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Kraft Liquor Corrosion
Margaret Gorog Federal Way, WA
Pulp and Paper Corrosion Symposium
Georgia Tech Renewable Bioproducts Institute
November 2014
• Brown Stock Corrosion
• Alkaline Liquor Corrosion
• Black Liquor Corrosion
– Evaporators
– Research
• High Solids Black Liquor Corrosion of Stainless Steel
• Material Selection Summary
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Topics
2
• The pH may drop to levels that may make carbon steel
unsuitable
• Pitting corrosion of carbon steel occurs in filtrate and seal
tanks
– Rather than spreading, pits tend to perforate the shell
in isolated locations.
– It can occur anywhere the metal is submerged, often
areas that are aerated
– Rust mounds or tubercles often cover the pits
– Microbiological corrosion
3
Brown Stock Corrosion
4
• Corrosion control consists of – Organic coatings and linings to
protect existing carbon steel tanks.
– All stainless steel construction, 304L or duplex stainless steel
• Watch out for galvanic corrosion of carbon steel, if repairs are made with stainless steel.
Brown Stock Corrosion
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5
Alkaline Liquor Corrosion
• Corrosion resistance of carbon steel depends on the
ability to maintain a protective oxide film on the metal
surface. As long as the passive layer remains intact,
corrosion rates are low.
• Higher temperatures and flow rates result in higher
corrosion rates. Increasing sulfidity increases corrosion
however its impact is less predictable. There is no
sulfidity number under which carbon steel corrosion will
not occur.
• Stainless steel remains passive in white, green and black
liquor. The exception for stainless steel is in high solids
(70% or higher) black liquor
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Alkaline Liquor Corrosion continued
• White liquor is corrosive to carbon steel. Anywhere where
the liquor is aerated, expect corrosion rates to increase
due to the formation of thiosulfate. Increased sulfidity can
augment this effect.
• Green liquor is less aggressive than white liquor though it
can form thiosulfates at the air to liquid interface
• Unlike green and white liquor, the formation of thiosulfate
in black liquor does not increase carbon steel corrosion
• Low silicon steel has improved resistance in less
aggressive liquors. Because of low demand it is
expensive and requires long lead times for delivery.
4
weld
Pitting at the liquid level. The average corrosion rate of
the upper two courses exceeded 20 mils/yr over 12 years.
Side A Side B
White Liquor Storage Tank Examination
• The pitting tends to go to the same depth
• Side B has fewer pits but is thinner overall
• The weld stands proud even though it has higher silicon
Side A Side B
Side view of the WL Tank Samples
5
• As sulfidity increases, the corrosion rate of
carbon steel increases
• The steel must be in the active state i.e.
already corroding to see an effect
– If existing corrosion rates are low, an
increase in sulfidity will not raise the
corrosion rates high enough to require an
upgrade in materials
– If existing corrosion rates are high, an
increase in sulfidity will make the situation
worse
The Sulfidity Question ???
CORROSION RATES OF MATERIALS EXPOSED IN LOW (30%)
AND HIGH (40%) SULFIDITY KRAFT LIQUORS (MPY).
