Listing description
Chromium (
/ˈkroʊmiəm/ KROH-mee-əm) is a chemical
element which has the symbol Cr and atomic
number 24, first element in Group
6. It is a steely-gray, lustrous, hard metal that takes a
high polish and has a high melting point. It is also odorless, tasteless, and
malleable. The name of the element is derived from the Greek word
"chrōma" (χρώμα), meaning color, because many of its compounds are intensely colored. It
was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead
chromate) in 1797. Crocoite was used as a pigment, and after the
discovery that the mineral chromite also contains chromium this latter mineral
was used to produce pigments as well.
Detaild description
Characteristics
Physical
Chromium is remarkable for its
magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it
transforms into a paramagnetic state.[1]
Passivation
Chromium metal left standing in air
is passivated by oxygen, forming a thin protective oxide surface layer.
This layer is a spinel structure only a few atoms thick. It is very dense, and
prevents the diffusion of oxygen into the underlying material. This barrier is
in contrast to iron or plain carbon steels, where the oxygen migrates into the
underlying material and causes rusting.[2] The passivation can be enhanced by short contact with oxidizing acids like nitric
acid. Passivated chromium is stable
against acids. The opposite effect can be achieved by treatment with a strong
reducing reactant that destroys the protective oxide layer on the metal.
Chromium metal treated in this way readily dissolves in weak acids.[3]
Chromium, unlike metals such as iron
and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement,
reacting with nitrogen from air and forming brittle nitrides at the high
temperatures necessary to work the metal parts.[4]
Occurrence
Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm.[5] Chromium compounds are found in the environment, due to erosion
of chromium-containing rocks and can be distributed by volcanic eruptions. The
concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5
to 800 µg/liter, and in rivers and lakes 26 µg/liter to
5.2 mg/liter.[6]
Chromium is mined as chromite
(FeCr2O4) ore.
The relation between Cr(III) and
Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III)
is the dominating species,[6] although in some areas the ground water can contain up to
39 µg of total chromium of which 30 µg is present as Cr(VI).[12]
History
Weapons found in burial pits dating
from the late 3rd century BC Qin
Dynasty of the Terracotta Army near Xi'an, China have been analyzed by archaeologists. Although buried more
than 2,000 years ago, the ancient bronze tips of crossbow bolts and swords found at the site showed no sign of
corrosion, because the bronze was coated with chromium.[20]
.
Applications
Metallurgy
The strengthening effect of forming
stable metal carbides at the grain boundaries and the strong increase in
corrosion resistance made chromium an important alloying material for steel.
The high
speed tool steels contain
between 3 and 5% chromium. Stainless
steel, the main corrosion-proof metal
alloy, is formed when chromium is added to iron in sufficient, usually more than 11% concentration. For its
formation, ferrochromium is added to the molten iron. Also nickel-based alloys
increase in strength due to the formation of discrete, stable metal carbide
particles at the grain boundaries. For example, Inconel 718 contains 18.6%
chromium. Because of the excellent high temperature properties of these nickel superalloys,
they are used in jet
engines and gas
turbines in lieu of common structural
materials.[27]
The relative high hardness and
corrosion resistance of unalloyed chromium makes it a good surface coating,
being still the most "popular" metal coating with unbeatable combined
durability. A thin layer of chromium is deposited on pretreated metallic
surfaces by electroplating techniques. There are two deposition methods: Thin, below
1 µm thickness, layers are deposited by chrome plating, and are used for decorative surfaces. If wear-resistant
surfaces are needed then thicker chromium layers of up to mm thickness are
deposited. Both methods normally use acidic chromate or dichromate
solutions. To prevent the energy consuming change in oxidation state, the use
of Chromium(III) sulfate is under development, but for most applications, the
established process is used.[24]
Dye
and pigment
The mineral crocoite
(lead
chromate PbCrO4) was used as a
yellow pigment shortly after its discovery. After a synthesis method became
available starting from the more abundant chromite, Chrome yellow was, together with cadmium
yellow, one of the most used yellow
pigments. The pigment does not degrade in the light and has a strong color. The
signaling effect of yellow was used for school buses in the United States and
for Postal Service (for example Deutsche
Post) in Europe. The use of chrome
yellow declined due to environmental and safety concerns and was substituted by
organic pigments or other lead-free alternatives.[32] Other pigments based on chromium are, for example, the
bright red pigment Chrome red, which is a basic lead chromate (PbCrO4•Pb(OH)2).[32] Chrome green is a mixture of Prussian blue and chrome
yellow, while the Chrome oxide green is Chromium(III) oxide.[32]
Glass is colored green by the
addition of chromium(III) oxide. This is similar to emerald,
which is also colored by chromium.[33] A red color is achieved by doping chromium(III) into the
crystals of corundum, which are then called ruby. Therefore, chromium is used in
producing synthetic rubies.[34]
Tanning
Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking
the collagen fibers within the leather.[36] Chromium tanned leather can contain between 4 and 5% of
chromium, which is tightly bound to the proteins.[7] Better management of chromium in tanning industry such as
recovery and reuse, direct/indirect recycling,[37] use of less chrome or chrome less tanning are practised to
better manage chromium in tanning.
Refractory
material
The high heat resistivity and high melting
point makes chromite and chromium(III) oxide a material for high temperature
refractory applications, like blast
furnaces, cement kilns,
molds for the firing of bricks and as foundry sands for the casting of metals. In these applications, the refractory materials
are made from mixtures of chromite and magnesite. The use is declining because
of the environmental regulations due to the possibility of the formation of
chromium(VI).[26]
Other
use
- Chromium(III) oxide is a metal polish known as green rouge.
- Chromic
acid is a powerful oxidizing agent
and is a useful compound for cleaning laboratory glassware of any trace of
organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to
wash the apparatus. Sodium dichromate is sometimes used because of its higher solubility (5
g/100 ml vs. 20 g/100 ml respectively). Potassium dichromate is a chemical
reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
PRICE
$2.43/KG
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