Meguerdijian 1
Saro Meguerdijian
Writing 340
Section 66801
Assignment: Illumin Magazine Paper
3 May 2013
Imagining Chromium: The Diversity of Applied Chromium Chemistry
Abstract
Chromium has four main abilities: protecting, oxidizing, catalyzing, and manipulating
electromagnetism. From stainless steel, to dyes and polyethylene (PE), to cassette tapes,
engineers have found a variety of applications for chromium to improve society. However,
hexavalent chromium (Cr(VI)) groundwater pollution continues to be a major public concern.
Future developments in applied chromium chemistry include innovations in PE productions,
economical treatment of Cr(VI), new chromium-containing metal alloys, and novel biomedical
implants.
Key words: chromium, hexavalent chromium, stainless steel, Nozaki-Hiyama-Kishi reaction,
Phillips catalyst, polyethylene, dyes, pigments, chromium dioxide.
Author bio:
Saro Meguerdijian is a student majoring in Chemical Engineering (Petroleum
Engineering Emphasis) with a minor in Business Law and plans to graduate in May 2015. He
independently conducted research on treating hexavalent chromium in California’s groundwater,
resulting in a potential treatment concept for which he was awarded the status of 2010 Siemens
Competition Semifinalist. His interests include: the hydrocarbon industries, applied chromium
chemistry, and computational modeling.
Email: smeguerd@usc.edu
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Introduction
Engineers often appear to be extreme specialists. From petrochemical pipe corrosion
engineers to water-soluble polymer experts with an emphasis on fraccing, if one can imagine the
engineer, he or she probably exists. Extreme specialization may be a consequence of engineers’
training and desire to dominate a niche in industry, but one should keep in mind that engineers
are more than specialists: engineers are problem solvers. Sometimes, they use the same natural
phenomenon to solve many different problems. Getting a clock to tick mechanically at fixed
intervals is not so different from making a car’s wheels spin reliably. The concept of controlled
rotation has been applied to both, albeit with widely varying results.
Similarly, applied chromium chemistry – the study and application chromium’s
properties – have snuck into nearly every aspect of life, usually by the initiative of some
inventive engineer. “Inventive” is the key word here: chromium can only do so much. It may be
extremely versatile (as the reader will soon see), but chromium has basically four main
“abilities” that engineers use: protecting, oxidizing, catalyzing, and manipulating
electromagnetism. These abilities, like the primary colors, may be limited in number but have a
virtually infinite number of degrees and variations. Engineers harness these abilities in numerous
applications to help make society safer, happier, and more economical. However, every science
has its drawbacks, and applied chromium chemistry is no exception. Hexavalent chromium is
widely useful but highly toxic, and many have suffered from excessive exposure. Fortunately,
the other abilities of chromium more than compensate for this drawback.
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Ability #1: Protecting
The modern explosion in the demand for chromium can be largely attributed to Harry
Brearley, a British metallurgist. While experimenting with gun-barrel erosion during World War
I, he discovered a type of steel that did not rust [1]. This alloy, or mixture of two or more metals,
became known as “stainless steel.” Brearley essentially created an entire industry – the stainless
steel industry – with his discovery. Society has greatly benefited from the discovery of stainless
steel, as demonstrated in the ubiquity of the alloy in cutlery, buildings, and other applications.
Stainless steel production depends on chromium for the steel’s protective properties [2], creating
a major demand for chromium. Over $2 billion of chromium products were imported and
exported by the United States in 2009, the vast majority of which is attributable to the stainless
steel industry [3]. Chromium is clearly big business, largely due to stainless steel.
The economic importance of stainless steel and its key component, chromium, is based
upon chromium’s ability to protect. Perhaps surprisingly, pure chromium is a highly reactive
metal. Chromium happens to be exceptionally “greedy” for particles called “electrons.” As a
result, chromium often grabs electrons from generally corrosive substances (a process known as
“oxidation”) and forms a thin, protective layer on its surface [2]. This layer protects the stainless
steel from corrosive materials, increasing the steel’s usable life.
