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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 thanpenetration 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 and pitting 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 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.
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