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Steel is an alloy, or mixture, of iron with carbon and small amounts of other metals. Pure iron can be melted and shaped, but tends to be soft. Adding carbon will strengthen the metal, and most carbon steels contain one to two percent carbon. Black carbon steel is created during the manufacturing process when high temperatures create a thin layer of oxidized iron on the outer surface.
Carbon bonds chemically with the iron in steel alloys, creating a much harder material than pure iron. As the carbon content increases, the material becomes harder but more brittle, or prone to breaking under stress or load. Steel containing carbon in an amount over 2% is considered cast iron; this can be used for piping and non-structural materials, but is considered too brittle for structural steel.
Iron reacts easily with the oxygen in air, creating iron oxide or rust that will cause parts to fail, so it is often coated to prevent surface rust. One advantage of black carbon steel is the natural anti-corrosion property of the black iron oxide coating, because the oxide acts as a barrier to keep oxygen from the iron below. The thin oxide layer is created at high temperatures, forming a durable layer requiring no further treatment or coating.
For applications where corrosion protection is critical, carbon steel can be painted or galvanized. Steel is galvanized by acid washing it, then dipping the steel into a molten zinc bath. The zinc forms a layer on the outside of the steel, and will corrode first when exposed to air or moisture. Galvanizing will improve the working lifetime of steel parts, but is not used on black carbon steel, because the oxide layer already protects the steel.
Black carbon steel is often used for gas or water utility piping, because it has low cost and can be welded using common methods. Some long-distance oil pipelines have used black carbon steel piping, because the pipe can be connected in the field and will not rust quickly. This steel can be used in climates or ground conditions that can accelerate corrosion, but additional protection such as anodes may be needed. Anodes are buried rods made of zinc that corrode first when connected to the steel pipe.
Cookware can be called black steel, but the dark color is a product of an oil treatment called seasoning, rather than manufacturing. Carbon steel is often coated with oil to prevent rusting, and steel cookware can be oiled and heated to absorb the oil molecules. The steel will darken and gain some rust-preventive properties, but seasoning is a temporary coating and must be repeated periodically.
Bear in mind that seasoning those "black steel" pieces of cookware also makes them "anti stick." Seasoning that cookware will also make them last a very long time. Seasoning is pretty easy to do and is critical.