How Does Black Liquor Corrosivity Change with decreasing REA Matthew Tunnicliffe Research Scientist Corrosion and Thermochemical Processes 604‐222‐5203 [email protected]Victor Padilla Manager Corrosion and Thermochemical Processes 604‐222‐5725 [email protected]Abstract Reducing the Effective Alkali (EA) targets in the continuous kraft digesters can reduce white liquor consumption, increase pulp yield, and decrease the load in the chemical recovery line. It is not certain whether this will have a negative effect on carbon steels assets, and therefore there are concerns with unpredictable rates of corrosion. The objective of the work is to measure the impact of reducing REA on the corrosion of carbon steel in both hardwood and softwood liquors using potentiodynamic (PDP), open circuit potential (OCP), and five day immersion studies in laboratory autoclaves. The REA in our studies ranged from 2 g/L to over 10 g/L as Na 2 O and were adjusted using weak sulphuric acid or caustic. The results of this work show that the corrosion rate of carbon steel in hardwood liquor was low and independent of REA, whereas carbon steel in softwood liquors generally increased with decreasing REA. This work provides insight for mill operations to consider with respect to liquor corrosivity and how one may manage process changes without an increase in risk to fixed equipment
Transcript
How Does Black Liquor Corrosivity Change with decreasing REA
carbon steel cylindrical working electrode (WE). The chemical composition of the carbon steel is given in Table 1.
The WE surface were ground to 600 grit with SiC paper prior to each test for the open circuit potential (OCP) and
potentiodynamic (PDP) test, whereas the disc shaped coupons were ground to 120 grit with SiC paper prior to
disc immersion. All tests were run at 150°C ± 2 in an autoclave using an electric heater. All samples were
cleaned off with ethyl alcohol and dried prior to immersion in the test solution.
Table 1. Chemical composition of test coupons
Test coupon C (%) Mn (%) P(%) S(%) Si(%)
A516 Grade 70 0.25 0.99 0.008 0.15 0.19
Black liquor samples were obtained from three different mills from the digester extraction or wash recirculation
line. The three mills pulp furnishes varied from single species hardwood, single species softwood, or a blend of
the two. Table 2 outlines the non‐coastal mill locations, pulp furnish, as received REA, sampling location within
the digester, and the rate of carbon steel corrosion in laboratory immersion experiments using as received
liquor. The REA of the black liquor samples were adjusted as needed by the test matrix by mixing weak sulphuric
acid or caustic to the black liquor samples received from the three mills and re‐titrating the new liquor sample.
Generally, this report targeted a range of REA from 2 to 10 g/L for each liquor sample.
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Table 2. Black liquor samples were obtained from three mills
Mill A
Hardwood Mill A
Softwood Mill B Softwood Mill C Hardwood Mill C Softwood
Pulp furnish Trembling
Aspen, Black Poplar
Jack Pine Hemlock, Cedar, Fir,
Larch Spruce, Balsam, Pine
Trembling aspen Spruce, Jack pine, and trace balsam
fir
Sample location
Extraction line Upper extraction line
/ lower extraction lines
Extraction line /
Wash line
Liquor REA as received (g/L as Na2O)
8.2 7.6 14.1 / 8.6
8.1 /
14.1
4.5 / 2.0
Corrosion rate of Carbon steel in as received liquor (mpy)
4.1 27.7 6.9 / 4.6
0.05 /
0.02
13.4 /
0.74
Immersion testing was used to assess the short term mass loss of carbon steel in mill liquors and assist in
identifying the REA limits. The coupons were ground to a 120 grit surface finish and weighed to the nearest
hundred thousandth of a gram. The coupons were then left in the autoclave for 120 hours at 150°C in 800 mL of
black liquor. When the coupons were removed from the cell, they were acid cleaned using inhibited
hydrochloric acid and reweighed to calculate a corrosion rate in mils per year (mpy), as outlined in ASTM
Standard Method G1.[12]
Results
Electrochemical results
Prior to each potentio‐dynamic polarization (PDP) sweep, the carbon steel samples were held at ‐1300 mVAg/AgCl
while the black liquor was introduced to the autoclave and heated. This was done to remove corrosion product
that may have formed as a result of being exposed to air or from the alkaline sulphide due to the delays
introduced by the heating. Once the autoclave reached the desired temperature, the PDP scan was performed
from ‐1300 mVAg/AgCl to ‐500 mVAg/AgCl at a scan rate of 0.167mV.s‐1. Each experiment was performed using a
fresh 800mL liquor sample and resurfaced working electrodes.
