See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/235937736
Corrosion behaviour of 316L stainless steel in PTA slurry
Article in Corrosion Engineering Science and Technology · November 2012
DOI: 10.1179/1743278212Y.0000000059
CITATIONS
READS
2
1,382
4 authors, including:
Junlei Tang
Institute for Carbon Neutrality & School of Chemistry and Chemical Engineering Southwest Petroleum University
83 PUBLICATIONS 941 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Coatings View project
Corrosion inhibitors for CO2 corrosion in oil and gas production View project
All content following this page was uploaded by Junlei Tang on 03 April 2016.
The user has requested enhancement of the downloaded file.
Corrosion behaviour of 316L stainless steel in
PTA slurry
P. F. Ju, Y. Zuo*, J. L. Tang and Y. M. Tang
The corrosion behaviour of 316L stainless steel (SS) in acetic–10% formic acid mixture containing
Br2 and in simulated purified terephthalic acid (PTA) slurry was investigated to understand the
corrosion mechanism of 316L SS in the drying process of PTA slurry. The results show that in
acetic–10% formic acid mixture containing Br2, the corrosion rate of 316L SS increases with the
increase in Br2 concentration and temperature. In the PTA slurry, the corrosion rate of 316L SS
changes in a parabolic form with an increase in the solid/liquid ratio of the slurry. The slurry has
the strongest corrosivity when the solid/liquid ratio of the slurry reaches ,6?25. In a half dry–half
wet PTA slurry, when the 316L steel surface remained at a suitable temperature to avoid
continuous liquid film on the surface, the corrosion rate of the steel will be greatly reduced.
Keywords: Stainless steel, Acetic acid, Corrosion, PTA, Solid/liquid ratio
Introduction
The polyester industry has been developing rapidly in
recent years. As the main raw material of polyester,
purified terephthalic acid (PTA) is in great demand.1 In
the PTA process, paraxylene is oxidised to terephthalic
acid. Manganese acetate and cobalt acetate are used as
catalysts, tetrabromoethane is used as accelerator and
acetic acid is the dissolvent.2 Hence, acetic acid containing Br2 is the primary solution in PTA slurry, which is
strongly corrosive to stainless steel equipments. During
the drying process of PTA slurry, the solvent is gradually evaporated in the dryer. It is frequently observed
in practical industries that the middle part of the dryer
was most severely corroded, which indicates a relationship between the solid/liquid ratio of the PTA slurry and
the corrosion rate.
The corrosion behaviour of stainless steel in acetic acid
with or without halide ions has been reported by many
authors.3–9 However, a few researches were focused on
the corrosion behaviour of stainless steel in PTA slurry,
which is important to industrial production. In this
research, the corrosion behaviour of 316L stainless steel
(SS) in the simulated drying process of PTA slurry was
studied. The effects of the solid/liquid ratio of the slurry
and temperature on the corrosion behaviour of 316L SS
were investigated.
Experimental
The studied material was 316L SS with the following chemical composition (wt-%): Fe–0?75Si–2?00Mn–
0?045P–2?28Mo–17?14Cr–12?58Ni–0?030C–0?030S. The
School of Materials Science and Engineering, Beijing University of
chemical Technology, Beijing 100029, China
*Corresponding author, email zuoy@mail.buct.edu.cn
ß 2013 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 7 June 2012; accepted 17 September 2012
DOI 10.1179/1743278212Y.0000000059
sample surface was abraded with abrasive papers up to
no. 1000 and then was degreased with acetone.
