Material of the month – Chromium
Anna Ploszajski looks at the metal that gives American school buses their unique yellow shade and Harley Davidson engines their flawless sheen – chromium.
In 1798, the French Chemist Louis Vauquelin first isolated chromium metal by mixing its ore, crocoite, with hydrochloric acid to produce chromium trioxide (CrO3), and then heated it with carbon. He named the metal after the Greek meaning for colour, chroma, due to the colours of its associated compounds.
The vivid hues of chromium-based materials are exploited to this day. Crocoite, or lead chromate (PbCrO4), is used to make chrome yellow pigment, famously giving American school buses and European postal vans their characteristic canary yellow shade. These pigments are popular because they don’t photodegrade, although paints do experience some gradual darkening
over time due to the formation of chromium(III) oxide. Similarly, basic lead chromate (PbCrO4) gives a bright chrome red colour, although its use has been limited due to its toxicity.
The mineral corundum (aluminium oxide) is the colourless component of many precious stones. Doping trace amounts of Cr3+ ions onto Al3+ sites in corundum gives the crystal a deep red colour in rubies. This phenomenon occurs when the violet and yellow-green portions of the incoming light are absorbed by the 3D electrons in the Cr3+ ions, meaning that only the red and a small amount of blue light is transmitted.
The doping of chromium in corundum occurs naturally, but rubies can also be produced synthetically by growing artificial corundum crystals which are doped with Cr3+ cations in the laboratory. One such synthetic crystal was the crucial component in the first laser, made in 1960, which made use of stimulated emission of photons from the chromium ions in the crystal.
As well as these reds, oranges and yellows of the lead chromates and trivalent chromium ions, chromium oxides are used to give a green colour in glassmaking and ceramic glazes. They are the main component in infrared reflective paints which are painted onto military vehicles for stealth – the paint has the same infared reflectance as green leaves.
The oxygen in air, which forms a few-atom-thick protective oxide surface layer with a spinel structure, passivates chromium. The density of this layer prevents the diffusion of further oxygen into the metal underneath, and even renders chromium resistant to hydrogen embrittlement, unlike iron and nickel.
It is this layer that makes chromium-plating so protective and, combined with hardness and a highly reflective sheen when polished, makes for a very decorative coating on car parts and plumbing fixtures. The chromium is electroplated onto metallic surfaces from a solution of chromium salt. A renowned example of this is in Harley Davidson motorbikes, whose engine and exhaust pipes are beautifully designed using chrome plating.
Chromium also features on motorcyclists’ backs, since chromium salts are used in tanning leather. When leather is dipped in chromium salt solution, the Cr3+ ions crosslink the collagen fibres in the hide, stabilising the molecular structure and making it weather resistant, stronger and elastic. A piece of leather tanned in this way contains 4–5% chromium.
Although chrome plating is a symbol of high-tech modernism, using chromium as an anti-corrosion coating dates back millennia. The oldest known example is the tips of the metallic weapons found with the Terracotta Army, dating back to the Chinese Qin dynasty, more than 2,000 years ago. However, it is not believed that the inclusion of chromium was necessarily deliberate, but that it was present in the original ore.
Chromium is the 22nd most abundant element in the Earth’s crust, and most commonly occurs as chromite ore (iron chromium oxide, FeCr2O4), mined in South Africa, Kazakhstan and India. The ore is refined into either ferrochromium or metallic chromium in an aluminothermic reaction or a two-step roasting and leaching process respectively.
Surely the most industrially indispensable application of chromium is in stainless steel, an industry that accounts for the use of around 85% of chromium currently produced. The advantages brought to steel by adding 11–20% chromium are twofold. Firstly, it enhances the strength of the metal by forming stable metal carbides at the grain boundaries, an effect which is also exploited in nickel-based superalloys. Secondly, as the name suggests, chromium bestows unrivalled corrosion resistance to its alloys.
The discovery of stainless steel
The birthplace of stainless steel is Sheffield, UK – its inventor Harry Brearley having produced it first in 1913. Brearley worked for the Brown Firth Laboratories, which were seeking a solution to the erosion of the interior gun barrels for the military war efforts. He chose to investigate the addition of chromium in steels since it was known to increase the metal’s melting temperature. As any undergraduate materials science student these days will know, examining the grain structure of metals involves the time-consuming grinding and polishing of samples, followed by a chemical etch. Brearley noticed that his chromium steels were particularly resistant to the chemical etch, and thus realised the anti-corrosion properties of his materials.
The original material Brearley produced lacked the machinability required, so the Brown Firth company was not initially interested in his work, and Brearley filed the patents in his own name. Once it was discovered that adding nickel to the stainless steel made it much more machinable, the business minds at the Brown Firth Laboratories pricked up their ears, but it was too late for the intellectual property, which brought about a dispute over the rights to the patents. Brearley left Brown Firth Laboratories in 1915 due to these disagreements, but his successor, William Herbert Hatfield, continued the research. The development of ‘18/8’ stainless steel, containing 18wt% chromium and 8wt% nickel is attributed to Hatfield, and is probably the most widely used stainless steel alloy today.
Meanwhile, Brearley took his material to R.F. Mosely and Co, a local cutlery producer in Sheffield – it recognised the potential of the metal in its industry, and became the world’s first global producer of stainless steel cutlery under the brand name Rusnorstain.
The two most common forms of chromium are trivalent (Cr3+) and hexavalent (Cr6+). In human biology, the difference between them is a matter of life or death. Trivalent chromium is generally considered to be completely benign, neither providing any biological role, nor causing us harm. In 2014, the European Food Safety Authority removed it from the list of essential elements. Nevertheless, the activity of trivalent chromium in the body is a matter of controversy, and chromium-containing dietary supplements exist and some studies protest their health benefits for diabetes, lower blood lipid levels, weight loss and body composition. However, these studies are altogether inconclusive, and there is insufficient data to confirm the benefits or harm provided by chromium-containing dietary supplements.
But for hexavalent chromium the case is clear – it is well known to be acutely toxic and mutagenic when inhaled because of its strongly oxidising nature. When entering the blood stream, hexavalent chromium can cause damage to the kidneys, liver and blood cells. The most famous case of the havoc wreaked by hexavalent chromium was Erin Brockovich’s case against Pacific Gas and Electric (PG&E), which was settled in 1996 for US$333m. PG&E operates a natural gas compression station in Hinkley, California. The compression of the gas generates a lot of heat, so the station uses water to cool the gas before subsequent transmission. Adding hexavalent chromium to the cooling water prevented corrosion in the machinery, but the contaminated water was stored in unlined ponds in the vicinity of the station, which percolated into the local groundwater. Between 1952–1966, the local area was poisoned by this toxic waste, despite the US$700m clean-up efforts from PG&E. By 2013, the toxic plume was believed to be more than seven miles long and PG&E estimates that it could take another 40 years before the area is safe to live in.
Chromium is a multi-faceted, multicoloured material. From the humble kitchen knife and giant structures in remote oceans to the Chrysler building in New York and the jet engine that gets you there, chromium’s crucial role in stainless steels has truly transformed the way we live and the world we live in.