|How stainless steel resists
Because stainless steel contains at least
10.5% chromium, the oxidation of the iron is changed to produce a complex
oxide that resists further oxidation and forms a passive layer on the surface.
This is a very thin layer (microns in thickness) but very tenacious and
will reform if it is removed by scratching or machining. The addition of
nickel to the structure (8% minimum in 304 and 10% minimum in 316) broadens
the range of passivity established by the chromium. The further addition
of molybdenum (2% minimum in 316) further expands the passivity range and
improves corrosion resistance, notable in acetic, sulfuric, and sulfurous
acids and in neutral chloride solutions including sea water.
If stainless steel is properly selected
and maintained it should not suffer any corrosion. Stainless steel will,
however, corrode under certain conditions. It is not the same type of corrosion
as experienced by carbon steel. There is no wholesale “rusting” of the
surface and subsequent reduction of thickness. If stainless steel corrodes,
the most likely form of corrosion is “pitting.” Pitting occurs when the
environment overwhelms the stainless steel’s passive film and it cannot
heal the interruption. It usually occurs in very tiny dark brown pits on
the surface (hence the name pitting), and does not interfere with the mechanical
properties of the stainless steel.
Stainless steel can also become subject
to crevice corrosion when the deposits or other material (like a washer)
creates a “crevice” on the surface. It is similar to pitting but over a
larger area where again
the environment has overwhelmed the ability
of the passive layer to heal itself when deprived of oxygen. It is not
attractive, but in most cases it should not affect the mechanical properties
of the stainless steel.