Mill scale
Encyclopedia
Mill scale, often shortened to just scale, is the flaky surface of hot rolled
steel
, iron oxide
s consisting of Iron(II,III) oxide
, hematite
, and magnetite
.
Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in rolling mills. Mill scale is composed of iron oxides mostly ferric and is bluish black in color. It is usually less than 1 mm (0.0393700787401575 in) thick and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating.
Because it is electro-chemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of steel exposed at the break. Mill scale is thus a boon for a while until its coating breaks due to handling of the steel product or due to any other mechanical cause.
Mill scale is a nuisance when the steel is to be processed. Any paint applied over it is wasted since it will come off with the scale as moisture laden air get under it. Thus mill scale can be removed from steel surfaces by flame cleaning
, pickling
, or abrasive blasting
, which are all tedious operations that waste energy. This is why shipbuilders used to leave steel delivered freshly rolled from mills out in the open to allow it to 'weather' till most of the scale fell off due to atmospheric action. Nowadays most steels mills can supply their produce with mill scale removed and steel coated with shop primers
over which welding can be done safely.
Rolling (metalworking)
In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling...
steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s consisting of Iron(II,III) oxide
Iron(II,III) oxide
Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...
, hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
, and magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
.
Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in rolling mills. Mill scale is composed of iron oxides mostly ferric and is bluish black in color. It is usually less than 1 mm (0.0393700787401575 in) thick and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating.
Because it is electro-chemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of steel exposed at the break. Mill scale is thus a boon for a while until its coating breaks due to handling of the steel product or due to any other mechanical cause.
Mill scale is a nuisance when the steel is to be processed. Any paint applied over it is wasted since it will come off with the scale as moisture laden air get under it. Thus mill scale can be removed from steel surfaces by flame cleaning
Flame cleaning
Flame cleaning, also known as flame gouging, is the process of cleaning a structural steel surface by passing an intensely hot acetylene-oxygen flame over it...
, pickling
Pickling (metal)
Tarnision is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called Tarnision liquor, which contains strong acids, is used to remove the surface impurities...
, or abrasive blasting
Abrasive blasting
Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically air, or a centrifugal wheel is used to...
, which are all tedious operations that waste energy. This is why shipbuilders used to leave steel delivered freshly rolled from mills out in the open to allow it to 'weather' till most of the scale fell off due to atmospheric action. Nowadays most steels mills can supply their produce with mill scale removed and steel coated with shop primers
Primer (paint)
A primer is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.-When primers are used:...
over which welding can be done safely.