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Fundamentals of Corrosion
In industrial or marine atmospheres, or from contamination with highway de-icing salts, chloride pitting may occur. If the contamination is severe,
overall rusting can occur rather than localized pitting.
Design of the installation in an industrial atmosphere is important. If the
stainless steel is partly sheltered so that deposits are not washed away by rain,
and the deposits are hygroscopic and corrosive, some attack may take place that
should not otherwise be encountered. In tests, exposed specimens were virtually unattacked, whereas for specimens in a sheltered location under a roof, only
those containing molybdenum (types 316 and 316L) remained unattacked.
In marine atmospheres, types 304 and 304L stainless steels may develop
substantial staining in the form of scattered patches of yellow-brown films.
There is little evidence of attack beneath the films. The discoloration will
develop during the first few months of exposure, after which it does not
appear to develop further. Even after exposure of 10 yr or more, the stain
may easily be removed, revealing a bright surface with little evidence of
attack. If the staining is unacceptable, a more highly alloyed stainless steel
such as type 309 will evidence a greatly reduced amount of staining. The use
of type 316 will practically eliminate all staining.
As the distance from the shoreline increases, the intensity of the staining
will decrease. However, the type 316 alloy remains free from staining for
long periods of time even when exposed as close as 80 ft from the shoreline.
Any of the austenitic stainless steels can be used in a marine atmosphere if
they are cleaned periodically. If cleaning is not practical, then type 316 alloy
will provide the greatest resistance to staining.
4.7.8.2 Ferritic Grades
The ferritic grades contain 15 to 30% chromium with a low carbon content
(0.1%). Resistance to atmospheric corrosion of the ferritic grades will depend
on the chromium content as well as the condition of exposure.
Types 405 and 409 have approximately the same resistance to chloride pitting
as type 410, whereas higher alloyed types such as 430 are more resistant.
Rural atmospheres are less corrosive to the ferritic grades than marine
atmospheres, while the corrosiveness of industrial atmospheres depends on
the nature of the contaminants.
Type 430 will rust in a sheltered location when exposed to an industrial
atmosphere. If exposed to washing by rain, the rusting can be reduced or
eliminated.
4.7.8.3 Martensitic Grades
The martensitic grades normally contain 11 to 13% chromium. Martensite has
a body-centered tetragonal structure that provides increased strength vs. the
annealed stainlesses with other lattice structures. Sufficient carbon is added
Fundamentals of Corrosion
In industrial or marine atmospheres, or from contamination with highway de-icing salts, chloride pitting may occur. If the contamination is severe,
overall rusting can occur rather than localized pitting.
Design of the installation in an industrial atmosphere is important. If the
stainless steel is partly sheltered so that deposits are not washed away by rain,
and the deposits are hygroscopic and corrosive, some attack may take place that
should not otherwise be encountered. In tests, exposed specimens were virtually unattacked, whereas for specimens in a sheltered location under a roof, only
those containing molybdenum (types 316 and 316L) remained unattacked.
In marine atmospheres, types 304 and 304L stainless steels may develop
substantial staining in the form of scattered patches of yellow-brown films.
There is little evidence of attack beneath the films. The discoloration will
develop during the first few months of exposure, after which it does not
appear to develop further. Even after exposure of 10 yr or more, the stain
may easily be removed, revealing a bright surface with little evidence of
attack. If the staining is unacceptable, a more highly alloyed stainless steel
such as type 309 will evidence a greatly reduced amount of staining. The use
of type 316 will practically eliminate all staining.
As the distance from the shoreline increases, the intensity of the staining
will decrease. However, the type 316 alloy remains free from staining for
long periods of time even when exposed as close as 80 ft from the shoreline.
Any of the austenitic stainless steels can be used in a marine atmosphere if
they are cleaned periodically. If cleaning is not practical, then type 316 alloy
will provide the greatest resistance to staining.
4.7.8.2 Ferritic Grades
The ferritic grades contain 15 to 30% chromium with a low carbon content
(0.1%). Resistance to atmospheric corrosion of the ferritic grades will depend
on the chromium content as well as the condition of exposure.
Types 405 and 409 have approximately the same resistance to chloride pitting
as type 410, whereas higher alloyed types such as 430 are more resistant.
Rural atmospheres are less corrosive to the ferritic grades than marine
atmospheres, while the corrosiveness of industrial atmospheres depends on
the nature of the contaminants.
Type 430 will rust in a sheltered location when exposed to an industrial
atmosphere. If exposed to washing by rain, the rusting can be reduced or
eliminated.
4.7.8.3 Martensitic Grades
The martensitic grades normally contain 11 to 13% chromium. Martensite has
a body-centered tetragonal structure that provides increased strength vs. the
annealed stainlesses with other lattice structures. Sufficient carbon is added