Chemical Engineering February 2023 - 24

Facts At Your Fingertips
Stainless Steels and Corrosion
Department Editor: Scott Jenkins
S
tainless steels are common materials
for
process equipment
and components in the chemical
process industries in large part
because of their corrosion resistance.
This one-page reference reviews the
properties of stainless steels with regard
to corrosion resistance.
Passive layer
Stainless steels are not fundamentally
inert materials, but rather, derive
their corrosion resistance from a thin,
invisible and insoluble layer of chromium
and iron oxides and hydroxides,
commonly called the passive
layer (Figure 1).
Although the passive layer is only
a few nanometers thick, it effectively
keeps the metal beneath isolated
from its surroundings, and electrochemical
reactions causing corrosion
are effectively slowed down. The
passive layer reduces the corrosion
rate to a fraction of what it would be
without it.
The passive layer forms spontaneously
in environments containing
enough oxidants. If the metal beneath
the passive layer is exposed
by mechanical damage, it spontaneously
repassivates. The oxygen content
of air, and most aqueous solutions,
is enough for both the creation
and maintenance of the passive layer
of stainless steels.
However, there are environments
that cause permanent breakdown
of the passive layer, allowing corrosion
on the unprotected surface.
For the passive layer to break down
in its entirety, stainless steels need
to be exposed under strongly reducing
conditions. More commonly,
the passive film is attacked locally,
while the rest of the surface remains
intact, causing various types of localized
corrosion.
The situation that then occurs with
an unfavorable anode-to-cathode
area ratio can lead to rapid propagation
once corrosion has initiated. Stainless
steel is generally either passive or
tends to corrode rapidly, in contrast to
carbon steel, where gradual, uniform
corrosion is expected.
24
Categories of stainless steel
Traditionally, stainless steels are categorized
according to their microstructure
into the following categories:
Ferritic stainless steels. Ferritic
stainless steels, also known as chromium
(Cr) steels, are alloyed with Cr,
and sometimes a small amount of
nickel (Ni). Molybdenum (Mo) is added
to some steels to improve corrosion
resistance, while alloying with niobium
(Nb) and titanium (Ti) improves
weldability. These steels have high
resistance to stress corrosion cracking
(SCC) but moderate resistance to
uniform, pitting and crevice corrosion.
Austenitic stainless steels. Austenitic
stainless steels are the largest
group of stainless steels and have
moderate to excellent corrosion resistance.
This group can be divided
into five subcategories:
* Cr-Ni steels (18-8). These " general
purpose " stainless steels are mainly
alloyed with Cr and Ni but with no Mo.
The " 18-8 " designation refers to their
approximate Cr and Ni content. They
have moderate resistance to uniform
corrosion, as well as pitting and crevice
corrosion, but rather poor resistance
to SCC.
* Cr-Mn steels (200-series). These
steels are also referred to as " 200-series "
steels in reference to the AISI/
ASTM nomenclature. Their Ni content
is decreased and the austenitic
microstructure is maintained by replacing
some of the Ni with manganese
(Mn) and nitrogen. They have
moderate resistance to uniform, pitting
and crevice corrosion, but rather
poor resistance to SCC.
* Cr-Ni-Mo steels. Also " general
purpose " stainless steels, this type
is alloyed with Cr and Ni, but have
increased corrosion resistance because
of their Mo content. They are
sometimes referred to as " acid-proof "
stainless steels. They have good resistance
to uniform, pitting and crevice
corrosion, but rather poor resistance
to SCC.
*
High-performance
austenitic
steels. These steels were developed
for use in highly demanding environments
and contain high Cr, Ni and
O2
O2
Oxide
Metal
Oxide
O2
O2
O2
O2
Cr
Cr
Cr
Cr2O3
AIR
O2
O2
O2
FIGURE 1. The passive layer of stainless steel is
generated when oxygen from the air combines with
chromium at the metal-air interface
Mo content. Many steels of this type
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 SCC.
* High-temperature austenitic steels.
These steels, developed for use at
temperatures exceeding 550°C,
have high Cr and Ni content, but also
contain Mo. 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. Duplex steels
combine many of the beneficial
properties of ferritic and austenitic
stainless steels. Compared with austenitics,
they typically have high Cr
content, but rather low amounts of
Ni. Mo and nitrogen are incorporated
to improve corrosion resistance
and balance the microstructure. This
group of steels has high resistance
to SCC, while resistance to uniform,
pitting and crevice corrosion varies
between moderate and high, depending
on the steel.
Martensitic and precipitation-hardening
stainless steels. This type
contains Cr without the addition of Ni
and Mo (or sometimes only very tiny
amounts). The corrosion resistance
of these steels is generally lower than
other stainless-steel families, and they
are used mainly when high hardness
is a key requirement.
n
Editor's note: This column is adapted from the following article:
Signorelli, R. and Li, J. Materials of Construction: Stainless Steel Versus
Nickel Alloys, Chem. Eng., July 2022, pp. 31-36.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2023
AIR
Cr2O3
Cr
Cr
O2
O2
O2
http://WWW.CHEMENGONLINE.COM

Chemical Engineering February 2023

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