Previous work has been helpful to identity a critical potential for carbon steel in alkaline sulphide
environment.[9] When carbon steel stabilizes above this critical potential, passivation of carbon steel and low
rates of corrosion would be anticipated. Conversely, high rates of corrosion would be anticipated when the
potential of carbon steel falls below this critical potential. Generally, this critical potential corresponds to ‐900
mVAg/AgCl; however specific liquor chemistries might shift this peak and careful interpretation of each PDP curve
is needed.[10]
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Mill A Softwood and Hardwood Lower Extraction Liquor
Figure 1 and Figure 2 shows the OPC and PDP curves for the electrochemical behaviour of carbon steel exposed
to hardwood and softwood extraction liquors from mill A.
In Figure 1a, the OCP of carbon steel in the mill A hardwood extraction liquor was generally stable over 5 hours
at potentials anodic to ‐900mVAg/AgCl (the formation of Fe3O4) in all REA tests. This suggests that carbon steel
formed a protective film for all of the tested REA. When looking at the PDP curves in Figure 1b, it is possible to
see an increase in current density as the REA is decreased. The behaviour of the carbon steel when exposed to
the as‐received liquor (REA of 8.2) is in good agreement with the typical behavior of carbon steel exposed to
black liquor presented in an earlier study.[10] The current peaks observed correspond to the formation of FeS
(location A), Fe3O4 (location B), and the oxidation of sulfur compounds from within the corrosion product
(location C). However, these peaks become less apparent when the REA was modified; which indicates that
other reactions may dominate the electrochemical behaviour of carbon steel.
Conversely, in Figure 2a the OCP of carbon steel in mill A softwood extraction liquor showed less stable
behaviour than carbon steel in the hardwood extraction liquor. With the exception of the REA 1.1 experiment
the OCP remained at or below ~ ‐900mVAg/AgCl for the majority of the 24 hour period (not shown) where higher
corrosion rates of corrosion would be theoretically possible. When looking the PDP curves in Figure 2b, there
was a general increase in corrosion current density and the disappearance of the anticipated current peaks.
Nevertheless, the corrosion current density when exposed to the softwood liquor was higher than when
exposed to the hardwood liquors for all of the tested REA as compared to Figure 1b.
Figure 1. Mill A: OCP and PDP curves of carbon steel when exposed to hardwood extraction liquor with changing REA at 150°C
a) b)
C
A B
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Figure 2. Mill A: Carbon steel OCP and PDP curves when exposed to softwood extraction liquor with changing REA at 150°C
Mill B Upper and Lower Softwood extraction Liquor
Figure 3 and Figure 4 shows the OCP and PDP curves for the electrochemical behaviour of carbon steel exposed
to upper and lower softwood extraction liquors from mill B. In general, the PDP curves of carbon steel in mill B
are shifted to higher current densities when compared with the other mill liquors tested. The OCP in Figure 3a
readings revealed a shift towards more anodic potentials with a decrease in REA. This shift has been attributed
to the formation of a protective film free of sulphur impurities, however the OCP readings also showed
fluctuations (REA 2.2 experiment) that may indicate instability of the corrosion product.[11] Similar observations
were made when carbon steel was exposed to REA 4.4, and 8.6 after longer OCP exposures. Figure 3b shows
that upon decreasing the REA of the lower extraction liquor the PDP curve remained at high current densities
and electrochemical noise became apparent between ‐1000mV and ‐800 mV. However, upon decreasing the
REA of these liquors further, the PDP of the lower extraction liquor better revealed the oxidation of sulphur at ‐
800mV and the anodic peaks at lower potentials were less visible.
Figure 3. Mill B: Carbon steel OCP and PDP curves when exposed to lower extraction softwood liquor
a) b)
a) b)
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The results for the upper extraction liquor shown in Figure 4 are in good agreement with the results from the
lower extraction liquor. Decreasing the REA generally shifted the OCP to more anodic potential (Figure 4a). The
decrease in REA generally shifts the PDP curves to lower current densities (Figure 4b). Similar to the lower
extraction liquor, the lowest current density was observed for the REA value of 4.6.
The behaviour observed in the liquors from Mill B suggests the formation of a protective film in the first 5 hours
of OCP experiments decreasing the REA. It is not clear how lowering the REA caused this effect, and more work
is needed to elucidate the cause.
Figure 4. Mill B: Carbon steel OCP and PDP curves when immersed in upper extraction liquor at 150°C
Mill C Hardwood and softwood extraction Liquor
Figure 5 and Figure 6 shows the OCP and PDP curves for the electrochemical behaviour of carbon steel exposed
to hardwood and softwood extraction liquors from mill C.
