The special chemical composition of stainless steel creates a protective layer on its surface, preventing oxygen from contacting the iron and causing rust.
The chemical composition of stainless steel prevents oxygen in the air and environment from coming into contact with the iron in the steel, thereby preventing harmful oxidation reactions. Steel usually rusts when the iron reacts with oxygen, forming iron oxide. Although rust is not harmful to humans, it can corrode the iron, making the material less safe and less attractive.
Steel is an alloy that contains 99% iron and about 0.2–1% carbon, while stainless steel typically contains about 62–75% iron, 1% carbon, and more than 10.5% chromium. Stainless steel also contains a few percent nickel, which makes the material more durable and easier to process.
Chromium plays a key role in preventing rust in stainless steel, according to materials scientist Tim Collins, secretary general of Worldstainless, a Belgian nonprofit that works with the World Steel Association. Chromium reacts with oxygen in the environment, both in air and under water, creating a passive layer of chromium oxide (Cr2O3) on the metal’s surface. This layer prevents oxygen from reaching the iron in the steel and causing rust, Collins explains.
The passive layer on stainless steel is only a few nanometers thick and is invisible. The chromium oxide layer also heals itself if damaged. It is inert, does not react chemically with other compounds, and does not leach out, making stainless steel well suited for food production, surgery, and many other applications.
Stainless steel was developed in 1912 by British metallurgist Harry Brearle while researching steel alloys to prevent corrosion in gun barrels. Brearley created an alloy of iron, carbon, chromium and nickel, but it was not suitable for gun barrels, so he threw it out in the backyard. After a few weeks, Brearley noticed that the alloy in his yard did not rust, so he developed the material and introduced the product in 1915.
Stainless steel now accounts for about 4% of the world’s steel use each year, or nearly 2 billion tons, according to Collins. But stainless steel is complex and expensive to produce, costing three to five times as much as regular steel, and the inclusion of special metals in the alloy (such as molybdenum for underwater applications) makes it even more expensive. As a result, most applications that require steel use regular or carbon steel.