316 stainless failure

Corrosion of Stainless Steel in High Solids Black Liquor Margaret Gorog Metallurgical Engineer Weyerhaeuser Federal Way, WA 98063-9777 ABSTRACT Austenitic stainless steels are typically used for heavy black liquor service containing at least 68% solids. Reports in the literature describe corrosion and cracking of these stainless steels. This paper covers specific mill experiences and confirms the result that 304L has improved corrosion resistance over 316L. 304L is sufficient in most applications, even up to 80% solids. Where it has not performed well, 2205 duplex stainless steel has been used in its place. The use of 316L has resulted in catastrophic failures that could have seriously injured mill personnel. Equipment affected includes concentrator tubes, piping and tanks. Though corrosion is the cause of thinning, all the failures are affected by turbulence and high flow rates. Cold work from bending or cutting has been a factor in some of the failures involving cracking. BACKGROUND At the 1996 TAPPI Engineering Conference, one paper(1) presented experimental results indicating that at high enough concentrations, sodium hydroxide causes grain boundary corrosion of austenitic stainless steel in heavy black liquor. At the time this type of liquor typically had a solids content between 65 and 70%. It was noted that this definition was changing as mills were pushing black liquor evaporation upwards to 80% solids which demanded an increase in the liquor temperature. This was expected to lead to higher corrosion rates. Other conclusions stated that there was no gain by alloying stainless steel with molybdenum. Chromium on the other hand had a positive effect on improving corrosion resistance. In 2001, three papers (2, 3, 4) were presented at the 10th International Symposium on Pulp and Paper Industry Corrosion Problems. They identified alkali content and temperature as the cause of corrosion. Other important information presented was the improved corrosion resistance of 304L (nominal 18% Cr, 8% Ni) over 316L (nominal 16% Cr, 10% Ni, 2% Mo). 304L however was subject to stress corrosion cracking. 2205 (nominal 22% Cr, 5% Ni, 3% Mo) duplex stainless steel has even lower corrosion rates than 304L. To raise awareness of heavy black liquor corrosion to pulp mills in North America, TAPPI published a journal article(5) summarizing the earlier Scandinavian research. At the 11th ISPPICP, it was concluded that the degradation of lignin at higher solids and temperature releases sulfurous gases that increase the risk for stress corrosion cracking of austenitic stainless steel(6). It also confirmed once again that 316L does poorly in high solids black liquor(7). Weyerhaeuser began experiencing occasional failures of stainless steel in high solids liquor by the late 1990’s. The damage had the appearance of erosion corrosion. The failures were occurring in liquors with a nominal concentration of 70 -73% solids. The catastrophic rupture of six year old 316L piping in 2000 was an unexpected introduction to high solids corrosion. The estimated corrosion rate was 20 mils per year. Since that time there have been numerous failures of stainless steel. This paper gives examples of these failures. The resulting investigations offer findings that can be used to deal with this type of corrosion. MILL EXPERIENCES INVOLVING HIGH SOLIDS BLACK LIQUOR CORROSION Piping The first example involves piping associated with a high solids concentrator that started up in 1994. 316L was a major material of construction for the piping in this system. This was not the first failure but demonstrates the importance of stainless steel composition. The nominal solids concentration is 73% and temperature 230 ºF. A ¼” thick, 316 stainless steel suction reducer leaked after nine years where the base material thinned to a hole. An adjacent area is show in Figure 1. The interior surface appears to be polished. Initially erosion was thought to be the thinning mechanism however SEM examination of the ID revealed intergranular attack as seen in Figure 2. This was indicative of corrosion. It was in an area of higher flow so there was some element of erosion combined with

corrosion. The weld between the pipe sections had not thinned to the same degree. This is another sign that corrosion was driving the failure. The chemistry of the plate and welds in the area of thinning is presented in Table I. It shows that most thinning occurs where the chromium is less than 18%. The thinnest plate most closely matches 316L. The parts that matched 304L did not thin. All failed piping in the concentrator area has been replaced with 304L. There have not been any repeat failures since 2003. Table 1 Chemistry comparison of various pipe segments. The thinned pipe matches 316L stainless steel.

