Chemical Engineering July 2022 - 32

rive their corrosion resistance from
a thin, invisible and insoluble layer
of chromium and iron oxides and
hydroxides, commonly called the
passive film, or passive layer. Even
though the passive layer is only a
few nanometers in thickness, this
passive layer 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
only a fraction of what it would be
without it. Other metals, such as
chromium, aluminum and titanium,
also show passivity, and it is the
ability of chromium to passivate that
is utilized in stainless steels.
The passive
layer on stainless
steels forms spontaneously in environments
containing enough oxidants.
Moreover, if the metal beneath
the passive layer is exposed
by mechanical damage, such as
scratches, it spontaneously repassivates.
The oxygen content of air,
and also of 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. Under circumstances
where the passive layer cannot
be rebuilt, corrosion occurs 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 very rapid propaGrade
316L
TABLE
1. COST COMPARISON
Price indication*
UNS S31254
UNS S32654
Alloy 625
Base (1)
3-4
5-8
9-12
*Note: Illustrative example. UNS S32654 is considerably
less expensive than Alloy 625 according
to historical market-price-level indications, mainly
influenced by fluctuations in raw material costs.
32
gation once corrosion
has initiated.
This means that the
use of stainless steel
in a specific environment
or application
is usually an either/or
situation. That is, the
material is either passive
or tends to corrode
rapidly, with little
scope for intermediate
conditions. This
can be compared to
carbon steel, where
a corrosion allowance
is commonly used
and where gradual,
uniform corrosion is
expected. If the pasFIGURE
2. USN S32654 has more favorable r values in all directions,
which makes it suitable for pressed components, such as sheets for
PHEs
sive layer is maintained, stainless
steel can almost last forever.
Categories of stainless steel
Traditionally, stainless steels are
categorized according to their microstructure
into ferritic, austenitic,
duplex and martensitic and precipitation-hardening
stainless steels.
Ferritic stainless steels. Ferritic
stainless steels, also known as Crsteels,
are alloyed with chromium
(Cr) but with either a small amount of
nickel (Ni) or no nickel at all. Molybdenum
(Mb) is added to some steels
to improve corrosion resistance,
while alloying with niobium (Nb) and/
or titanium (Ti) improves weldability.
These steels
are magnetic and
have high resistance to stress corrosion
cracking (SCC) but moderate
resistance to uniform, pitting and
crevice corrosion.
High-temperature ferritic stainless
steels for use at temperatures
between 800 and 1,150°C are also
available.
Austenitic stainless steels. AusSteel
grade
Ultra 654 SMO
Ultra 254 SMO
Alloy 31
Alloy 625
Alloy 22
Alloy C-276
Alloy 59
EN
C
tenitic stainless steels are the largest
group of stainless steels and can be
divided into five subcategories:
* Cr-Ni steels
* Cr-Mn steels
* Cr-Ni-Mo steels
* High-performance austenitic steels
* High-temperature austenitic steels
Austenitic stainless steels are nonmagnetic
in the solution-annealed
condition. They have moderate to
excellent corrosion resistance, depending
on the steel.
Cr-Ni steels (18-8). These " general
purpose " stainless steels are
mainly alloyed with chromium and
nickel but with no molybdenum.
They are sometimes referred to as
18-8 type stainless steels in reference
to their approximate chromium
and nickel content. In the
past, titanium and niobium stabilization
was used to improve their
resistance to intergranular corrosion,
but this is not necessary for
modern low-carbon Cr-Ni steels.
They have moderate resistance to
TABLE 2. ALLOYING ELEMENTS, % COMPOSITION
UNS
1.4652
1.4547
1.4562
2.4856
2.4602
2.4819
2.4605
S32654
S31254
N08031
N06625
N06022
N10276
N06059
0.010
0.010
0.015
0.030
0.010
0.010
0.010
Cr
24
20
27
22
21
23
Ni
22
18
31
Mo
7.3
6.1
6.5
Min. 58 9.0
Min. 50
16 Min 52
Min. 60
13
16
16
N
0.5
0.02
0.2
-
-
-
-
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
Other
3.5 Mn, Cu
Cu
2 Mn, Cu
3.5 Nb + Ta
3 W, 2.5 Co
3.5 W, 2.5 Co
0.1-0.4 Al
JULY 2022
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Chemical Engineering July 2022

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