Ferritic stainless steels have certain useful corrosion properties,
such as resistance to chloride stress-corrosion cracking, corrosion
in oxidizing aqueous media, oxidation at high temperatures and pitting
and crevice corrosion in chloride media.
These steels contain above approximately 13% Cr and precipitate
a prime phase in 350oC to 540oC range, and the maximum effect is at
about 470oC. Because precipitation hardening lowers temperature ductility,
it must be taken into account in both processing and usage of ferritic
stainless steels, especially those with higher chromium content.
Structures of these steels are kept completely ferritic at room and high
temperature by adding titanium or columbium, or by melting to very low
levels of carbon and nitrogen, or both. Such microstructures provide
ductility and corrosion resistance in weldments. Molybdenum improves
pitting corrosion resistance, while silicon and aluminum increase
resistance to high temperature oxidation.
The newer ferritic steels with high content of chromium have become
possible through vacuum and argon-oxygen decarburization, electron-beam
melting, and large-volume vacuum induction melting. The representatives
of this group include ASTM designations 409 and 439.
Type 409 with 12% Cr is relatively low-cost and has good formability and
weldability. Recommended thickness is limited to approximately 3,8 mm
maximum if ductile-to-brittle transition temperature (DBTT) at room
temperature or lower is needed (Figure 1). Its atmospheric corrosion
resistance is adequate for functional uses, so applications of this
type of steel include automobile exhaust equipment, radiator tanks,
catalytic reactors, containerization and dry fertilizer trunks.
Type 439 with 18-20% Cr resists chloride stress-corrosion cracking.
Resistance to general and pitting corrosion is approximately equivalent
to that of austenitic types 304 and 316. This grade is suitable for
equipment exposed to the aqueous chloride environments, heat
transfer applications, condenser tubing for fresh water power
plants, food-handling uses and water tubing for domestic and
industrial buildings. Sheet thickness cannot exceed approximately
3,2 mm if DBTT (Figure 1) at room temperature or lower is needed.
|Figure 1. Ductile-to-brittle transition temperatures
(DBTT) for ferritic stainless steel rise with section thickness.
Bands for 409 and 439 indicate data scatter
Resistance to stress-corrosion cracking is the most obvious advantage of
the ferritic stainless steels. Ferritic steels resist chloride and caustic
stress corrosion cracking very well. Nickel and copper residuals lower
resistance of these steels to stress corosion.
Susceptibility of the ferritic steels to intergranular corrosion is
due to chromium depletion, caused by precipitation of chromium carbides
and nitrides at grain boundaries. Because of the lower solubility for
carbon and nitrogen and higher diffusion rates in ferrite, the synthesized
zones of welds in ferritic steels are in the weld and adjacent to the weld.
To eliminate the intergranular corrosion, it is necessary either to reduce
carbon to very low levels, or to add titanium and columbium to tie up the
carbon and nitrogen.
Pitting, an insidious localized type of corrosion occurring in halide media,
can put complete installations out of operation in relatively short time.
Resistance to this type of corrosion depends on chloride concentration,
exposure time, temperature and oxygen content. In general, resistance to
pitting increases with chromium content. Molybdenum also plays an important
role and it is equivalent to several percentages of chromium.
General corrosion resistance: The atmospheric corrosion resistance
of the ferritic steels is excellent. These steels have good corrosion
resistance in strongly oxidizing environments, such as nitric acid. In
organic acids, all ferritic steels are superior to austenitic, but in
reducing media general corrosion resistance of ferritic steels is worst
High-chromium ferritic stainless steels
High-chromium ferritic stainless steels - such as types 442 and 446 -
have excellent resistance to corrosion and to oxidation in many industrial
environments. These alloys are included in ASTM specifications A176-74
(Chromium stainless flat products), A 511 (Seamless stainless steel
mechanical tubing), A268-74 (Ferritic stainless steel tubing for general
service) and also in ASME code and AISI and SAE specifications.
High-chromium ferritic steels have 18-30% Cr and low content of carbon
and nitrogen. Titanium in these alloys prevents intergranular
chromium-carbide and nitride precipitation during welding or
processing. Because of the ferritic structure and controlled composition,
the alloys exhibit good resistance to general, intergranular and pitting
corrosion, and stress corrosion cracking. Similar to other high chromium
stainless steels, types 442 and 446 have excellent oxidation resistance
at elevated temperatures. They also have high thermal conductivity,
higher yield strength than austenitic stainless steels, and lower
The excellent resistance to chlorides, organic acids and chloride
stress-corrosion indicates that these alloys should be suitable for a
wide range of applications in which conventional stainless steels or
other materials are either inadequate or uneconomical. High-chromium
ferritic stainless steels are useful in heat exchanger tubing, feed-water
tubing and in equipment that operate with chloride-bearing or brackish
Available in sheet, strip, tubing and welding wire, alloys are finding
substantial application in replacing brass and cupronickel,
corrosion-resistant high-nickel alloys, and other materials in the food
processing, power, chemical, petrochemical, marine and pulp and paper