Chemical Engineering January 2017 - 30

Corrosion rate, mm/yr (mils/yr)
0.15
(6)
317L
Acid composition: 43.4% P2O5, 1.7%
F-, 100 ppm CI-, 3.5% SO2-, 0.6%
Fe3+, 0.8% AI3+, 0.5% Ca2+, 0.4%
Mg2+, 1.1% SiO2
Results of 1 + 3 + 3 days exposure at 90oC (194oF).
0.1 mm/yr = 4 mils/yr
though the value is equivalent to 317L. The
element Mo also facilitates passivation, but
does not have any marked effect in reducing
corrosion in the passive state. This is likewise
true of copper (Cu), which is included in this
grade, but not the standard grades AISI 316L
and 317L.
Alloy 20Cb3
0.10
(4)
Alloy 904
Further to our understanding of these elements,
the addition of nickel (Ni) also plays a
major role in corrosion resistance. Nickel is a
strong austenitic former, which, in combination
with chromium and the other elements,
helps to imbue the material with the desired
microstructure properties and also improved
mechanical integrity.
0.05
(2)
Alloy G

High-chromium alloy
20
FIGURE 3. The corrosion rate
of certain alloys are shown
in relation to the chromium
content
25
27
% Cr
mium are twofold: it markedly reduces
the rate of corrosion of a steel in the passive
state, which is also important in the
practice of preventing corrosion. Figure 3
shows the relationship between chromium
content and corrosion rate. A high-chromium-alloy
austenitic stainless steel that
is designed for service in highly corrosive
conditions is pictured alongside traditional
graphite tube in Figure 4. The corrosionresistant
austenitic stainless steel shown
in Figure 4 contains higher nominal levels
(wt. %) of chromium than other common
steel grades. As mentioned, this can reduce
the material's corrosion rate in the passive
state, thereby preventing corrosion.
High-chromium-alloy material also contains
a higher level of molybdenum (Mo) than 316L,
For operators, a practical advantage of mechanical
integrity is higher production time,
as it becomes unnecessary to plug failed or
broken tubes due to operational or cleaning
damage. It is also possible to clean the material
mechanically, if necessary, without risk
of fracturing the heat exchanger tubes. It is
clear from corrosion-rate data that, due to
the enhanced high-alloying contents of chromium
and nickel, a high-chromium alloy has
considerably better resistance than standard
stainless steels of type AISI 304, AISI 316L
and 317L.
FIGURE 4. A high-chromium
alloy austenitic stainless steel
tube (left) pictured alongside
standard graphite tube (right).
30
Environmental contaminants
As previously mentioned, levels of contaminants
in the fertilizer production environment
are a major concern when assessing
the capabilities of replacement tubes. Figure
5 illustrates the strong negative influence of
contaminating chloride and fluoride ions on a
steel's corrosion resistance when combined
at high temperatures. As shown, a maximum
chloride ion level of around 700 ppm can be
tolerated by a high-chromium alloy without
risk of active corrosion. The significant influence
of temperature upon the reaction is
shown in Figure 6. An increase of 10°C (50°F)
can nearly double the corrosion rate and,
once again, the influence of a high chromium
content can be seen.
Real-world example. A full-scale trial was
conducted in order to assess the performance
properties of high-chromium alloys
compared to graphite tube. For the test, a
heat exchanger was equipped with a highchromium
alloy grade and run parallel to a
graphite block heat exchanger. A noticeable
difference occurred during the service period
when it became necessary to increase the
steam pressure in the graphite unit but not in
the high-alloyed unit. This difference can be
attributed to the reduced scaling rate of the
high-alloyed austenitic material. This, in comCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2017
http://WWW.CHEMENGONLINE.COM
Chemical Engineering January 2017

Table of Contents for the Digital Edition of Chemical Engineering January 2017
