What is Mill Scale?

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Mill scale is natural oxidative layer found on steel. It appears as a bluish coloured layer which is formed on the outer surface of the steel and created during the casting and rolling process. 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.

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 vustite, 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 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. (1)

Test method for the determination of Mill Scale using the copper sulphate test method can also be used in conjunction with ASTM A967.

  1. Measure an area of approximately 100mm2 on th blasted surface.
  2. Mix a solution of 4 mg of copper sulphate, 1 ml sulphuric acid and 250 ml of distilled water.
  3. Using a spray bottle capableof spraying a fine mist, spray the copper sulphate solution onto the surface area to be tested
  4. Allow to stand for 6 minutes
  5. Observe the test area which should turn a bright copper colour
  6. Any mill scae which is on the surface will show as black spots
  7. Report and document the findings of the test
  8. Test area is to be washed and re-blasted due to contamination regardless of whether mill scale is present or not (2)

REFERENCES

1.Finishing, What exactly is Mill Scale?, Erişim tarihi: 16 Haziran 2016 http://www.finishing.com/386/57.shtml

2.Wilson, L., The Paint Inspector’s Field Guide