The next two examples involve high solids concentrator (HSC) recirculation piping. The failure mechanism was catastrophic rupture due to thinning. The first came from a system that concentrates black liquor to 70% solids. The line that failed had been in service for two years. Figure 3 shows the ruptured section and Figure 4, a view of the internal surface. The pipe ID is grooved suggesting erosion. However the weld stands proud of the pipe indicating that corrosion is also occurring. Figure 5 shows a Scanning Electron Microscope (SEM) micrograph of the damaged surface. The pipe on each side of the weld reveals intergranular attack confirming the corrosion mechanism. Energy Dispersive X-ray (EDX) semi-quantitative results are listed in Table II. It shows that the pipe alloying matches 316L stainless steel. The weld, which was more corrosion resistant than the adjacent pipe had the highest concentration of chromium. It also had the highest concentration of molybdenum which did not affect corrosion performance. Table II Chemistry of 70% black liquor solids piping as determined using EDX. The closest match is 316L. % Element Thicker Pipe Weld Matrix Ruptured Pipe Chromium 18.3 19.61 17.1 Nickel 12.65 11.22 9.67 Molybdenum 2.22 2.31 1.97 The next example involves the failure of a section of 80% solids piping. The failed section was a smaller piece that was spliced into a horizontal run downstream from a ball valve that was frozen in the open position and upstream from an elbow. There were no records indicating when and why it had been installed. It may have been a replacement for earlier corrosion. 304L was specified for the original piping. Figure 6 shows the remnants of the ruptured pipe. Though no one was hurt, it’s obvious that this was a serious safety incident. The ID surface is polished smooth with the appearance of erosion. It thinned to about 10 mils before failing next to a weld. There is poor fit-up and incomplete weld penetration with the result that the joint of the thicker pipe was offset from the section that thinned. There was no apparent erosion of this edge, as shown in Figure 7. Figure 8 is a series of SEM micrographs showing areas on both sides of the weld. The metal next to the rupture site displays intergranular corrosion, similar to the failures already discussed. The adjacent piping which visually does not appear to be corroding also shows signs of chemical attack when magnified. EDX results listed in Table III show the ruptured pipe is a close match to 316L stainless steel. The thicker pipe matches 304L. Though there may be flow effects, corrosion is the primary mechanism. It is even affecting 304L. The failure probably occurred near a weld because it created localized turbulence that enhanced corrosion.

Chemistry thick reducer to thinned thick Element reducer side pipe weld pipe pipe weld Carbon, C% 0.027 0.022 0.002 0.031 Manganese, Mn% 1.94 1.81 1.8 1.21 Sulfur, S% 0.01 0.016 0.006 0.013 Silicon, Si% 0.43 0.47 0.47 0.58 Nickel, Ni% 8.23 11 10.5 10.4 Chromium, Cr% 18.1 17.9 17 19.1 Phosphorus, P% 0.023 0.019 0.027 0.017 Copper, Cu% 0.23 0.11 0.27 0.11 Molybdenum, Mo% 0.33 2.13 2.36 0.93 Closest match 304L 316L 316L 304

Table III EDX chemistry of 80% black liquor solids piping.

