Why are other metals added to stainless steel?

Answer 1

Stainless steel is an alloy of steel and chromium. The chromium is added for what is called passivation -- that is, to make passive or unreactive. When the chromium metal in stainless steel meets air, it forms a very thin layer of chromium (III) oxide. This layer is inert, impermeable to water, and extremely resistant to all forms of corrosion (since it is already oxidized). It protects the inner structure of the steel from rust cavities, since it prevents oxidation in the iron itself. Stainless steel is easily sterilized, and any harsh treatments that remove parts of the material are met with innate regeneration of the protective chromium oxide layer. These properties make it ideal for biological uses, such as eating utensils and surgical instruments, and for uses where corrosive materials are present, such as industrial process equipment that deal with acids.

Though it is the chromium that makes stainless steel, there are many grades of stainless steel which many contain a few other additives. Nickel is the most widely-used one, which is used to make the metal less brittle. Most stainless steel products contain considerable amounts of nickel. Other additives are manganese, which is typically used more or less as a substitute for nickel, and molybdenum, which adds strength, thermal resistance, and corrosion resistance to the stainless steel.