The OCP values depicted in Figure 5a show that there is little change in the potential of carbon steel when
exposed to hardwood liquors, even when the REA is decreased over 24 hours (only 5 hours shown). The OCP
values are constant and anodic to ‐900mVAg/AgCl which corresponds to where the formation of Fe3O4 should form
as a protective film. This is in good agreement with the hardwood results from mill A. Figure 5b shows that
decreasing the REA shifts the PDP graphs to lower current densities, and Ecorr becomes more anodic. This is in
agreement with the previous results.
In contrast, Figure 6 shows the electrochemical performance of carbon steel when exposed to softwood liquors.
The OCP values in Figure 6a, while initially high, drop after a few hours below ‐900mVAg/AgCl in all the tests longer
than 8 hour. This drop in OCP suggest that carbon steel is not able to form a protective and tenacious protective
film when exposed to these liquors and appears to be independent of REA.
When looking at the PDP curves in Figure 6b, it is possible to see an increase in current density as the REA is
decreased.
a) b)
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Figure 5. Mill C: Carbon steel OCP and PDP curves when immersed in hardwood extraction liquor at 150°C
Figure 6. Mill C: Carbon steel OCP and PDP curves when immersed in softwood extraction liquor at 150°C
Mill C Hardwood and Softwood Wash liquor
Figure 7 and Figure 8 shows the OCP and PDP curves for the electrochemical behaviour of carbon steel exposed
to hardwood and softwood wash liquors from mill C. The results are in agreement with the results seen on the
Mill C extraction liquors. The OCP values depicted in Figure 7a show that the protective film formed by carbon
steel is stable for all tested REAs. However, in contrast, Figure 7b shows that decreasing the REA shifts the PDP
graphs to higher current densities, indicating faster reaction kinetics. The Ecorr values observed in Figure 7b
become more anodic as REA decreases; which is in good agreement with the OCP values observed in Figure 7a.
a) b)
a) b)
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Figure 7. Mill C: Carbon steel OCP and PDP curves when immersed in hardwood wash liquor at 150°C
Figure 8 shows the electrochemical performance of carbon steel when exposed to softwood wash liquors. The
OCP values in Figure 8a stabilized in potentials anodic to ‐900mVAg/AgCl after a couple of hours in all REA tests.
This behaviour suggest that carbon steel is able to form a protective and tenacious protective film when exposed
to these liquors and time to stabilize may be a function of REA. When looking at the PDP curves in Figure 8b, it is
possible to see an increase in current density as the REA is decreased. This also suggest an increase in reaction
kinetics which could explain the seemingly faster formation of a protective film observed in Figure 8a.
Figure 8. Mill C: Carbon steel OCP and PDP curves when immersed in softwood wash liquor at 150 °C
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Immersion tests results
Figure 9 shows the corrosion rates for carbon steel after immersion in black liquor samples for 120 hours at 150
°C. Generally a corrosion rate of less than 5 mpy is considered acceptable. To put this value in perspective: with
a corrosion rate equal or lower to 5 mpy, one could expect a life of 50 years or greater for equipment with 0.25
inch (6.3 mm) corrosion allowance, assuming general corrosion is the predominant damage mode. The 5 mpy
limit is indicated in Figure 9 with a dashed red line.
The results shown in Figure 9 suggest that there is no clear correlation between corrosion rate and REA, with the
exception for the softwood extraction liquors for mill A and C. Similarly, for hardwood liquors, the corrosion rate
remained relatively low for all tested liquors and different REA levels.
Figure 9. Corrosion rates for carbon steel after immersion in black liquor samples for 120 hours at 150 °C all mills A, B, and C
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Conclusions
The work done confirmed previous experimental results and empirical data on the relative corrosivity of pulp
furnish. Hardwood is less corrosive than softwood under laboratory conditions. Only Mill A hardwood extraction
black liquor corroded carbon steel at a rate exceeding 5 mpy. This is attributed to the presence of extractives
that act as chelates that disturb the protective passive film in carbon steel.
OCP testing revealed stable behaviour of carbon steel in hardwood liquors. Oscillations appeared in softwood
liquors in longer term studies not displayed in this report. The presence of oscillations generally resulted in
moderate to high rates of corrosion. The highest measured corrosion rate of carbon steel when OCP oscillations
were present was greater than 40 mpy; while the lowest corrosion rates produced from a liquor known to
produce oscillations was 6 mpy. As expected, the hardwood liquors did not produced potential oscillations;
which suggest the formation a stable corrosion product. Decreasing REA generally increased the OCP values.