The types of stainless steel include ferritic, austenitic, duplex-austenitic-ferritic, and
precipitation hardening [2]. Ferritic steels are have high chromium content and so are more
useful in applications requiring high corrosion resistance e.g. hot water tanks. Tough and
corrosion resistant, austenitic stainless steels are used in the chemical industry and home
appliances. Duplex-austenitic steel is stronger than austenitic steel and finds applications in
desalination and the petrochemical industry. Precipitation-hardening steels contain a variety of
metals, including chromium, and are known for their high strength and ability to be produced
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with precise dimensions, resulting in their use in the aerospace industry. Stainless steel has
numerous uses, all of which depend upon chromium’s ability to protect.
Chromium’s ability to protect is applied on a more personal level in chromium-alloy
implants. Chromium alloys have been used in pulmonary stents, hip implants, and orthopedic
implants [2, 4-6]. A pulmonary stent can be used to reinforce a blood vessel transplant when a
vein is used to replace an artery. Since veins are less able to resist high blood pressures than
arteries, a chromium-cobalt mesh can be used to support the vein without contacting the vein’s
interior [5]. Another type of stent designed by Boston Scientific consists of a platinumchromium tube that medicates the patient as blood flows [4]. Called a “drug-eluting stent,” these
tubes allow patients to lead normal lives during treatment, restoring patients’ sense of
independence while ensuring that they continually have a proper level of drugs in their blood.
The biocompatibility which permits these innovations emerges from chromium’s protective
ability. Still, there is considerable controversy over metal-on-metal bone implants because of
their ability to release metal ions into a patient’s bloodstream [6]. While the result on patient’s
health is not well understood, litigation has resulted from perceived dangers associated with
emissions of chromium and other metals from implants [6].
While implants are often made of chromium-containing alloys, other applications require
coating a metal with a layer of chromium. A chromium-coated metal (called a “plated metal”) is
more resistant to wear and corrosion, allowing the metal to be used for longer periods of time
and under harsh conditions [7]. Chromium is generally applied to a metal surface through a
process called “electroplating” in which an electric current is passed through a metallic object
that one wishes to coat. The metallic object is first submerged in a liquid that contains chromium
ions (chromium atoms that have lost electrons; usually hexavalent chromium [7]). When the
electric current passes through the metal, it gives electrons to the chromium ions in the liquid.
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Upon accepting the electrons, the chromium ions become chromium metal and plate onto the
metal. A basic visual introduction about chromium electroplating can be found at [8]. The
resulting plated metal serves two main purposes in society: decorative and functional [7].
Decorative chromium electroplating produces the familiar lustrous chromium finish and is meant
primarily for viewer’s pleasure. On the other hand, functional chromium electroplating (“hard
chrome”) is used by industry for protecting equipment [7].
Ability #2: Oxidizing
As discussed previously, chromium is a superb electron thief (“oxidizing agent”). The
ability to protect is itself based on the ability to oxidize. Because of chromium’s wide use as an
oxidizing agent for organic material, a separation section is included herein.
Chromium’s oxidizing ability finds use in the preparation of various organic compounds.
Organic compounds are those that contain the element carbon. These compounds consist of
atoms bound to one another via bonds. A group of atoms bonded together is called a molecule.
Chromium’s special role is in adding oxygen to organic molecules. At first glance, this may
appear to be a banal ability – after all, what does adding an oxygen atom to a group of atoms
based on carbon-carbon bonds achieve? A great deal! Many dyes are based on anthraquinone, an
intermediate vital in the dye industry [9]. See Figure 1 to see how a special type of chromium
called “hexavalent chromium” (Cr(VI)) takes hydrogen atoms from anthracene and replaces
them with oxygen to make anthraquinone:
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Figure 1. Hexavalent chromium (Cr(VI)) oxidizes anthracene to form anthraquinone, a
valuable base chemical for dye creation. Note that the Cr(VI) is very selective about which
hydrogen atoms (H) it replaces with oxygen atoms (O). The unmarked intersections of lines
represent carbon atoms, and the lines themselves represent single or double bonds.
