In metallurgy, stainless steel is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass.  Stainless steel does not stain, corrode, or rust as easily as ordinary steel, however it is not stain-proof.  It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry.  There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime.  Stainless steel is used where both the properties of steel and resistance to corrosion are required.

The difference between stainless steel and carbon steel is the amount of chromium present. When exposed to air and moisture carbon steel rusts. Iron oxide (or rust) is active and accelerates corrosion by forming more iron oxide.  Stainless steel contains sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.