White
Liquor
Green
Liquor
15% Weak
Black Liquor
Material Low High Low High Low High
A516-Grade 70 40 48 5.7 10 0.15 0.19
A285-Grade C 34 48 4.9 6.9 0.08 0.04
UNS S30403 0.04 0.04 0.00 0.00 0.02 0.03
UNS S32304 0.04 0.00 0.00 0.00 0.00 0.00
UNS S31803 0.04 0.00 0.00 0.00 0.00 0.03
26% Solids
Black Liquor
45% Solids
Black Liquor
Flash
Tank Liquor
Material Low High Low High Low High
A516-Grade 70 3.6 13.3 26 38 196 238
A285-Grade C 0.20 7.7 20 25 178 214
UNS S30403 0.08 0.03 0.00 0.04 - -
UNS S32304 0.00 0.00 0.00 0.04 - -
UNS S31803 0.03 0.00 0.00 0.04 - -
Angela Wensley, et al, “Effect Of Sulfidity On The Corrosivity Of White, Green, And Black Liquors”, NACE 1999
Liquor Corrosion Data
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11
Existing Situation After increasing sulfidity
• Corrosion rate < 2 mils/yr,
the metal is passive
No appreciable change in
corrosion
• Corrosion rate up to 10
mils/yr, carbon steel has
acceptable resistance
Rates could increase so that
carbon steel is marginal
• Corrosion rate 10-20 mpy,
carbon steel is marginal
Rates could further increase so
that carbon steel is unsuitable
• Corrosion rate > 20 mils/yr,
carbon steel is unsuitable
Rates will remain unacceptably
high
Affect of Sulfidity on Corrosion
Corrosion Control
• Anodic Protection
– For white and green liquor
• Full or partial replacement
– Carbon steel
– Stainless steel
• Linings
– Organic
– Inorganic
– Stainless steel
7
“Dry” side cracking of a 304L Liner
Intergranular Stress Corrosion Cracking of 304L
8
Stainless Steel Band is Too Narrow
16
Black Liquor Corrosion
• Carbon steel is suitable for weak black liquor
• The transition to marginal use is ~20% black liquor solids
• Carbon steel does not passivate in 45% and higher solids
black liquor
• Corrosion chemistry is not well understood
• Sulfidity is not a good measure for black liquor though
there appears to be some correlation with corrosion
• Anecdotally, the conversion from batch to continuous
cooking has resulted in the more claims for increased
black liquor corrosion
9
• All carbon steel is affected to some degree
– Liquor splashing causes erosion-corrosion
– It has little resistance in effects 1, 2 and 3
– Corrosion may be noticeable where liquor enters
the evaporator train, often the fifth effect.
• 304L stainless steel and duplex stainless steels are
resistant
Evaporator Corrosion
Pitting Corrosion in the Vapor Dome
10
Tube Sheet Wastage
Bands of Corrosion
11
• Severe carbon steel erosion-corrosion was detected
in the fourth effect after it was switched to a position
between the first and third effects.
Original order 6,5,4,3,2,1
New order 6,5,3,4,1 (no change in solids to 6,5,3)
• The entire vapor dome corroded with corrosion rates
of 60 mils/year and pits 1/8 inch deep.
• The fourth effect vapor dome was rebuilt with 304L
SS
Case Study
Note the baffle is corroding away from vapor dome wall
Severe Carbon Steel Corrosion
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Fine pitting of the vapor duct wall
Erosion Corrosion
• Weld overlay with 309L stainless steel
– Time consuming to apply
• Thermal spray alloy 625
– Works better in the lower solids effects
– Estimated coating life is < 10 years
• Linings
– Liquor gets between the lining and shell
• Section Replacement
Corrosion Control
13
Some blistering
Third Effect Arc Spray
3rd effect
1st effect
Stainless Steel Lining Plug Welded to the Shell
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Carbon Steel 516-Gr70
0
10
20
30
40
50
60
70
80
0% 10% 20% 30% 40% 50% 60% 70%
% Solids in Black Loblolly Pine Liquor
Corrosi
on
Rate
in
mil
s p
er y
ear
(mp
y)
516Gr70 at 170oC
516Gr70 at 140oC
516Gr70 at 100oC
Welded 516Gr70 at 170oC
Welded 516Gr70 at 140oC
Welded 516Gr70 at 100oC
170oC
100oC
140oC
Preet Singh, et al, IPST, Georgia Institute of Technology
Carbon steel in laboratory generated liquors.
Notice the peak in corrosion around 50% BL solids
Corrosion Research
Tests at 170oC
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0% 10% 20% 30% 40% 50% 60% 70%
% Solids in Black Loblolly Pine Liquor
Co
rro
sio
n R
ate
in
mil
s p
er y
ea
r
(mp
y)
304L (Stainless Steel)
316 (Stainless Steel)
2205 (Duplex Stainless Steel)
2304 (Duplex Stainless Steel)
304 (Sensitized)
Preet Singh, et al, IPST, Georgia Institute of Technology
Stainless steel in laboratory generated liquors.
