Chemical Engineering July 2022 - 33

FIGURE 3. Sea water testing of PHE plates of a plateand-frame
heat exchanger. The upper image is 654 SMO
and lower image is 254 SMO under the same conditions
and time frame
uniform, pitting, and crevice corrosion,
but rather poor resistance to
stress corrosion cracking.
Cr-Mn steels (200-series). These
steels are also referred to as
" 200-series " steels in reference to
the AISI/ASTM nomenclature. Their
nickel content is decreased and the
austenitic microstructure is maintained
by replacing some of the
nickel with manganese and nitrogen.
They have moderate resistance
to uniform, pitting and crevice corrosion,
but rather poor resistance to
stress corrosion cracking.
Cr-Ni-Mo steels. These are another
type of " general purpose "
stainless steels. They are alloyed
with chromium and nickel but have
increased corrosion resistance because
of their molybdenum content.
They are sometimes referred
to as " acid-proof " stainless steels.
In the past, titanium and niobium
stabilization was used to improve
resistance to intergranular corrosion,
but this is not necessary
for modern low carbon Cr-Ni-Mo
steels. They have good resistance
to uniform, pitting, and crevice corrosion,
but rather poor resistance
to stress corrosion cracking.
High performance austenitic
steels. These steels were developed
for use in highly demanding
environments and have high
chromium, nickel and molybdenum
content. Many are also alloyed
with nitrogen to further
increase corrosion resistance
and mechanical strength.
Some of these steels are alloyed
with copper to increase
the resistance to certain acids.
They have high resistance to
uniform, pitting, crevice and
stress corrosion cracking.
Within this category, superaustenitic
stainless steels are
normally used where greater
resistance to corrosion, especially
protection from chloride
pitting and crevice corrosion,
is needed. They are defined
as austenitic, iron-based alloys
that have pitting resistance
equivalent (PRE) greater than
40, such as UNS S32654, that
is also known as a 7Mo superaustenitic
grade.
High-temperature austenitic
steels. These steels, developed
for use at temperatures exceeding
550°C, have high chromium and
nickel content, but contain no molybdenum.
They have good oxidation
and creep resistance.
Duplex stainless steels. Duplex
stainless steels have a ferritic-austenitic
microstructure, with a phase
balance of approximately 50% ferrite
and 50% austenite. They are
magnetic due to their ferrite content.
Duplex steels combine many of the
beneficial properties of ferritic and
austenitic stainless steels. Compared
with austenitics, they typically have
high chromium content but rather
low nickel content. Molybdenum and
nitrogen are added to improve corrosion
resistance and balance the microstructure.
Nitrogen also increases
the mechanical strength.
Duplex steels should only be
used at temperatures between -40
and 250°C, because they become
brittle
outside
this
temperature
range. They have high resistance
to stress-corrosion cracking, while
resistance to uniform, pitting and
crevice corrosion varies between
moderate and high, depending on
the steel.
Martensitic and precipitation
hardening stainless steels. Martensitic
stainless steels is the smallest
group of stainless steels. They
contain chromium and either no or
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Chemical Engineering July 2022

Table of Contents for the Digital Edition of Chemical Engineering July 2022