The potentiodynamic tests were in good agreement with the OCP measurements, and the results were in
agreement with the expected behaviour of carbon steel exposed to hardwood and softwood liquor. However,
no strong conclusions can be drawn on the effect of reducing REA on the corrosion current density or the
reaction kinetics. More work is needed to understand the effect of reducing REA on the electrochemical
behaviour of carbon steel, and perhaps other electrochemical methods should be explored.
The immersion test results showed that there is no correlation between corrosion rate and change in REA. For
hardwood liquors, the corrosion rate remained relatively low for all tested liquors and different REA levels. With
the exception for the softwood extraction liquors for mill A and C, there was no significant change in corrosion
rate with decreasing REA. This suggest that there is an opportunity to carefully reduce the EA targets of black
liquor in a digester. This could possibly help debottleneck the chemical recovery process, and increase pulping
yield. Additionally, this would translate to a reduction of white liquor consumption, and conversely a reduction
in the amount of non‐process elements that are introduced with these purchases. Reducing EA targets in the
continuous kraft digesters offers several benefits.
It is highly recommended to approach this change with caution, and implement an online corrosion monitoring
program at the mill to measure the effect of reducing the REA real time. Transient conditions are hard to
replicate in the laboratory, and for those mills that pulp both hardwood and softwood species, there might be
an important effect that was not considered in this study during a species transition.
References
[1] J. Gullichsen and C.‐J. Fogelholm, Papermaking Science and Technology ‐ Chemical Pulping, vol. 6A. Fapet
Oy, 1999.
[2] R. G. Kelly, P.J. Ambrose, R. C. Wilson, K. A. Jefferies, and S. Kannan, “Results of corrosion coupon
exposures of ferrous materials in black liquor,” TAPPI J., vol. 79, no. 11, pp. 148–154, 1996.
[3] A. Wensley and D. Christie, “Corrosion of Evaporators,” in Engineering, Pulping, & Environmental
Conference, 2005.
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[4] A. Wensley and P. Champagne, “Effect of sulfidity on the corrosivity of white green and black liquors,” in
CORROSION, 1999.
[5] A. Wensley, “Corrosion of Batch and Continuous Digesters,” in The International Symposium on
Corrosion in the Pulp and Paper Industry, 1998, no. 1.
[6] A. Wensley, “Corrosion testing in evaporator liquors,” in Corrosion, 2006, no. 06242, pp. 1–19.
[7] P. M. Singh and A. Anaya, “Effect of wood species on corrosion behavior of carbon steel and stainless
steels in black liquors,” Corros. Sci., vol. 49, no. 2, pp. 497–509, 2007.
[8] A. F. Maclean, Harold; Gardner, “Heartwood extractives in Digester Corrosion,” Pulp Pap. Mag. Can., vol.
11, pp. 125–130, 1953.
[9] G.M. Barton and A.F. Gardner, “The Chemical Nature of the Acetone Extractive of Western Red Cedar,”
Pulp and Paper Magazine of Canada, vol. 55, no. September, p. 132, 1954.
[10] M. Tunnicliffe, “The Effect of Extractives on the Passivation of Carbon Steel in Synthetic Black Liquor
Environments,” in CORROSION 2018, Phoenix, AZ, 2018.
[11] D.C. Crowe and D. Tromans, “High‐Temperature Polarization Behaviour of Carbon Steel in Alkaline
Sulphide Solutions,” Corrosion, vol. 44, no. 3, p. 142, 1988.
[12] ASTM Standard G1 "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test
Specimens,” ASTM Int. 2012.
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How Does Black Liquor Corrosivity Change with Decreasing Residual Effective Alkali (REA)
TAPPI PEERS
October 27‐30
Session 29
Agenda
Introduction
Experimental Methods• Mill pulp Furnish and corrosion rates
• Liquor REA modification
Results• Short term electrochemical studies
• Longer term immersion studies
Discussion
Conclusion
Introduction
Introduction
The relative happiness of Engineering's and Mill Manager's is directly related to the amount of pulp they make in a year…
• Reducing liquor REA targets can influence reaction kinetics in the digester and is a tool that can be used to increase throughput.
• Decreasing white liquor usage can help debottleneck a mill
• Less chemical will be required if you use less white liquor
…but will lowering REA corrode their digester?
Introduction
Suppliers, consultants, research papers, and process engineers know they have target, so why should we explore this topic?
Authors reference unknown
Kraft black liquor Corrosivity
Inorganic compounds in black liquor
• Previous knowledge has established that corrosion by black liquor is related to temperature, sulphidity, and REA(?). Corrosion is thought to increases with:
Temperature Sulphidity REA
…but no studies could be found that establish a critical REA
Kraft black liquor corrosivity
But organic compounds also play a role!