The corrosion rates are low, < 1 mil/yr
316 has the highest corrosion rate of the stainless steel
Corrosion Research
15
SA516-Gr. 70
0.00
50.00
100.00
150.00
200.00
13.7% BL 47.5% BL 51.7% BL 52.3% BL 79.9% BL
Softwood Black Liquor
Av
g. C
orr
os
ion
Ra
teSA516-Gr. 70
0.00
50.00
100.00
150.00
200.00
250.00
16.7% BL 50.6% BL 52.4% BL 58.8% BL 79.5% BL
Hardwood Black Liquor
Avg
. C
orr
osio
n R
ate
(m
py)
Preet Singh, et al, IPST, Georgia Institute of Technology
Corrosion of carbon
steel in real black liquor
under static conditions
Mill A supplied liquor,
tested at 170ºC
The corrosion rate
peaks at around 50% BL
solids
Corrosion Research
Stainless Steels in Hardwood BL
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
304-L 2304 2101 2205 309 312
Alloys
Coro
ssio
n R
ate
(m
py) 16.7% BL
50.6% BL
52.4% BL
58.8% BL
79.5% BL
Stainless Steels in Softwood BL
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
304-L 2304 2101 2205 309 312
Alloys
Corr
osi
on
Rate
(m
py)
13.7% BL
47.5% BL
51.7% BL
52.3% BL
79.9% BL
Preet Singh, et al, IPST, Georgia Institute of Technology
Corrosion of
stainless steel in real
black liquor under
static conditions
Mill A supplied liquor,
tested at 170ºC
There is a peak just
below 50% BL
solids, however the
rates are low.
Corrosion Research
16
Stainless Steels in Hardwood BL
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
304 309 2205 2304 312
Alloys Tested
Av
e. C
orr
osi
on
Ra
te (
mp
y)
16.03% BL
44.45% BL
52.2% BL
73.45% BL
Stainless Steels in Softwood BL
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
304 309 2205 2304 312
Alloys Tested
Av
e. C
orr
osi
on
Ra
te (
mp
y)
13.36% BL
44.07% BL
53.47% BL
71.18% BL
Preet Singh, et al, IPST, Georgia Institute of Technology
Mill B supplied liquor,
tested at 170ºC
There is a peak around
50% BL solids, however
the rates are low.
Duplex stainless steels
have lower corrosion
rates than 304 or 309
Corrosion Research
Corrosion Rate for Tensile Samples and Coupons - Mill-B
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0%
% Solids in Softwood Black Liquor
Co
rro
sio
n R
ate
(m
py
)
516 Gr70 Tensile Samples
516-Gr70 Coupons
Corrosion Rate for Tensile Samples and Coupons - Mill-B
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0%
% Solids in Softwood Black Liquor
Co
rro
sio
n R
ate
(m
py
)
304 Tensile Samples
304 Coupons
Preet Singh, et al, IPST, Georgia Institute of Technology
Mill B supplied liquor,
tested at 170ºC
Tensile samples simulate
loss of passivity and
effects of flow
Little difference is seen
with carbon steel,
however the corrosion
rate increases significantly
with stainless steel.
Erosion Effect
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wash liquor#2 weak BL 4th effect conc feed conc prod
% solids 7 19 27 50 72
A36 0.575
A53-C 0.214 20.96
A285-C 0.066 9.36 38.27 1.90
304L 0.011 0.010 0.027 0.044 0.183
316L 0.013 0.030 0.140 2.05
2205 0.011
Ti 0.030
Note the peak around 50% BL
solids. Rates are more typical of
actual carbon steel corrosion.
