008
Q. As per some friends, the mill scale can be used in production of steel production, put in the trench when steel liquid get out, thus it will automatically smelt in the liquid, and no wastage, steel still proper, want to know if it is the best way to use mill scale?Howard Li
mineral resources development - Tianjin, P.R China
October 25, 2008
Q. Is there any free iron available in typical Mill scales.Dr. Zubair Alam
Chemist - Al Khobar, Eastern Province, Saudi Arabia
May 14, 2010
Q. If you are installing heavy, rotating equipment what are the policies for the base plates, regarding mill scale. I have been taught that it is poor engineering practice to install equipment on a mill scale surface. Can anyone corroborate my thinking.Les Pulman
Engineering - U.K.
Hi, Les. It may depend on what type of equipment, but to the limits of my experience base plates are always painted, and to properly paint them requires removal of mill scale.
Ted Mooney, P.E. RET
Brick, New Jersey
May 18, 2010
The base plates support a Gas Turbine weighing 370 tonnes. If the Mill Scale is on the plates, under vibration, the scale breaks down to dust and effectively leaves a void. The underside of the plate is coated with scale also and sits on a cementitious grout. The grout is water based which could result in the Mill Scale separating from the plate.
Engineering - Wales U.K.
May 18, 2010
Hi, Les. I never designed in that field and can't comment on codes or standard practices for heavy gas turbines, but I do agree with you in general that steel structures require removal of mill scales, and that same opinion has already been offered here by Burt Olhiser and Vijay Vijayaraghavan. Good luck.
Ted Mooney, P.E. RET
Brick, New Jersey
May 18, 2010
It is easy to understand why the oxides form in a specific sequence in a mill scale. The highest oxide (the one with maximum proportion of oxygen) forms on the surface as it is directly exposed to air (the source of oxygen) and the lowest oxide forms next to the base metal (the source of Fe). However, the lowest oxide, FeO, is said to form only above 570 °C. Therefore, its presence could be indicative of the scale being mill scale, which might be easy to confirm if FeO is retained at room temperature, without further oxidation to a higher oxide, for sufficiently long time.
If only mill scale was dense, adherent and continuous it could have provided necessary protection against corrosion but unfortunately it is not. Moreover, it is cathodic (and not anodic like zinc) to iron and hence will corrode much faster at locations of discontinuity due to unfavorably large area ratio of cathode to anode. In such a case, a bare metal surface might be better than a discontinuous mill scale (or any other cathodic coating) as the former will corrode more uniformly as against the possible severe localised corrosion of the latter.
- Sydney, NSW, Australia
August 17, 2010
I have a question about what happens to make scale? What is the chemical reaction that happens?Gus Wimmer-Brown
- St. Louis, Missouri, U.S.A.
June 11, 2011
Comments on applying finishes over mill scale gave me a sculptor's idea. I don't fight Mother Nature, I leave my outdoor steel sculptures to develop a handsome rust finish. So, if I was to remove some of the mill scale selectively, in a decorative pattern, the finished piece might develop a pattern as parts rust differently. However, would the mill scale and non-mill scale rust soon catch up with each other and soon look the same?
I posted an earlier question with no response - whether applying a rust converter in a selected pattern on a rusted surface would also allow you to add visual interests or does the rust margin creep under and push off the converted rust?
- Plymouth Massachusetts US
May 18, 2012
Q. I want to remove the oil from the mill scale; does anybody have an approved process which can cost less than 10 dollars/ton?Vk Murarka
Engineering Company - Marion, Ohio, USA
A. Hi Vk. Murarka,
Just some ideas to your question for de-oiling mill scale:
There are really many different processes. One method is using water to separate the oil content from the solid by high shear agitation. I refer you to the following US patent for details: US Patent 5047083, "Process for de-oiling mill scale".
Another patent suggests a mixing the mill scale with calcined lime followed by a burning process to form pellets. Please see US patent 4326883.
engineering - Sydney, Australia
October 4, 2012
Q. What is the effect of steel mill scale on shear blade if the steel plate is sheared without pickling or shotblasting.Mohankrishna G
- Jamshedpur, India
December 19, 2012
Q. Hello: I want to briquette Mill Scale Dust removed from a Shot Blasting Operation. Can these briquettes be used as material to be remelted for Industrial Steel Manufacturing in EAF type of process? What are the pros and cons?Brian Lehane
- Ramos Arizpe, Coahuila, Mexico