Cr(VI) oxidizes anthracene very selectively and with high conversion, which essentially means
that the production of anthraquinone from anthracene is highly efficient [9]. Interestingly, the
same oxidizing ability that allows Cr(VI) to facilitate this reaction also helps in wood
preservation [10]. The wood industry used Cr(VI) oxide, a powerful oxidant, to protect wood
from fungi and insects. A Cr(VI) treatment of wood may also make wood more fire resistant[11].
While Cr(VI) has found wide use in industry, from electroplating to dyes to wood
preservation, Cr(VI) has also caused widespread fear because of its high toxicity, which is
believed to result from its oxidizing ability [12] . In an episode publicized by the movie Erin
Brockovich, the residents of Hinkley, California were exposed to Cr(VI) by poor disposal
practices at Pacific Gas and Electric (PG&E) [13]. Cr(VI) had been used in the company’s
cooling towers during the 1950s and 1960s and, after disposal in unlined ponds, contaminated
the city’s groundwater. After much litigation, PG&E settled for $333 million with the residents
in 1996 [13]. Many residents had become ill due to the Cr(VI) and, tragically, much Cr(VI) still
remains in the city’s groundwater.
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The popularization of the incident and the State of California’s sense of responsibility for
protecting its citizens led to the establishment of a Public Health Goal for Cr(VI) in groundwater
[12]. Set at 0.02 parts per billion in drinking water, the standard is intended to specify an amount
of hexavalent chromium exposure that will not result in harm. Hopefully, further action on this
standard will help protect the people of California from further exposure and motivate national
discussions between government, industry, and watchdog organizations over how to safely and
economically implement Cr(VI) regulations. With the litigation over Sacramento’s
groundwater’s Cr(VI) contamination just ensuing [14], effective public-private partnerships will
be necessary.
While Cr(VI) contamination is a concern domestically, it is also a concern
internationally. Kellogg, Brown and Root (KBR) was hired to repair the Qarmat Ali water
treatment plant in Iraq [15]. Sodium dichromate, a form of Cr(VI), had been used to maintain the
plant’s pipes. KBR was later found to have knowingly exposed U.S. soldiers in Iraq to the
sodium dichromate, potentially causing numerous illnesses. As a result, KBR was ordered to pay
$85 million to those soldiers as compensation for their exposure. Litigation involving Cr(VI)
exposure often is often expensive because of Cr(VI)’s extreme toxicity.
A short note is necessary regarding the biological role of trivalent chromium (Cr(III)).
Unlike Cr(VI), Cr(III) is considered an essential trace metal, though this has been called into
question [16]. A hypothetical blood sugar regulator known as glucose tolerance factor (GTF) is
believed to include Cr(III), which is the reason why Cr(III) is currently classified an essential
trace metal. However, Cr(III) has not been found to perform any specific biological function in
humans and has even been suspected of being carcinogenic [16]. The biological role of Cr(III) is
highly uncertain but worth investigating due to its potential impact on human health.
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Ability #3: Catalyzing
Chromium’s third major ability is that of catalysis. Certain forms of chromium can make
reactions occur more quickly and under different conditions than they normally would. For
example, chromia (chromium (III) oxide) does exceptionally well at removing hydrogen atoms
from ethane to form ethylene (see Figure 2) [9]:
Figure 2. Dehydrogenation of ethane to ethylene using a chromium catalyst. Through an
intricate mechanism not shown here, the chromium catalyst causes the removal of two
hydrogen atoms each bound to one of the carbon atoms, resulting in a double bond
between the two carbon atoms.
Ethylene and other carbon chains with double bonds are valuable industrial feeds, finding use in
polymer and high-octane gasoline production [9]. Among the many reactions catalyzed by
chromium, two merit special attention here: the polymerization of polyethylene (PE) and the
Nozaki-Hiyama-Kishi reaction. These reactions were selected because PE production has major
economic consequences [9], and the Nozaki-Hiyama-Kishi reaction presents a significantly
different view of what is possible using chromium.
