Stainless steel is the generic term given to a family of corrosion resistant and high temperature steels. Their remarkable resistance to corrosion is due to a chromium-rich oxide film which forms on the surface. When enough chromium (more than about 10%) is added to ordinary steel, the oxide on the surface is transformed – it is very thin, virtually invisible and protective in a wide range of corrosive media. This is what we call stainless steel and there are several different types, and many different grades.
The majority of stainless steels contain nickel (Ni), which is added for a number of reasons but particularly to change the crystal structure from ferrite to austenite. Austenitic stainless steels are ductile, tough and, most importantly, easy to form and weld. These steels are not magnetic in the annealed condition. The most common example is Type 304 (S30400) or “18/8” – the most widely used stainless steel in the world. The lower carbon version, Type 304L (S30403) is always preferred in more corrosive environments where welding is involved. There are numerous applications for this grade of stainless steel.
Molybdenum (Mo) is added to some stainless steels to increase their corrosion resistance. It increases an alloy’s pitting and crevice corrosion resistance. When 2-3% molybdenum is added to Type 304 or 304L, we create Type 316 (S31600) or 316L (S31603) stainless steel. They are sometimes referred to as the medical grades of stainless steel but their range of applications is very wide.