Steel is one of the world’s most versatile and commonly used materials. Consisting primarily of iron and carbon, it’s used to make buildings, bridges, cell towers, automobiles, tools, instruments, appliances and countless other items. Statistics show that the world now produces nearly 2 billion metric tons of steel per year, attesting to its popularity.
While all steel contains iron and carbon, there are many different types. Some types of steel contain other alloying elements, with chromium being one of the most common. Stainless steel, for instance, is characterized by a high chromium content. It still contains iron and carbon, but roughly 10.5% to 20% of stainless steel consists of chromium.
What Is Chromium?
Chromium is a transition metal that’s prized for its strength and corrosion-resistant properties. On the Mohs Hardness Scale, it typically scores an 8.5 to 9 rating. In comparison, pure iron scores about 4 to 4.5, meaning chromium is nearly twice as hard.
Corrosion Resistance
The main reason chromium is added to steel is to protect against corrosion. Normally, steel can rust and corrode due to oxidation. Exposure to oxygen can trigger a chemical reaction (oxidation) that converts iron within steel into iron oxide. Chromium, however, shields the iron from environmental oxygen so that it doesn’t rust or corrode.
Chromium works to protect steel from rust and corrosion through a process known as passivation. All stainless steel contains chromium. During production, stainless steel is typically bathed acidic solution to eliminate superficial free iron. With the free iron removed, stainless steel has a cleaner surface that allows for the formation of a passive layer.
Chromium in the surface of stainless steel will react to environmental oxygen — but this doesn’t create rust or corrosion like with iron. Instead, it creates an invisible layer of chromium oxide. Chromium oxide is very stable and acts as a barrier between the steel’s other elements and the environment. As a result, the steel’s iron won’t oxide.
Strength
In addition to protecting steel from corrosion, chromium also makes steel stronger. Chromium forms carbides when combined with carbon. The presence of these chromium carbides enhances the steel’s strength and ability to resist wear and tear.
Stainless steel and other types of high-chromium steel can handle heavy loads. This is why manufacturing equipment is often made of high-chromium steel.
Heat Resistance
The chromium content in steel can affect its heat resistance. High-chromium steel is typically able to handle hotter temperatures than its low-chromium counterpart.
Chromium has a melting point of 3,465 degrees Fahrenheit, whereas iron has a melting point of 2,800 degrees Fahrenheit. With high levels of chromium, steel can withstand hot temperatures without succumbing to corrosion or otherwise degrading.