Failures of 316L have occurred
in 72% liquor
New Bern Black Liquor Corrosion 2003
0
5
10
15
20
25
30
35
40
45
0 10 20 30 40 50 60 70 80
Black Liquor Solids (%)
Co
rro
sio
n R
ate
(m
py
)
A36
A 53-C
A 285-C
304L
316L
Mill Corrosion Testing
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• Stainless steel corrosion occurs in black liquor with as low
as 70% solids
• Failures may be rapid, on the order of weeks or months,
and catastrophic
• The failure mechanism is flow assisted corrosion
• Black liquor chemical analysis reveals a correlation
between NaOH concentration
• Sulfide tends to be higher. Chloride and thiosulfate are
variable with little correlation to corrosion
• Stress Corrosion Cracking is also an issue
High Solids Corrosion of Stainless Steel
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35
• At least 18% chromium is required for corrosion
resistance
• With 16% chromium, 316L is not recommended for this
service. The molybdenum in 316L offers no benefit.
• 304L is sufficient for most applications.
• Duplex stainless steels are more resistant though not
immune to corrosion in high flow conditions
• Reducing stresses and deposition will improve resistance
to stress corrosion cracking.
High Solids Corrosion of Stainless Steel
80% Recirculation Line
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Step at the Butt Weld
Intergranular corrosion
near the rupture Corrosion of 304
pipe
316L 304L
Intergranular Corrosion
% Element Ruptured Weld Thicker
Chromium 17.84 18.87 19.37
Nickel 10.09 11.13 7.93
Molybdenum 1.84 1.63 0.53
20
0.140 0.075 0.015 0.330
0.275 0.015 0.365
70% Recirculation Line
Thicker Ruptured
% Element Thicker Weld Ruptured
Chromium 18.30 19.61 17.10
Nickel 12.65 11.22 9.67
Molybdenum 2.22 2.31 1.97
316L Intergranular Corrosion
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70%, Concentrator Tube Corrosion and Cracking
316L Enhancer, 304L Tube
Concentrator Tube Thinning
22
43
Corrosion next to the
enhancer bar
The metal under the
bar is protected from
corrosion.
Tube Corrosion Next to the Enhancer
Spiral Tube Cracking
23
Enhancer Cracking
46
Enhancer Stress Corrosion Cracking
24
74% Black Liquor Storage Tank
Cracking in the Heated Area
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Widespread Crack Indications
Intergranular Attack – Grains are Popping Out
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70% Flash Tank
Intergranular Corrosion
Original Shell Wear Plate
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Product Liquor Flash Tank
2205 plate
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%Solids 70 – 80; Temperature > 230 ºF; All flow related
Equipment Failure Mode Metal Fix
Product liquor flash tank Corrosion 304L 2205
BL heater pipe reducer Corrosion/Cracking 316L 304L
Ring header piping Corrosion 316L 2205
HSC tubes Corrosion/Cracking 304L 304L/2205
HSC spiral enhancers Cracking 316L Removed
Tubel concentrator tubes Corrosion 304L 2205
Liquor Heater Nozzle Corrosion 316L/304L 304L/design
HSC valves (cast) Corrosion CF3M CF3M
HSC piping Corrosion/Cracking 316L/304L 304L
Liquor Solids Meter Corrosion 316L Alloy 20
74% storage tank Corrosion/Cracking 304L 2304
Equipment Failures
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• Guideline Based on Carbon Steel Corrosion Rate
• Corrosion rates cannot be predicted by knowing
liquor composition or sulfidity
• Both carbon and stainless steel can fail by erosion
Material Selection Summary
Corrosion Rate (mpy) Severity Material
>20 High 304L
10 – 20 Moderate Prefer 304L
< 10 Low Carbon Steel
• White liquor
• H,S,I black liquor
• Green liquor
• Weak black liquor or less
than 20% solids
• Other choices
304L stainless steel
304L stainless steel
304L preferred
Carbon steel
Lean Duplex stainless steel
A weldable 12% Cr stainless
steel for mildly corrosive liquors2
Material Selection - Service
2 .Bennett, D, “Corrosion damage mechanisms and their prevention in tanks containing Alkaline
pulping liquors”, 2002 TAPPI Engineering Conference
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Questions