The polymerization of PE essentially consists of joining a large number of ethylene
molecules into a long chain. This chain of molecules is called a polymer [17]. PE-type polymers
have a wide variety of uses. A non-exhaustive list: heavy-duty sacks, packing materials,
construction, gellants for mineral oils, lubrication, toys, electrical fittings, bottles, wire and cable
coating, and ink tubes for ballpoint pens [18]. Chromia-on-silica (silicon dioxide, or SiO2)
catalysts were found to be an effective catalyst for joining ethylene molecules to one another to
form long chains of PE [17]. These catalysts can create PE of high densities (known as high
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density polyethylene, or HDPE) [17] and low densities while maintaining a linear structure
(linear low density polyethylene, or LLDPE) [9]. However, chromium catalysts used in the
production of PE are known to give varying results dependent on minor differences in their
preparation [9]. While this may initially seem like a disadvantage, this high sensitivity is actually
a major advantage. It means that the properties of PE produced can be changed simply by
modifying the chromium catalyst, without changing the raw materials. This allows for
tremendous leverage in creating niche grades of PE.
While PE polymerization provides exceptional value to society, the Nozaki-HiyamaKishi (NHK) reaction finds a more specialized role in the laboratory. The NHK reaction
essentially consists of chromium (II) chloride acting in conjunction with nickel (II) chloride to
attach a group of atoms to a special group containing a doubly-bonded oxygen (known as a
ketone)[19]. See Figure 3:
Figure 3. A highly simplified presentation of the Nozaki-Hiyama-Kishi reaction. Each R
represents a different carbon-containing group of atoms. X represents chlorine, bromine,
or iodine. The reader should focus on the transfer of the R1 group to the center carbon of
the other molecule (ketone) and the addition of a hydrogen atom to the oxygen atom. The
transfer of the R1 group is the main objective of the Nozaki-Hiyama-Kishi reaction.
The NHK reaction in-and-of-itself has limited utility, but it is important as a tool for organic
synthesis. Organic chemistry is essentially a “game” for scientists and engineers to use reactions
such as this one to discover and manufacture new compounds, such as drugs or environmentallysafe chemicals. The NHK demonstrates chromium’s ability to be involved in this quest for new
chemical discovery in a way other than polymerizing PE.
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Ability #4: Manipulating Electromagnetism
Chromium’s fourth main ability consists of manipulating electromagnetic radiation (e.g.
light) and magnetic fields. Chromium dioxide, a black, magnetic pigment, found widespread use
for its ability to manipulate magnetic fields [10]. Chromium dioxide began to be used in tape
recording in the late 1960s [20]. Chromium-dioxide pigments are less noisy than other pigments
in cassette tapes and are the only pigments (as of 1990), that allow for thermomagnetic
duplication [20]. Heating the chromium dioxide crystals in the tapes causes them to lose their
magnetic orientation [21]. Once this occurs, the tapes can be magnetized in the desired format,
permitting rapid duplication [21]. This became of commercial importance when bulk duplication
of videotapes was needed prior to the CD and DVD [21].
Chromium’s ability to manipulate electromagnetic radiation, such as light, has found use
in chromium-based dyes and pigments. Fundamentally, a dye or pigment absorbs certain
wavelengths of light while reflecting others. As might be inferred by its name [22], chromium
forms numerous colorful compounds. The colors of chromium pigments include various types of
black, green, red, and yellow [10]. Notably, hydrated chromium oxide green reflects light in the
infrared region, giving it importance in military camouflage paints [10]. One can essentially use
chromium compounds to selectively absorb and reflect different wavelengths of light simply by
modifying the compounds. Still, despite chromium’s utility in the creation of novel dyes and
pigments, concern over the toxicity of hexavalent chromium may cause a decline in chromebased dye and pigment use.