The corrosion rate of 316L SS in boiling 90 wt-%
acetic plus 10 wt-% formic acid mixture containing
400 ppm Br2 was measured by weight loss test. The
addition of formic acid was to accelerate the corrosion
rate. The PTA slurry was composed of 150 mL PTA and
60 mL acetic/formic acid mixture containing 400 ppm
Br2. Figure 1 shows the schematic diagram simulating
the PTA slurry drying process. The solution was
evaporated from the slurry at 105uC and collected by a
measuring cylinder; therefore, by measuring the evaporated volume, the solid/liquid ratio of the slurry can
be calculated (Table 1). The solid/liquid ratio was
calculated by dividing the purified terephthalic volume
by the rest acetic/formic acid mixture in the slurry. After
the solid/liquid ratio of the slurry reached the required
value, a vertical reflux condensation tube was connected
with the flask to keep the solid/liquid ratio of the slurry
stable. The specimens were immersed in slurries with
different solid/liquid ratios for 72 h at 105uC. Scanning
electron and optical microscopes were used to observe
the surface of the specimens after immersion.
A special experimental equipment was designed to
study the influence of the steel surface temperature on
the corrosion behaviour in PTA slurry, as shown in
Fig. 2. The container was made of 316L SS. A 316L SS
specimen, which has the same size as the container
bottom, it was put closely at the bottom and sealed with
polytetrafluoroethylene. Then, PTA slurry was added in
the container, and the whole container was heated to
150uC by an oil bath. Three other 316L SS specimens
were immersed in the middle part of the slurry. The
temperature of the middle part of the slurry was 105uC,
as measured by a thermometer. To keep the solid/liquid
ratio of the slurry stable, the acetic/formic acid mixture
containing 400 ppm Br2 was added into the slurry at the
rate of 40 mL h21, as the slurry was evaporated during
Corrosion Engineering, Science and Technology
2013
VOL
48
NO
3
207
Ju et al.
Corrosion behaviour of 316L stainless steel in PTA slurry
1 Schematic diagram simulating PTA slurry drying process
the testing process. During the test, the Br2 concentration of PTA slurry may increase gradually due to the
solution evaporation; however, the results on the influence of surface temperature on corrosion rate would
not be influenced.
1: container made of 316L SS; 2: 316L specimen at container bottom; 3: 316L specimens immersed in middle
part of PTA slurry; 4: constant temperature oil bathing;
5: mechanical stirring; 6: solution replenish; 7: flowing
regulator
2 Schematic diagram of experiment facility to study influence of temperature on corrosion behaviour of 316L
SS in PTA slurry
Results and discussion
Corrosion behaviour of 316L SS in boiling acetic
and formic acid mixture
Figure 3 shows the corrosion rate of 316L SS in boiling
acetic and formic acid mixture containing different Br2
concentrations at 105uC. According to Fig. 3, the corrosion rate increases as the Br2 concentration increases. The
corrosion resistance of stainless steels is dependent on the
passive film on the surface. In boiling acetic and formic
acid mixture, which is a reducing medium, the passive film
is not stable, and Br2 ions would adsorb on the surface
defects10 and damage the passive film. Figure 4 shows
the morphology of the 316L samples after immersion
in boiling acetic and formic acid mixture containing
400 ppm Br2. It can be seen that the corrosion morphology of 316L SS in the medium is mainly uniform with
some shallow pits, which is in accordance with a previous
report.11
3 Relationship between corrosion rate of 316L SS and
Br2 concentration in boiling acetic and formic acid
mixture
Effect of temperature on corrosion rate of 316L
steel in acetic/formic acid mixture containing
Br2
The conductivity of non-aqueous acetic acid increases
rapidly with increasing temperature. On the other hand,
at higher temperature, more Hz tends to ionise from
acetic acid, which accelerates the dissolve rate of stainless
steel.11 In addition, the porosity of the passive film
increases with temperature, which can accelerate the
permeation of Br2 in the passive film.12
Figure 5 shows the corrosion rate of 316L SS in the
acetic/10% formic acid mixture containing 10 ppm Br2 at
different temperatures. As shown in Fig. 5, the corrosion
rate rises as the temperature increases. Sekine4 reported
that the corrosion of 430 SS in aqueous formic and acetic
acid systems is determined by the solution conductivity.
