Chemical Engineering January 2017 - 33

Current density, log i
Active
zone
imax
Cr
Cr
1
Tube sheet
2
First condensate formed: 120-130oC
Reboiling: corrosion increases with
increasing gas inlet temperature
Cr
Epp
FIGURE 8. This schematic anodic polarization curve illustrates
the effect of chromium on the electrochemical parameters of
stainless steel. In nitric acid, the corrosion potential is near
the transpassivity potential, Etr.
By comparing the Huey test results of
several materials, it is possible to rank the
corrosion resistance of standard and special
steels. Huey tests for various grades show
that standard 304L is better in the quenchannealed
state, but exhibits an unacceptable
variation in the corrosion rate for the
sensitized condition.
Huey tests also show that high-chromium
stainless steel displays a slower corrosion
rate on the order of 0.12 mm/yr, equivalent
to 4.7 milli-inch (mils) per year. This can
be attributed to the material's extremely
low carbon content and its lower impurity
content, as well as closer tolerances
for impurities than similar, standard steels.
These properties equip the high-alloyed nitric
acid service grade with excellent resisTABLE
1. INFLUENCE OF ELEMENTS ON THE
CORROSION RESISTANCE OF STAINLESS STEEL IN
NITRIC ACID
≤ 65%
HNO3
QA/QA +
Sensitized*
Positive
Cr
Si
Negative C
Si
P
S
Mo
Ti
Nb**
x
x
x
x
x
x (0.5)
x
x
> 65%
HNO3
QA
x
x (2)
x (0.02)
x (0.1)
x (0.01)
x (0.01)
x
x
x (0.01)
x
80
100
UNS S32304
> 65%
HNO3
QA +
Sensitized*
120
Boiling point
curve
Etr
Potential, E
tance to both corrosion in nitric acid and
intergranular corrosion.
Recognizing high-corrosion situations
One of the locations in nitric acid plants
where the most severe corrosive conditions
are found is normally the device or
unit that is used to concentrate the oxides
of nitrogen (NOx) from a gaseous to liquid
state - usually a condenser or cooler
(see Figure 7). Other very
FIGURE 9. Hot-dewpoint corrosion can commonly occur in the
coolers and condensers in nitric acid plants
Passive
zone
Transpassive
zone
NOx gas
1
2
Tube
severe locations
are the tail-gas preheaters and boiler
feedwater heaters.
A common corrosion case occurs in coolers
or condensers with process gas on the
tube side. Corrosion takes place at the inlet
where the first condensate is formed, the
reboiling of which causes very severe corrosive
conditions. Especially harsh conditions
are found where condensation and evaporation
take place (as in graphite block heat
exchangers). This leads to the kind of attack
illustrated in Figure 9.
It is useful to recognize the high-corrosion
situations that may occur in equipment durTemperature,
oC
140
UNS S304/S30400
High-chromium alloy
UNS S32906
FIGURE 10. An isocorrosion
diagram is a useful tool for
identifying high-corrosion
situations during the processdesign
phase
x
x
60
* Quench-annealed (QA) and sensitization at 650°C for 1 h
** Pronounced end-grain attack
( ) denotes minimum level for influence
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
40
0%
20%
JANUARY 2017
40%
60%
HNO3, wt. %
33
80%
100%
http://WWW.CHEMENGONLINE.COM
Chemical Engineering January 2017

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