The Self-Healing Ability of Cerium Coating (Chromium Substitute)

In recent years, several research efforts are targeted on the utilization of rare earth elements, especially on cerium thin film coatings. Cerium is a soft, ductile and silvery-white metal that tarnishes when exposed to air, and it so soft that can be cut with a knife. Cerium has no biological role and is not very toxic. Many surface treatments, like sol-gel, chemical vapor deposition (CVD) and physical vapor deposition (PVD) technique, based on the use of cerium and cerium compounds have been investigated because of their low toxicity. In other words, consumption or inhalation of those compounds is not considered harmful to health.

Cerium compound physical vapor deposition permits to improve corrosion protection performance of the surface it is deposited on. The composition of the films has an impact on the corrosion properties of the cerium-based layer. In general, the coatings obtained by PVD are composed of Ce compound in trivalent or tetravalent states. The ratio between these 2 oxidization states is strongly depending on the oxidizing ability of the medium. However, no clear correlation between the Ce oxidation state and corrosion properties was found nowadays.

cerium film
cerium film

What’s more, these cerium coatings have an active mechanism similar to that observed for chromate coatings that they both have the amazing self-healing ability when damage occurs. Chromate coatings have the self-healing properties because of the presence of unreacted Cr6+ ions that are able to migrate to the exposed metal (for example a scratch) and can be further reduced to create a Cr3+ based compound that seals the scratch or the defect. However, the chromate compounds are extremely toxic and carcinogenic. Since cerium is not toxic, it is a perfect substitute for chromate. When it comes to cerium, the contact between a CeO2 film and solution induces the formation of Ce(OH)22+ ions. The existence of oxidizable metal would reduce these ions into Ce3+. Then the precipitation of trivalent cerium oxide occurs; it can be enhanced by the local increase of alkalinity. Therefore, this precipitated oxide seals the film and decreases the corrosion rate of metal. Since cerium is not toxic, it is a perfect substitute for chromate.

In conclusion, cerium is good, but some people would concern their price. Is rare earth element—Cerium—very expensive? The answer is not, actually Cerium is one of the least expensive rare earths and is the major component of “mischmetal”. So don’t care too much about the price.

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Related Blog: How was cerium discovered? | History of Cerium

How was cerium discovered? | History of Cerium

Cerium is the most abundant rare earth elements. It is a silvery gray active metal, whose powder is easily oxidized in the air and soluble in acid. Cerium has been widely used in the automotive industry as a catalyst to reduce emission, and in glass industry as glass polishing materials. Cerium sputtering target is an important material in optical coating.

Discovery History

In 1803, when the German chemist Martin Heinrich Klaproth analyzed an ore, he determined the existence of a new metal oxide and called it ochra (ocha-colored soil). and the ore ochroite because it appears to be ochre when burning.

In the same year, the Swedish chemist Jöns Jakob Berzelius and the Swedish mineralogist Wilhelm Hisinger also analyzed the same new metal oxide, which is different from yttrium. Yttrium is dissolved in ammonium carbonate solution and appears red when burning on gas flame. However, this metal oxide is insoluble in ammonium carbonate solution and does not exhibit characteristic flame color when burning.

The ore is thus called ceria (bauxite), and the element is named cerium to commemorate the discovery of an asteroid, Ceres.

Discovery of cerium

Three Early Applications of Cerium

Carl F. Auer von Welsbach
Carl Auer von Welsbach

Eighty-three years after the discovery of “cerium”, in 1886, the Austrian Carl Auer von Welsbach found the first application of cerium (also rare earth) as a luminescent enhancer for steam hoods. He found that heating 99% thorium oxide and 1% cerium oxide would give off a strong light, so cerium used in coal gas lamp gauze can greatly increase the brightness of the gas lamp. The gas lamps in Europe, where electric lights were not yet popular, were the main source of lighting and were essential for industrial production, commerce, and life.

After the First World War, electric lights gradually replaced gas lamps, but cerium continued to open up new applications. In 1903, Welsbach once again discovered the second largest use of cerium. He found that cerium iron alloys can generate sparks under mechanical friction and therefore can be used to make flints. This classic use of cerium has been around for 100 years. Everyone who smokes knows that a lighter uses a flintstone, but many people they that it is cerium that brings fire to people.

cerium arc carbon rods
cerium arc carbon rods

In 1910, the third important application of cerium was discovered for arc carbon rods in searchlights and film projectors. Similar to the steam cover, cerium can improve the efficiency of visible light conversion. Searchlights were once an important tool in war air defense. Arc carbon rods have also been an indispensable source of light for filming.

Modern Applications of Cerium

Since the 1930s, cerium oxide has been used as a glass decolorizer, clarifier, colorant, and abrasive polishing agent.

As a chemical decolorizer and clarifier, cerium oxide can replace the highly toxic white magnetic (oxidation) to reduce operational and environmental pollution.

The use of cerium titanium yellow pigment as a glass colorant produces a beautiful bright yellow art glass.

