Stainless Steel: A Definition
Stainless steel is principally known and considered for use because of its high level of corrosion resistance in both wet and dry conditions, particularly in comparison with standard carbon steels.
What determines whether a steel is stainless or not is dependant on its composition; a steel must have a minimum content of 11% chromium by mass to be considered a stainless steel.
On exposure to air and moisture a carbon steel will form a film of iron oxide (rust) around it. This film is chemically active and therefore accelerates corrosion by forming more iron oxide. Likewise, the chromium in stainless steel will form an oxide film on its surface. This film, in contrast to carbon steel, makes the surface inert to chemical reaction, rendering it passive. This is Stainless Steel's natural inbuilt corrosion resistance.