Stainless steel is known for its corrosion
resistance in many environments in which carbon and low-alloy tool steels would
corrode. The
corrosion resistance is a result of a very thin (about 5 nanometers) oxide
layer on the steel’s surface. This oxide layer is referred to as a passive
layer since it renders the surface electrochemically passive in
the presence of corrosive environments.
The passive layer forms because of the chromium added
to stainless steel. Stainless steel must have at least 10.5% chromium in order
for the passive layer to form. The more chromium that is added, the more stable
the passive layer becomes, and the better the corrosion resistance. (For more
on chromium, see The Role of Chromium in Intergranular Corrosion.) Other
elements such as nickel, manganese and molybdenum can be added to enhance
stainless steel corrosion resistance.
Another requirement for the formation and maintenance of the
passive layer is that the steel surface must be exposed to oxygen. Corrosion
resistance is greatest when the steel is boldly exposed and the surface is
maintained free of deposits. If passivity is destroyed under
conditions that do not permit restoration of the passive film, then stainless
steel will corrode much like a carbon or low-alloy steel. For example, covering
a portion of the surface—for example, by biofouling, painting or installing a
gasket—produces an oxygen-depleted region under the covered region. The
oxygen-depleted region is anodic relative to the well-aerated boldly
exposed surface, possibly resulting in the corrosion of the covered region.
Pitting in 304 stainless steel.
Under certain circumstances, the passive layer can break
down at localized spots on a well-exposed stainless steel surface. When this
happens, the metal can corrode in the localized spots. This is called pitting
corrosion. One common cause of pitting corrosion is exposure to aqueous
environments that contain chloride. Examples are coastal atmospheres, road salt
combined with rainwater, and even tap water containing high levels of chloride.
During the fabrication of stainless steel components or
structures, it is possible to degrade the corrosion resistance. This occurs
when austenitic stainless steels (e.g., 304 grade) are exposed
to temperatures between about 797°F (425°C) and 1598°F (870°C). If the exposure
time is too long, then the areas near the metal’s grain boundaries lose
their corrosion resistance and can be preferentially attacked when exposed to a
corrosive environment. The grains fall out and the metal loses strength. The
increased susceptibility to corrosion by this change in microstructure is
called sensitization.
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