Cobalt: a Wonderful Metal


Have you ever seen such a wonderful flower? It is blue on a sunny day and turns into purple when it is going to rain, but becomes bright rosy on a rainy day. This wonderful “flower” is not a real flower, and it is made by the filter paper. People soak the filter paper in a solution of cobalt dichloride, and then dry it to make the flower.

Cobalt dichloride is an important cobalt compounds with a special property. It is blue in the anhydrous state; but it becomes rosy once it absorbs water and forms an aqueous crystal (CoCl2·6H2O). Therefore, people take advantage of this property to make the “weather flower”: with less moisture in air on the sunny day, cobalt dichloride can remain anhydrous state, so it is blue; when it is going to rain, more and more moisture in air will make cobalt dichloride partly become aqueous compounds with both red and blue, and finally becomes purple; there is a lot of moisture in air when it rains, most cobalt dichloride become aqueous compounds, so it is rosy. In a word, people can predict the weather depending on changes in color.

Colors of cobalt dichloride are sometimes red and or blue, while cobalt metal is silver. Cobalt is not only very hard, but also magnetic just like iron and able to be picked up by the magnet. In addition, cobalt is heavier than iron, with specific gravity of 8.8, melting at 1,490°C, and it is more chemically stable than iron without being corroded in air and water at room temperature. Meanwhile, it is difficult to be dissolved in dilute acid. However, it can react with chlorine, oxygen and sulfur to generate chlorides, oxides and sulfides when heated. In the earth’s crust, cobalt content is about one hundred thousandth. And important cobalt ores are smaltite, cobaltite, linnaeite, erythrite and so on. There is about five thousandths of cobalt in the meteorites, which indicates a lot of cobalt in other celestial bodies. In nature, there is not only stable cobalt, but also radioactive cobalt. Radioactive cobalt-60 is now used to treat cancer instead of radium, and is widely used as a tracer atom.

Cobalt has few uses in the industry, mainly used for the production of various cobalt alloys. Cobalt has superior wettability with many metals at whether high temperatures or low temperatures, thus cobalt powder is widely used as adhesives, almost as irreplaceable matrix adhesives of diamond tools and hard alloys. Cobalt alloys have very high hardness. Among them, the alloy with tungsten of 78-88%, cobalt of 6-15% and carbon of 5-6% is known as “superhard alloy”, which is able to withstand the high temperature of 1,000°C without loss of its original hardness for the manufacture of cutting tools; and the “satellite” made by cobalt of 35%, chromium of 35%, tungsten of 15%, iron of 13% and carbon of 2% is also a famous hard alloy used to make high-speed cutting tools and drills. Besides, cobalt alloys have magnetism. The famous permanent magnet is the cobalt steel made by cobalt of 15%, chromium of 5-9%, tungsten and carbon of 1%. In some magnetic alloys, cobalt content is as high as 49%. In addition, cobalt is usually used to make some heat-resistant and acid-resistant alloys.

In the biology, cobalt is an important trace element. According to tests, the absence of cobalt in the sheep feed will cause severe alopecia; however, the alopecia can be cured just by adding a trace of cobalt in the feed—-1 milligram per day and night. Furthermore, vitamin B12 is cobalt organic compounds with cobalt of 4.5%. Now, a lot of vitamin B12 has been artificially synthesized to treat pernicious anemia, asthma and spinal disease, etc.

For the colorless glass, a deep blue glass can be made by adding some cobalt compounds. This glass can block ultraviolet rays very well, so welders and steelworkers often wear these cobalt glasses to protect the eyes at work. In addition, the cobalt compound is used as a blue pigment in the manufacturing process of cloisonne, enamels and ceramics.