Chromium is a chemical symbolized by Cr. It¡¯s a hard metal that has a steely grey appearance while maintaining a shiny gleam and polish. It also can withstand high heat, is odorless, and very pliable when at melting point. Chromium metal is different from native chromium; it is mined from an ore called chromite or iron chromium oxide which is available in commercial quantities. Pure chromium is a rare metal and cannot be used for manufacturing anything because it¡¯s too expensive and hard to find.
The word ¡°Chromium¡± comes from ¡°chroma¡± which is a Greek word for color. It was used as far back as the Qin Dynasty, 2000 years ago in China for coating the weapons of the army. It came to the western world as a mineral for coating and pigmentation in 1761.
To produce chromium metal, the chromite has to be roasted or leached. Once this is done, the metal develops a high resistance to corrosion and this led to the start of the stainless steel industry. Researchers found out that by adding metallic chromium, stainless steel is formed which is a hard material that does not discolor easily. Aside from being used to produce stainless steel products, this metallic element is also used for processing leather and nonferrous alloys.
The other elements found in stainless steel are iron, nickel, carbon, titanium, niobium, and molybdenum. The minimum amount of chromium needed to create the passive film used for stainless steel is 12%. It is this passive film that prevents corrosion. Unfortunately, the passive film requires oxygen to stay anti-corrosive so it does not perform well in places where there is poor oxygen or poor ventilation. For example, it will break down when exposed to sea water because of the salt and low oxygen environment.
There are 3 types of stainless steel: ferritic, austenitic, and martensitic. They are distinct from each other by their microstructure. For example, austenitic stainless steel is made from the basic formula of 18% chromium and 8% nickel and does not undergo heat treatment. Ferritic is made of 17% chromium and iron while martensitic has 12% chromium and .12% carbon. There are other grades, texture and finishing that can be achieved through minor adjustments in the formulation.