Chromium, chemical symbol Cr, atomic number 24, atomic weight 51.9961, belongs to Group VIB in the periodic table of elements. It is a body-centered cubic crystal. It is the hardest metallic element in nature. Common valencies are +3, +6, and +2.
Chemical properties
Chromium is an inactive metal that is chemically stable at room temperature and will not be corroded in humid air. Therefore, it is often used to make stainless steel and chrome plating. Chromium can slowly dissolve in dilute hydrochloric acid and dilute sulfuric acid to form a blue solution. When exposed to air, it turns green because it is oxidized into green Cr2O3. Chromium reacts with concentrated sulfuric acid to produce sulfur dioxide and chromium sulfate. However, chromium is insoluble in concentrated nitric acid and is passive due to the formation of a tight oxide film on the surface. Metallic chromium is generally characterized by surface passivation in acid. Once depassivated, it is easily soluble in almost all acids.
At high temperatures, chromium can directly combine with halogens, sulfur, nitrogen, carbon, etc., react with nitrogen, and be corrupted by alkali. It is soluble in strong alkaline solutions. Chromium has high corrosion resistance and oxidizes very slowly in the air, even in a red-hot state. It is oxidized by carbon monoxide at 1000°C.
Physical properties
Chromium is a silver-white shiny metal. Due to its large number of single electrons and strong metal bonds, chromium has a high hardness and melting point. It is the transition metal with the highest hardness. Pure chromium is ductile, while chromium containing impurities is hard and brittle. Density 7.20g/cm3. The melting point is 1890℃, and the boiling point is 2482℃. Chromium is insoluble in water.
Chromium is used in steel alloys due to its good mechanical strength and corrosion resistance. Stainless steel contains the highest chromium content, up to about 20%. Many metal surfaces are chrome-plated, rust-proof, and bright.
Plating on metal can provide protection. The thinner the chromium layer during electroplating, the closer it will be to the metal surface and less likely to fall off. On the inner walls of some gun barrels and barrels, the chromium layer is only 0.005 mm thick. However, after firing hundreds of rounds and bullets, the chromium layer is still there. If you want to plate chromium on copper, it is best to plate a layer of nickel first and then plate it with chromium. This can make the copper plating more durable.
The sword unearthed from the Qin Mausoleum Terracotta Warriors
Professor Walkerland
1770: The Russian mineralogist Pallas collected an unseen ore in the Ural region. He sent the ore to a chemical laboratory in Europe, hoping to analyze and study it.
1794: French chemist Vauquelin began analyzing the ore and found no new discoveries except that it contained common metal elements such as lead, iron, and aluminum.
1797: Vauquelin changed his research strategy again. He mixed the ore powder with potassium carbonate, added water to boil it, and added different reagents to the product. The solution actually appeared in multiple colors! Vauquelin realized that behind these color changes was a new chemical element.
1798: Vauquelin indeed isolated a new metal and named it Chrom, inspired by the Greek word Chroma (color). As a result, the chromium element has the Latin name Chromium and the element symbol Cr.
Tips: The earliest application of chromium in China was more than 2,000 years ago:
In 1974, a pit of pottery figurines of the First Emperor of Qin was discovered in Lintong County, Shaanxi Province. Three swords were unearthed in one of the tombs. The sword's body is black and translucent, and the cold light is threatening. These three swords were buried in tidal soil five to six meters deep for more than 2,000 years. Not only were they rust-free when they were unearthed, but they were still sharp. Why have they not rusted after thousands of years?
After careful study, scientists found that the surfaces of these three swords had been specially treated. The ancients used the chromium salt oxidation method. Chromate is a very strong oxidant that can form a dense and stable oxide film on the surface metal of the sword, thereby protecting the metal inside. However, this chromic acid treatment technology began to be used for metal corrosion resistance abroad in the 1930s, while the Chinese people had mastered it more than 2,000 years ago. This has to be said to be a miracle.
Chrome plated metal
Chromium oxide, also known as chromium green, chromium trioxide, and chromium oxide green. It is a common green pigment and inorganic compound with a chemical formula of Cr2O3 and a molecular weight of 151.99 g/mol. It is widely used in ceramic coatings.
Chromium Oxide Chemical Properties
1. Chromium oxide has good chemical and thermal stability and is not prone to chemical reactions with other substances. At normal temperature, it has strong corrosion resistance to water, acid, and alkali.
2. It reacts with oxygen at high temperature to form a stable chromium oxide layer, a dense protective layer that prevents further oxidation reactions. This oxide layer can effectively isolate chromium trioxide from the external environment, providing additional chemical stability.
Chromium oxide physical properties
1. Two adjacent octahedrons share the face composed of three oxygen atoms, making chromium oxide highly stable.
2. Mohs hardness 9, harder than quartz and steel
3. The crystal structure makes chromium trioxide have good thermal expansion properties. At high temperatures, this structure can effectively withstand stress caused by thermal expansion and is less prone to lattice damage.
4. The melting point is 2266±25℃, the boiling point is above 3000℃, and its chemical properties are extremely stable.
5. Refractive index: 2.551 (the refractive index of air is 1.0, the refractive index of water is 1.33, the refractive index of resin is 1.51) - hiding power
6. Solubility: Insoluble in water, acid and alkali solutions, soluble in hot alkali metal bromate solution.
Ceramic coating preferred
Ceramic coatings have excellent wear resistance, corrosion resistance and heat resistance. Generally speaking, there are certain differences in physical and chemical properties between ceramic coatings and metal substrates, especially the difference in linear expansion coefficients between the two. This difference causes a sudden change in the structure and performance of the interface between the coating and the substrate, resulting in large residual stress and thermal stress after coating, which in turn can cause the coating to peel off. Chromium oxide has the characteristics of low thermal expansion, and the thermal stress generated during friction is small. Adding chromium oxide to the coating is not prone to severe fracture, which reduces the shortcomings of ceramic coatings to a certain extent.
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