Stainless steel is a steel alloy containing at least 10% chromium, which makes it resistant to corrosion. Adding chromium to stainless steel forms a thin film of metal oxide on the surface of the metal, protecting the material from corrosion.
One popular stainless steel grade is stainless steel 316, which is typically composed of 16 to 18 percent chromium, 10 to 14 percent nickel, 2 to 3 percent molybdenum, and a small amount of carbon. The addition of molybdenum to 316 stainless steel improves its corrosion resistance compared to other grades of stainless steel. The addition of other alloys further enhances its properties.
The performance and characteristics of stainless steel 316 make it the second most widely used stainless steel grade after stainless steel 304. It is used in corrosive environments such as chemical plants, refineries and marine equipment.
Stainless steel 316L has a lower carbon content and is used in applications where the metal is at risk of sensitization. Stainless steel 316H, which has a higher carbon content, has greater thermal stability and creep resistance. Another widely used stainless steel 316 grade is stabilized 316Ti. Stainless steel 316Ti has better resistance to intergranular corrosion.
Stainless steel utilizes the passivation principle, in which the metal becomes "passivated" or unresponsive to oxidation by corrosive compounds found in the atmosphere and process fluids. Passivation is accomplished by exposing the stainless steel to air, where chromium oxide forms on the stainless steel's surface.
To promote the formation of a passive film, the alloy is chemically treated by immersing it in an acidic passivation bath of nitric acid for thorough cleaning. Contaminants such as exogenous iron or free iron compounds are removed to prevent them from interfering with the formation of the passivation layer.