Previous studies/experience has demonstrated that softwood liquors are generally more than hardwood black liquors, but non have studied specific REA targets
The objective of this paper is to determine if changing black liquor REA targets influences the corrosivity of black liquor
Experimental MethodsNot all experiments need to take a long time
Experimental Methods
Black liquor samples were obtained from three mills across Canada. Two of these mills swing from hardwood to softwood, and the third is a softwood mill.
Mill A
Softwood Jack Pine
Hardwood Trembling Aspen, Black poplar
Mill B
Softwood Hemlock, Cedar, Fir, Larch Spruce, Balsam, Pine
Mill C
Softwood Spruce, Jack pine, and trace balsam fir
Hardwood Trembling aspen
Experimental Methods
Sample location.
• Mill liquor samples were obtained from the Extraction screens and wash zone
Laboratory experiments setup• All experiments were performed in an autoclave at 150°C
• Ag/AgCl reference electrodes were used
Electrochemical Techniques
• 5 hour Open circuit potential
• Potentiodynamic experiments
• 5 day immersion experiments were used
White liquorChips
Extraction liquor
Wash liquor
Continuous Digester
Experimental Methods
Process liquor characterization
• REA titrations were performed using the SCAN technique
• Black liquor samples were modified by adding weak sulphuric acid to target REA
Electrochemical experiments were perform with ASTM practices in mind
• Carbon steel in hardwood liquor was generally stable, and the OCP increased with decreasing REA
• Similar trends with softwood were observed, with the exception of oscillations in OCP
Hardwood
Softwood
Mill B –Extraction Liquor
• Carbon steel in mill be softwood was quite low, and appear to oscillate (unstable?)
Lower extraction
Upper
In longer term studies, and online corrosion monitoring these oscillations were more regular
Mill C – Extraction liquor
• The OCP of carbon steel in Mill C extraction liquor was generally stable
Hardwood
Softwood
Mill C – Wash Liquor
• The OCP of carbon steel in hardwood was stable in both hardwood and softwood
• …however, the carbon steel in the softwood wash liquor took longer to reach a stable OCP
Hardwood
Softwood
Results
The OCP experiments are useful to predict the performance of carbon steel in longer term studies
• High and Stable OCP > ‐900 mV are likely to result in low rates of corrosion
• Low OCP < ‐900mV would likely result in higher rates of corrosion
• OCP Oscillation tend to produce much higher rates of corrosion
Mill A
Softwood Oscillations, and below ‐900mV
Hardwood OCP > ‐900 mV
Mill B
Softwood OCP < ‐900, and limited oscillations
Mill C
Softwood OCP > ‐900 mV
Hardwood OCP > ‐900 mV
ResultsLonger term experiments
Immersion ResultsA corrosion of 5 mpy (red line) represents a low rate of corrosion of carbon steel in black liquor environments
• The rate of carbon steel in hardwood liquors did not appear to be related to REA
Of the hardwood liquors tested; there was no strong correlation between the corrosion of carbon steel and REA
Immersion ResultsA corrosion of 5 mpy (red line) represents a low rate of corrosion of carbon steel in black liquor environments
• Mill C softwood liquor was much more corrosive than the liquor from Mill B and C
• Mill B and C softwood liquors did not show a strong correlation with decreasing REA
• Wash liquor did not appear to be corrosive
Of the liquors tested; there was no strong correlation with REA, but in most cases the corrosion rate of carbon steel was independent of REA
Discussion
Discussion
Could oscillations be a product of laboratory experiments? Yes, but they also occurred in mill liquor in online corrosion monitoring practices at mill A and B
Softwood vs. Hardwood black liquor corrosivity Softwood black liquor was generally more corrosive than hardwood, but limited correlation could be established in this study as to why
Extraction liquor vs. Wash liquor corrosivity Extraction liquor was generally more corrosive than wash liquor
What is in softwood, and how does REA change how it reacts with carbon steel Papers have been written on this subject and it remains unclear at this time
Future work• It would be great to install a corrosion probe close to a device that measures online REA
measurement device• Confirm sulphidity
Conclusion
Conclusions
• This work confirmed previous experimental results relating to the relative corrosivity of pulping liquors to carbon steel when processing hardwood vs. softwood
• The OCP experiments were able to good for predicting relative corrosivity of process liquors with knowledge of PDP experiments
• From immersion testing; • The carbon steel hardwood liquors had low rates of corrosion and were independent of REA
• Mill A had apparent good correlation with liquor corrosivity, whereas Mill B and C did not
