What is black steel? Black steel is carbon steel that was heated / specially treated upon being made, forming a thin layer of oxidized iron on the surface. This thin layer of black is actually magnetite. Magnetite is an oxidized form of iron that is resistant to corrosion. Corrosion resistance is very important in the chemical and oil industries. For example, some oil pipelines and chemical plant pipelines are constructed of black steel because the magnetite layer provides the extra protection against corrosion, or essentially rust.
Black steel is less expensive than stainless steel and therefore usually used in applications that require some resistance to rust, but nowhere near the level to which stainless steel would be required.