% Element Thicker Pipe Weld Matrix Ruptured Pipe Chromium 19.37 18.87 17.84

Nickel 7.92 11.13 10.09 Molybdenum 0.53 1.63 1.84

Stress corrosion cracking, SCC, is also an issue for piping. Figure 9 shows an example of SCC of 316L long radius 5D bends in 73% liquor. This system was periodically steam cleaned to remove thick deposits. It appears that sulfur plays a role in the cracking as it was one of the major elements in the deposit as identified by the EDX scan in Figure 10. Chlorides were not detectable. The pipe hardness is Rockwell C 35. In the annealed condition it is typically softer, in the upper Rockwell B range. There is a variation in thickness around the circumference resulting in raised bands of material. This is not a result of corrosion but cold work introduced during bending. Figures 12 – 14 show a variety of cracks observed on the inner surface. Physical marks such as the bands, scratches or punches were often initiation sites for cracking. Figure 14 is a micrograph showing branched cracking typical of SCC. At higher magnifications, the cracks are primarily transgranular. Individual grains of austenite are filled with parallel deformation lines indicating severe cold work. This piping was replaced with 304L that was fully solution annealed before going into service. Concentrator Tube 304L concentrator tubes also experience corrosion and cracking. The time to failure has been from six months to about ten years. Solids concentration varies but all are at least 70%. 2205 has been the replacement material for the tubes that failed rapidly. So far, the service life has exceeded four years. The longer term failures have been replaced in kind. The examples here show several types of degradation. Some tubes are filled with spiral bars called enhancers or turbulators. Their purpose is to increase the turbulence in the tube. The bars were made out of 316L. They rapidly cracked and fell out of the tubes. The mills that have not replaced the enhancers have not seen a change in concentrator performance. Figure 15 shows a cracked enhancer. SCC initiated along the sheared edge of the bar. This is another example of cold work providing the stress component for initiating SCC. Additionally the tube cracked along a diagonal line that followed the path of the enhancer. This is shown in Figure 16. Figure 17 is a close-up photograph revealing fine branched cracking. Figure 18 is a micrograph showing that crack initiation occurred on the OD. This is the steam side but there were reports of black liquor contamination due to carryover. The ID is corroding. Figure 19 shows fine pitting. Figure 20 shows a diagonal line that marks where the enhancer rested against the tube wall. It is actually a raised line of material. The stainless steel has thinned all around it. Figure 21 in cross section shows this step. The smooth surface of the tube ID was examined with the SEM and again the intergranular effect is seen in Figure 22. The other example is from a concentrator in which the liquor flows on the outside of the tube elements. The liquor solids are 71%. 304L tubes are joined to 2205 headers. At the location of greatest splashing 304L had thinned to failure. Turbulence was a major influence on corrosion. The 2205 tubes shown in Figure 23 have etched welds. Though there is some chemical effect no measurable thinning was detected. The replacement material for 304L was 2205. Tank The last example is a product liquor flash tank at 70% solids. The liquor enters a tank via a tangential nozzle and hits one section of the wall. Figure 24 shows the damaged plate where it was affected by liquor impingement. The area had been covered with an additional 304L wear plate which was perforated. The shell had corroded to the point of needing replacement. Similar to the other failures, the surface is shiny and polished with telltale intergranular corrosion visible with the SEM (not shown). The corroded area was replaced with 2205 along with a ½” 2205 wear plate. Two years later a small area of this plate has lost half its thickness. The discoloration noted in Figure 25 is the location of the thin section where the liquor splashes against the wall.

DISCUSSION Stainless steel failures continue to occur in high solids black liquor service. The perception is that more failures will occur as the solids concentration approach 80% however the experience is that corrosion is possible in heavy black liquor at all solids concentrations of at least 70%. The failures can be rapid, on the order of weeks or months, and catastrophic. Piping, concentrator tubes and tanks were provided as examples but other affected equipment not discussed in detail here includes nozzles, meters and cast parts in ball valves. Table IV provides a larger sampling of failures in high solids black liquor. Most of the failures involve 316L. Though there is widespread knowledge that this material is not suitable for high solids black liquor service, it is often selected for spur of the moment repairs. 304L has done well except in high velocity, turbulent or splashing situations. It thins but not at the same rate and certainly provides much better service than 316L. The one exception is CF8M, the cast equivalent of 316, for the body of ball valves. They are in widespread use largely because of the expense of upgrading to a cast duplex. This is a situation where the inspection frequency must be stepped up in anticipation of failure, with plans for more frequent change outs. Stellite liners have improved the longevity of the valves making this an acceptable approach. The upgrade for 304L has been 2205 and in all cases it has been an improvement though it is not immune to corrosion. Lean duplex stainless steels such as 2101, 2304 or 2003 have been recommended as alternates to 2205, however they are not readily available. Corrosion testing indicates they will do well in alkaline liquors(8). For any stainless steel, austenitic or duplex, a corrosion allowance should be considered for use in high solids black liquor service. In all cases involving thinning, there is a flow affect from high velocity or turbulence. The failure mechanism visually appears to be erosion corrosion. Metallographic examination shows that the metal is indeed corroding. Flow assisted corrosion may be a better term describing the wastage. Turbulent liquor prevents passivity by continuously removing oxides and exposing fresh metal to the corrosive environment. Intergranular corrosion suggests that sodium hydroxide is a cause. In order to better understand the corrosion mechanism, data from in-house liquor testing has been analyzed to determine specific corrodents. There is a tendency for those mills that experience corrosion to have a higher sodium hydroxide concentration compared to other mills without high solids failures. The mills having reported the most corrosion have made additions of caustic to the black liquor. In one case caustic was added to neutralize acidic saltcake coming from the chlorine dioxide R8 process. In another case, alkali residuals were kept high to control digester corrosion. In both cases, adjustments to caustic levels appeared to reduce corrosion, however at the same time material changes were made that also improved equipment performance Chlorides are variable. Some failures involve high chlorides relative to other mills that have low chlorides. The affect of sulfur is difficult to assess but it may play a role in stress corrosion cracking as it was detected at high levels in the deposits on cracked piping. Regardless of liquor chemistry, material selection has been the most important factor in controlling corrosion. CONCLUSIONS Corrosion of stainless steel occurs in black liquor with as low as 70% solids. The failure mechanism can be described as flow assisted corrosion. Design changes to reduce the flow affects

will reduce or eliminate corrosion. Reducing stresses and deposition will improve resistance to stress corrosion cracking. At least 18% chromium is required for corrosion resistance. Molybdenum is not a significant factor. 304L is sufficient for most applications. Duplex stainless steels are more resistant though not immune to

corrosion in high flow conditions. Stainless steel in high solids black liquor service needs to be inspected for thinning and cracking on a regular

basis.

