What is stainless steel?

August 23, 2016

Stainless steel will not easily corrode, rust, or stain like carbon steel but is not completely resistant to stains and is vulnerable in certain environments.

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Stainless steel is susceptible to corrosion in certain environments,
though the addition of nickel, manganese, carbon all effect how it performs.

Stainless steel is also known as inox steel which comes from the French word for stainless steel, “inoxydable.” Stainless steel requires a minimum chromium content of 10.5% by weight and will not easily corrode, rust, or stain like ordinary carbon steel does. However, stainless steel is not completely resistant to stains and can become vulnerable in certain environments such as those with a low oxygen content, high salinity, or poor air circulation. Stainless steel is typically used in applications where both the favorable mechanical properties of steel and corrosion resistance are needed.

Stainless Steel vs Carbon Steel

Carbon steel is vulnerable to corrosion and rust because the surface of the metal quickly oxidizes to form an iron oxide film, which in turn accelerates the corrosion even further by forming more iron oxide. As the corroded layer of rust grows, it flakes and falls away. Stainless steel differs from carbon steel by the much larger concentration of chromium present in the alloy. The chromium serves to reduce corrosion by forming a passive film of chromium oxide on the surface of the metal. This film blocks oxygen’s ability to diffuse to the surface of the steel and keeps the corrosion from spreading into the internal structure of the metal. However, this passivation only happens if the chromium concentration is high enough, and if oxygen is present to form the protective chromium oxide film. Because oxygen is required to form the passive layer, corrosion resistance can be reduced if the component is used in a non-oxygenated environment such as an underwater structure.

Types of Stainless Steel

Different types of stainless steel can be characterized in many different ways. For example, they can be categorized by their elemental alloy composition. Stainless steel with added nickel experience a stabilized austenite structure which causes them to be less brittle at low temperatures. Similarly, steel with added manganese also preserves the austenitic structure but at a much lower cost, making it preferable to nickel in some applications. Stainless steel with a greater carbon content, on the other hand, allows for greater hardness and strength at the cost of making the alloy more brittle.

Crystal Structure

Alternatively, stainless steels can be categorized by their microstructure. Austenitic steels display a face-centered cubic crystal structure and make up 70% of total stainless steel production. Superaustenitic steels have a high molybdenum content (greater than 6% by weight) and nitrogen additions which allow them to have a greater resistance to chloride pitting and crevice corrosion. Additionally, these stainless steels have a higher nickel content, which reduces their vulnerability to stress corrosion cracking. Ferritic stainless steels have a body-centered cubic crystal structure and offer better mechanical properties than their austenitic counterparts, but pay for that with reduced corrosion resistance due to their lower chromium and nickel contents. Martensitic steels are not as corrosion resistant, but they are very strong and tough. They are also easily machinable and able to be hardened by heat treatment.

Generally, stainless steel is only used in applications where a corrosion resistant material is needed, because it is more expensive than carbon steel. However, stainless steel is not favorable in all environments and should be avoided in specialized applications, such as underwater or enclosed spaces. As always, engineers must carefully consider material choices during the design process as they are crucial to the success of a project.

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