Table 1 Corrosion rate of 316L SS at different PTA humidities
Volume of evaporated solution from PTA slurry/mL
Volume of rest solution in PTA slurry/mL
Solid/liquid ratio/a.u.
Corrosion rate/g m22 h21
208
Corrosion Engineering, Science and Technology
0
60
2.50
0.05
2013
VOL
12
48
3.12
0.18
48
20
40
3.75
0.25
NO
3
25
35
4.28
0.41
30
30
5.00
0.50
36
24
6.25
0.52
42
18
8.33
0.48
48
12
12.5
0.43
54
6
25.0
0.22
Ju et al.
Corrosion behaviour of 316L stainless steel in PTA slurry
4 Corrosion morphology of 316L SS after immersed in boiling acetic and formic acid mixture containing 400 ppm Br2
(10 wt-% formic acid)
5 Relationship between corrosion rate of 316L SS and
temperature of acetic and formic acid mixture containing 10 ppm Br2
Corrosion behaviour of 316L SS in PTA slurry at
different solid/liquid ratios
Table 1 and Figure 6 show the corrosion rate of 316L SS
in PTA slurry with different solid/liquid ratios. The
solid/liquid ratio was calculated from the evaporated
solution volume. It is seen that as the solid/liquid ratio
of the PTA slurry increases from the initial value to
,6?25, the corrosion rate rises quickly until a peak
value. After the peak value, the corrosion rate decreases
as the solid/liquid ratio increases.
The colour of the initial acetic/formic mixture
containing Br2 was orange yellow, while the evaporated
solution was colourless and transparent. When AgNO3
was added in the two solutions, different results were
observed. In the initial solution, white and yellow
precipitation was produced, and the solution turned
colourless and transparent, which is due to the reaction
6 Relationship between corrosion rate of 316L SS and
solid/liquid ratio of PTA slurry
between Br2 in the solution and AgNO3, while in the
evaporated solution, white precipitation was produced,
and the solution remained colourless. The white precipitation is due to AgAc, which is insoluble in HAc. This
experiment confirms that the evaporated solution did not
contain Br2; hence, during the heating process, Br2 in the
rest of the PTA slurry was concentrated. This may explain
the increase in the corrosion rate with the increased solid/
liquid ratio. When the solid/liquid ratio exceeds a critical
value (,6?25 in Fig. 6), the liquid film on steel surface
would turn thinner and discontinuous, which leads to
decreased corrosion rate, although the Br2 concentration
is at high levels.
From the above results, the corrosion phenomenon in
a PTA slurry dryer may be explained. At the initial
section, the temperature of the PTA slurry is low, and
the corrosion is low. As the slurry is put forward in
the dryer and heated, both the slurry temperature and
Table 2 Corrosion behaviour of 316L SS in PTA slurry at different temperatures
Specimen immersion situation
Immersion time/h Corrosion rate/g m22 h21 Corrosion phenomenon
316L SS at the bottom (150uC)
316L SS in the slurry (105uC)
316L SS in the slurry (105uC)
316L SS in the slurry near the solution supply (105uC)
72
72
72
72
8.3361024
0.1372
0.1075
0.6655
Glossy and slightly corroded
Rugged and severely corroded
Rugged and severely corroded
Rugged and severely corroded
Corrosion Engineering, Science and Technology
2013
VOL
48
NO
3
209
Ju et al.
Corrosion behaviour of 316L stainless steel in PTA slurry
a specimen at container bottom (150uC); b specimen in middle part of slurry (105uC)
7 Morphology of 316L SS specimens after immersion in PTA slurry at different locations
the Br2 concentration increase, and the corrosivity of the
slurry increases. However, with the drying process going
on, the slurry gradually turns to solid PTA, and corrosion
decreases again. Therefore, at the middle section of the
drying process, the slurry shows the most severe
corrosivity to the stainless steel dryer.
