Why is stainless steel stainless?

Stainless steel is found in everything, including the kitchen sink. So what exactly is stainless steel, and why is it stainless?

Discovery

In 1913, just before the First World War, English metallurgist Harry Brearly accidentally discovered that chromium gave stain resistance to low carbon steel when he was working on a munitions project.

What is stainless steel made from?

Modern stainless steel still includes chromium, but it now also contains other elements in varying degrees, depending on the type of steel and its intended use. Modern stainless steel includes nickel, iron, manganese, carbon, silicon, and molybdenum.

What is stainless steel?

Stainless steel, by definition, contains at least 10% chromium. The chromium reacts with oxygen in the air to create a thin, stable film that acts as a barrier to prevent corrosion.

The process is actually a form of corrosion, but because the layer is so thin, it’s impossible to see with the naked eye. On the other hand, regular steel that hasn’t undergone this process has another reaction with oxygen from water. It causes an unstable layer of iron oxide to form as the metal is exposed to air and water. This film thickens and is known as rust. The corrosion is typically unappealing and can be seen with the naked eye.

Why doesn’t stainless steel rust?

Stainless steel has already undergone a corrosion process that has strengthened it against further corrosion. The thin layer of chromium is strong, but so thin that it is only a few atoms in thickness, so is completely invisible. Even if the layer is scratched or disrupted, more oxide will form as a reaction to the chromium, so the structure will still maintain its stainless properties.

Stainless steel does need good air circulation to prevent stains from occurring, as oxygen is required for the chromium corrosion process.

Types of stainless steel

There are five different types of stainless steel, with each being made using a slightly different process with different amounts of the components of stainless steel. Stainless steel can be classified as austenitic, ferritic, martensitic, duplex, or precipitation-hardening martensitic.

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