Solar cells are a promising area for chromium’s ability to manipulate light. Chromium
has a history of use in manipulating light energy; in fact, chromium was used in early lasers and
masers (which are like lasers, except that they emit microwaves instead of light) [23]. In meeting
the National Academy of Engineering’s (NAE) Grand Challenge of making solar energy
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economical, lanthanum strontium chromium oxide (LSCO) and “black” chromium plating
emerge as potential players [7, 24, 25]. Many metals that could be useful in solar cells are
damaged via reactions with oxygen in air [26]. On the other hand, black chromium deposits and
oxides such as LSCO are stable in air and so can be used in solar cells with minimum damage.
Solar cells based on LSCO or black chromium could help make solar cells more economical.
Future Progress
The engineering applications of chromium are primarily limited by ingenuity. Although a
few main abilities of chromium have been exploited in a variety of applications, the fundamental
problems of applied chromium chemistry still remain unsolved. From the healthcare and
environmental perspective, these problems include the bioactivity of trivalent chromium and
effective remediation of chromium pollution. Trivalent chromium’s role or effect in the human
body, if any, still remains unknown. An unequivocal answer to this long unsolved problem
would have major immediate impacts in groundwater regulations and in medical
recommendations concerning chromium.
With respect to hexavalent chromium, the main problem is pecuniary: protecting the
public from hexavalent chromium economically. If California’s Public Health Goal of 0.02 parts
per billion for hexavalent chromium in drinking water becomes a legally enforceable limit,
California’s water utilities’ compliance costs could rise significantly. Similarly, if federal levels
of permissible hexavalent chromium in drinking water were reduced dramatically, water utilities
nationwide would suffer greater compliance costs. Clearly, the safety of the public’s drinking
water is vital, but the challenge will be to protect the public without placing excessive pressure
on water utilities.
In addition to healthcare and environmental issues, other important chromium-related
problems include determining the oxidation number of chromium on the chromia-on-silica
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catalyst for the polymerization of polyethylene [9]. The scientific literature is rife with debate
over this issue. A better understanding of chromia-on-silica could facilitate the discovery of new
grades of PE or cost-saving methods of PE production. Further research could also lead to a
better understanding of the properties of chromium, leading to new applications heretofore
unimagined. With an increased understanding of chromium’s most basic behavior, chromium’s
applicability could be extended.
Additional areas of innovation in applied chromium chemistry will include the creation of
dyes and pigments, medical implants, and extraterrestrial hexavalent chromium treatment. Dyes
and pigments can be created to absorb various wavelengths or combinations of wavelengths.
Chromium-based dyes are especially versatile in this respect. With growth in nanotechnology,
engineers should explore methods to selectively control light absorption from the infrared to
ultraviolet region using chromium nanoparticles. Because these nanoparticles can have sizes
comparable to the wavelength of visible light, they may be able to manipulate light in novel
ways. With the need for stronger and safer medical implants, chromium alloys should be
thoroughly explored. Chromium-cobalt is currently “the standard” in biocompatible implants,
though it is controversial in some instances [2,6]. However, Platinum-Chromium and other
alloys may be able to provide alternatives that will help patients live healthier lives. The
biocompatibility of many chromium alloys, stemming from their corrosion resistance, will
ensure that chromium plays a major role in the field of metal implants. Non-metal implants may
supplant chromium alloys if they are found to be more cost effective. However, given the high
strength of chromium alloys, this will be challenging.
Hexavalent chromium treatment costs may also become an important factor in
extraterrestrial colonization. A report found that hexavalent chromium may be present in trace
amounts in Martian dust [27]. If trace amounts of Cr(VI) are found in Martian dust, any human
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habitation of Mars would need to be strictly controlled for Cr(VI) exposure. Depending on the
concentration of Cr(VI) in Martian dust, individuals inhabiting Mars may be burned simply via
contact with the dust and potentially develop cancers. This threat could potentially thwart the
creation of a long-term Mars colony for a considerable period of time. Assuming that Mars
colonization would entail the creation of closed buildings for human habitation, highly efficient
and effective air filtration and dust removal systems would be needed. Therefore, combining
efforts toward creating air filtration systems for industrial settings and for Martian bases would
be mutually beneficial.
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Works Cited
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