Influence of 316L SS surface temperature on
corrosion rate in PTA slurry
Table 2 shows the measured corrosion rates of 316L SS
specimens in PTA slurry at different temperatures. It is
seen that the corrosion rate of the specimen at the
container bottom, which remained 150uC during the
test, is only 8?3361024 g m22 h21, three orders of
magnitude lower than that of the specimens in the
middle part of the slurry. The surface morphology of
the specimens after immersion is shown in Fig. 7. The
specimen at 150uC was only slightly corroded and the
surface remained smooth, while for the specimens
immersed in the slurry, where the temperature was only
105uC, the corrosion rate was obviously higher. The
highest corrosion rate was observed for the specimen
located near the solution supply (Fig. 2). The above
results may be explained as follows: corrosion reactions
as electrochemical processes depend on the presence of
the electrolyte on the surface. In the solid/liquid mixed
slurry, when the surface temperature of the specimen
is high enough, the liquid film on the surface may
be evaporated quickly. Hence, corrosion is greatly
decreased. While for the other specimens in the middle
part of the slurry, because the specimen surface is heated
by the PTA slurry, the liquid film would be always
present on the surface, resulting in a continued corrosion
reaction. The above results show that in a half dry–half
wet PTA slurry, if the 316L steel surface remained
at a high temperature to avoid continuous liquid film on
the surface, the corrosion rate of the steel may be
reduced.
210
Corrosion Engineering, Science and Technology
2013
VOL
48
Conclusions
1. In acetic/10% formic acid mixture containing Br2,
the corrosion rate of 316L SS increases with the
increases in Br2 concentration and temperature.
2. In PTA slurry, the corrosion rate of 316L SS shows
a parabolic relationship to the solid/liquid ratio of the
slurry. The slurry has the strongest corrosivity when the
solid/liquid ratio of the slurry reaches ,6?25.
3. In a half dry half wet PTA slurry, if the 316L steel
surface remained at a high temperature to avoid
continuous liquid film on the surface, the corrosion rate
of the steel may be greatly reduced.
Acknowledgement
The authors are grateful to the National Natural Science
Foundation of China (contract no. 51210001) for
support to this work.
References
1. G. R. Pophali, R. Khan, R. S. Dhodapkar, T. Nandy and
S. Devotta: J. Environ. Manage., 2007, 85, 1024–1033.
2. S. Q. Mu, H. Y. Su, T. Jia and Y. Gu: J. Chu. Comput. Chem. Eng.,
2004, 28, 2219–2231.
3. I. Sekine, S. Hatakeyama and Y. Nakazawa: Corros. Sci., 1987, 27,
275–288.
4. I. Sekine, S. Hatakeyama and Y. Nakazawa: Electrochim. Acta,
1987, 32, 915–920.
5. I. Sekine and K. Senoo: Corros. Sci. 1984, 24, 439–448.
6. I. Sekine, T. Kawase, M. Kobayashi and M. Yuasa: Corros. Sci.,
1991, 32, 815–825.
7. T. Alan, R. Mary Ryan, W. Anthony and S. Q. Zhou: Corros. Sci.,
2003, 45, 1051–1072.
8. M. A. Veloz and G. González: Electrochim. Acta, 2002, 48, 135–144.
9. X. Q. Cheng, X. G. Li and C. F. Dong: Int. J. Miner. Metall.
Mater., 2009, 16, 170–176.
10. T. Suter and H. Bohni: Electrochim. Acta, 1997, 42, 3275–3280.
11. G. Q. Liu, Z. Y. Zhu and W. Ke: Corros. Sci. Prot. Technol., 2000,
12, 296–299.
12. W. Bogaerts, A. van Hanute and M. J. Brabers: ‘Localized
corrosion’, 31; 1981, Houston, TX, NACE.
NO
3
Copyright of Corrosion Engineering, Science & Technology is the property of Maney Publishing and its content
may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express
written permission. However, users may print, download, or email articles for individual use.
View publication stats