Good corrosion resistance due to a high chromium content is the key characteristic. Opinions vary on the level of chromium (Cr) at which a steel becomes stainless, but it is at least 10.5%. Nickel (Ni) and molybdenum (Mo) are often present, and manganese, copper, titanium, silicon + other alloying elements may be added.
The principal grades are austenitic (typically 16-26%Cr, 6-22Ni); ferritic (10.5-28%Cr with no/low Ni); martensitic (higher carbon content than ferritic and typically 12-19%Cr with low/no Ni); and duplex, a dual-phase austenitic/ferritic steel (Cr>21%, Ni <8%).
Austenitics are non-magnetic, easily formed, but harden rapidly during processing (typically used in the process industries, heat exchangers, cutlery). Ferritics are less corrosion resistant, easily formed and magnetic (catering, architectural, materials handling). Martensitics are magnetic, have higher strength, are less easily worked (surgical instruments, shafts, fasteners). Duplex is strong with good impact resistance (desalination, heat exchangers).