Before delving into the industrial chain and applications of metallic chromium, we first need to understand the metal itself - where it comes from, what properties it possesses, and in what forms it exists in the industrial world.
1. Basic Properties: A "Colorful" Hard Substance
Chromium (chemical symbol Cr, atomic number 24) is a transition metal with a silver-white color and a slight sky-blue tint. Its "hardness" lives up to its reputation - it is the hardest metal in nature. Pure chromium has good ductility, but industrial chromium containing impurities is hard and brittle.
Chromium has a melting point of 1907°C, a boiling point of 2679°C, and a density of approximately 7.19 g/cm³. At room temperature, it is stable against oxygen and moisture, as a dense chromium oxide film quickly forms on its surface, preventing further corrosion. However, when the temperature rises (e.g., above 1200°C), this protective film is destroyed.
Almost all chromium compounds display beautiful colors - chromium sulfate is green, potassium dichromate is orange-red, and lead chromate is yellow. For this reason, when the French chemist Vauquelin discovered it in 1797, he named it after the Greek word "chroma" (meaning "color").
2. Origin and Mystery: Hidden Deep in the Earth's Crust, Yet Remarkably Present on Ancient Qin Swords
Chromium accounts for only 0.01% of the Earth's crust, ranking 17th in abundance. Metallic chromium does not exist in a free state in nature; it is mainly "hidden" in chromite ore.
Interestingly, bronze swords unearthed from the No. 2 Pit of the Terracotta Army of China's Qin Dynasty remained free of rust and as bright as new after 2,000 years. Scientific testing revealed that the sword surfaces had an oxide film containing 2% chromium. This chromate conversion coating process is a modern advanced technique - Germany first invented it in 1937, and the United States in 1950, with patents filed thereafter. The casting skills of the Qin people are truly astonishing.
3. Two Industrial Forms
Metallic chromium produced industrially comes in two main forms:
Aluminothermic chromium: in lump form, bright silvery in color with a metallic luster, with a chromium content > 98%.
Electrolytic chromium: in flake form, with a dark brown surface; after hydrogen refining, the surface becomes bright, with higher purity (> 99%).
China began developing metallic chromium in 1958, and the Jinzhou Ferroalloy Plant built the country's first aluminothermic chromium production facility in the early 1960s.
