| Corrosionist The Website of Corrosion and Corrosion Control |
| You are here >>> Corrosion Prevention Home - Materials Index - Stainless Steel |
|
Stainless Steels.
Stainless steels can be defined as a ferrous alloys that contain more than 11% chromium and are resistant to general rusting in mild atmospheric conditions. They are generally subject to non-uniform attack in chloride containing environments with crevice corrosion and pitting attack being common on many alloys. Due to the high rate of localized attack that occurs at the start of the attack, the incubation time for attack is often a more significant measure of corrosion resistance than penetration rates. The stainless steels exhibit two potential states:
The passive state is substantially cathodic to the active state. It is this
potential difference that drives non-uniform attack, such as pitting and crevice
corrosion on these alloys. For purposes of evaluating possible galvanic
corrosion between the
stainless steels and other alloys, the potential of the passive state should
be used. As these alloys polarize readily, their potentials can have a wide
range in some conditions and may be of little value in assessing galvanic
corrosion problems based on field measurements.
200 Series Stainless Steels.
This group of alloys is similar to the more
common 300 Series alloys described below as they are non-magnetic and have an
austenitic structure. The basic alloy contains 17% chromium, 4% nickel and 7%
manganese. Although these alloys are somewhat more resistant to attack than the
more common 300 series stainless steels, their overall performance is similar.
Some proprietary grades related to the 200 series, have performance equal or
superior to the best 300 series stainless steels. These alloys are, however, not
immune to attack and are very susceptible to
concentration cell corrosion and
pitting
corrosion attack. When corrosion starts they usually corrode rapidly and
nonuniformly. In seawater immersion, the incubation time for these alloys is in
the range of 1 to 3 months with some of the Nitronic grades having incubation
times of up to 1 year.
300 Series Stainless Steels.
This group of alloys are non-magnetic and have
an austenitic structure. The basic alloy contains 18% chromium and 8% nickel.
These alloys are subject to crevice corrosion and pitting. They have a range of
incubation times in seawater ranging from essentially zero in the case of the
free machining grades, such as Type 303, to 6 months to 1 year for the best
alloys, such as Type 316. They have been widely used in facilities with mixed
results. If used in an application where chloride levels are low or where
concentration cell corrosion has been prevented through design, they are likely
to perform well. When chloride levels are high and where concentration cells can
occur, the performance of these alloys is often poor. They must always be
selected with care for a specific application and the effect of potential
non-uniform attack on system performance must be addressed.
400 Series Stainless Steels.
This group of alloys are magnetic and have a
martensitic structure. The basic alloy contains 11% chromium and 1% manganese.
These alloys can be hardened by heat treatment but have poor resistance to
corrosion. They are subject to both uniform and non-uniform attack in seawater.
The incubation time for non-uniform
corrosion attack in chloride containing environments is very short, often
only hours or a few days. Unless protected, using these alloys in seawater or
other environments where they are susceptible to corrosion is not recommended.
600 Series Stainless Steels.
This series of stainless steels is commonly
referred to as “Precipitation Hardening” stainless steels. These steels can be
heat treated to high strength levels. They are subject to crevice corrosion and
pitting in chloride containing environments and are also subject to stress
corrosion cracking and hydrogen embrittlement.
The incubation time for crevice corrosion and pitting in seawater is
relatively short, often only a few days. The incubation time for stress
corrosion cracking can be very short, sometimes measured in hours. The use of
these alloys in chloride containing environments is not normally recommended
unless they are carefully selected, their heat treatment is carefully specified
and controlled, and the effect of pitting and crevice corrosion is properly
addressed.
Miscellaneous Cast and Wrought Stainless Steels.
Alloy 20, a proprietary cast and wrought
stainless steel has superior corrosion resistance to Type 316 stainless steel.
Many newly developed proprietary stainless steels have given good service in
many applications
and have corrosion resistance superior to Type 316 stainless steel. The use
of these alloys must be evaluated based upon their specific corrosion
properties. While the resistance of these alloys is superior to many other
grades of stainless steels, the benefits of using these alloys in critical
applications instead of more corrosion resistant or corrosion immune alloys must
be balanced against the consequences of failure.
Stainless Steel corrosion tables
Stainless Steel corrosion by waters
|
||