Cerium oxide as a main component to manufacture various specifications of polishing powder has completely replaced iron red polishing powder, greatly improving polishing efficiency and polishing quality.

As a glass additive, cerium can absorb ultraviolet light and infrared rays and thus has been widely used in automotive glass. It not only protects against UV rays but also reduces the temperature inside the car, thus saving air conditioning power.

cerium polishing powder
cerium polishing powder

This history column aims at introducing the history of different metal elements. If you are a metal lover or history lover, you can follow our website. For previous posts of metal history, you can look them up in the “history” category.

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Short introduction to the element: Scandium

SAM®Scandium was first discovered by Lars Nilson in 1879. The origin of the name scandium comes from the Latin word ‘scandia’ meaning Scandinavia. It is a bright, silvery-white metal with active chemical properties that it easily oxidizes in air and reacts strongly with water. It has many of the characteristics of the rare earth elements, particularly yttrium.

In absolute terms, however, scandium is not rare. Scandium is abundant in minerals that it is found in concentrated amounts in the minerals euxenite, gadolinite and thortveitite; however, most of them existed as the form of scandium oxide (Sc2O3); thus due to the difficulties in the preparation of metallic scandium, global trade of the pure metal Scandium is very limited.

Scandium is usually alloyed with aluminum. Aluminum scandium alloys are used in the aerospace industry and other applications such as bicycle frames, fishing rods, golf iron shafts and baseball bats, etc. When used as an alloying element, adding a small amount of scandium to the aluminum alloy can promote grain refinement and increase the recrystallization temperature from 250 ° C to 280 ° C. Scandium is a strong grain refiner and an effective recrystallization inhibitor for aluminum alloys. It has a significant effect on the structure and properties of the alloy, and greatly improves the strength, hardness and corrosion resistance of the alloy.

Aluminum Scandium alloy

In addition to scandium alloys, garnets containing scandium are used as gain media in lasers, including those used in dental surgery, and scandium-stabilized zirconia has been recognized as a high-efficiency electrolyte in solid oxide fuel cells. Finally, scandium oxide is used in metal-halide lamps that are used to produce high-intensity white light that resembles sunlight.

Basic specification of scandium

Symbol: Sc
Atomic Number: 21
Atomic Weight: 44.95591
Color: silvery white
Other Names: Skandium, Skandij, Scandio
Melting Point: 1541 °C, 2806 °F, 1814 K
Boiling Point: 2836 °C, 5136 °F, 3109 K
Density: 2.985 g·cm3
Thermal Conductivity: 15.8 W·m-1·K-1

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An overview of rare earth element

The content of rare earth elements in the earth’s crust is not that scarce as the name suggests. The total Clark value of rare earth is 234.51%, which is more than common elemental copper (Clarke value 10%), zinc (Clarke value 5%), tin (Clark value 4%) and cobalt (Clarke value is 3%). However…

Rare earth definition

Rare earth contains 17 metal elements of lanthanides as well as scandium and yttrium. Scandium and yttrium are included in rare earth element because they are often symbiotic with lanthanide elements in mineral deposits, thus have similar chemical properties.

rare earth element

Discovery of Yttrium

There are 250 kinds of rare earth minerals in nature. The first to discover rare earths element was the Finnish chemist John Gadolin, who separated the rare earth element yttrium (Y) from a bituminous heavy ore (Yttria, Y2O3) in 1794.

Origin of the name

There were few rare earth minerals discovered in the 18th century, and using  chemical methods can only a small amount of water-insoluble oxides could be produced. At that time, people often referred to water-insoluble solid oxides as earth. For example, aluminum was called “ceramic earth” and calcium oxide was called “alkaline earth”. At that time, rare earths are generally separated as oxides, and because of the small quantity, they were thus named Rare Earth (RE or R).

Rare earth is not rare

The content of rare earth elements in the earth’s crust is not that scarce as the name suggests. The total Clark value of rare earth is 234.51%, which is more than common elemental copper (Clarke value 10%), zinc (Clarke value 5%), tin (Clark value 4%) and cobalt (Clarke value is 3%). However, the distribution of rare earth elements is too scattered, and these seventeen elements always exist at the same time, so the total purity is not high. In general, minerals containing 10% rare earths can be called rare earth-rich ores; pure rare earth products are expensive due to their similar properties and difficulty in extraction.

Physicochemical property

  1. Lack of sulfides and sulfates, indicating that the rare earth elements have oxytropism;
  2. The silicate of rare earth is mainly island-shaped, without layered, frame-like and chain-like structures;
  3. Some rare earth minerals (especially complex oxides and silicates) exhibit an amorphous state;
  4. The rare earth minerals mainly include silicates and oxides in magmatic rocks and pegmatites, and fluorocarbonates and phosphates in hydrothermal deposits and weathering crust deposits;
  5. Rare earth elements are often symbiotic in the same mineral due to their similar atomic structure, chemical and crystal chemical properties. That is to say that the rare earth elements of the cerium and the yttrium are often present in one mineral. But these elements do not coexist in equal amounts– some minerals are mainly composed of cerium, and some minerals are mainly yttrium.

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