Table IV Summary of stainless steel corrosion failures in high solids black liquor Equipment Failure Mode Material Comments

Liquor solids meter corrosion 316L All corrosion is assisted by flow

Thin walled pulled Tee - HSC piping

corrosion 316L Catastrophic rupture. The failure occurred approx. six years after startup. The estimated corrosion rate is 20 mils per year.

Manual Ball Valve corrosion CF8M The 316 cast equivalent valve body failed after 6 years. It was replaced in kind with an additional stellite lining. The replacement has been in service for 6 years.

Liquor heater corrosion 316L/304L Failures occurred on the order of weeks. The nozzle design was changed to reduce flow and now 304L is satisfactory.

HSC piping corrosion 316 Carbon steel installed in Oct 02 failed within 2 years. It was replaced with 316L with some sections failing in less than 12 months. Pipe thinning led to rupture in the most recent failure in spring 06.

Suction reducer - HSC piping

corrosion 316 The pipe leaked. There have been other instances of piping thinning including some 304L

HSC tubes/enhancers corr/cracking 304/316 304 tubes have thinned over 10 years. 304 is the replacement material. Enhancers cracked and fell out some years after startup. They were never replaced.

Concentrator tubes/ Enhancers

cracking 304/316 The enhancers are made out of 316. The tubes cracked after 6 months. Thermal expansion issues contributed to the failure. 2205 is the tube replacement material.

Tube element concentrator tubes

corrosion 304 The failures occurred 2 years after startup. 2205 is the replacement material. This has occurred at another location resulting also in an upgrade to 2205. The duplex stainless steel is performing well

Concentrator tubes/ Enhancers

cracking 304/316 Cracking occurred after 7 months. The new tubes are made out of 2205.

Reducer cracking/corr 316L The pipe leaked after 6 - 8 months. 304L is the replacement material.

Product Liquor Flash Tank corrosion 304L Corrosion occurred next to a tangential nozzle. The thinned section was replaced with 2205 stainless steel which is now thinning in a small area about 8” square.

HSC recirculation piping corrosion 316L Catastrophic rupture – safety incident

REFERENCES 1. Klarin, Anja, Kottila, Mika, ”Caustic Corrosion in Black Liquor Evaporators”, proceedings from the 1996

Engineering Conference, TAPPI. 2. Klarin, Anja, “Corrosion Phenomena in Black Liquor Evaporators”, Proceedings from the 10th International

Symposium on Corrosion in the Pulp and Paper Industry, Helsinki, 2001. 3.Kottilla, Mika, Rauscher, John, “Selecting the Proper Material for Black Liquor Concentrators”, Proceedings from

the 10th International Symposium on Corrosion in the Pulp and Paper Industry, Helsinki, 2001. 4.Andreasson, Pernilla, “Corrosion Problems with Stainless Steels in Evaporator Plants” Proceedings from the 10th

International Symposium on Corrosion in the Pulp and Paper Industry, Helsinki, 2001. 5. Bennett, Dave, Reid, Craig, “Corrosion of Stainless Steel in High Solids Black Liquor Service”, September 2002

Solutions, TAPPI. 6. Klarin-Henricson, Anja, “Corrosivity of Black Liquor from a Modern Digester to A High Dry Solids

Concentrator”, Proceedings from the 11th International Symposium on Corrosion in the Pulp and Paper Industry, Charleston, SC, 2004.

7. Troselius, Lars, “Corrosion in Evaporator Plants and Heavy Black Liquor Tanks”, Proceedings from the 11th International Symposium on Corrosion in the Pulp and Paper Industry, Charleston, SC, 2004.

8. Singh, Preet, Institute of Paper Science and Technology at Georgia Tech, IPST report, 2006.

Figure 1: 72% solids, 1/4” 316L stainless steel suction piping leaked after 9 years of service. The weld has not lost any thickness. The surface appears to have been polished.

Figure 2: An SEM micrograph reveals intergranular corrosion.

Figure 3: Ruptured 70% heavy black liquor line.

Figure 4: A close-up of the rupture site shows grooving. The weld has not corroded to the same degree as the pipe.

Figure 5: The pipe matches the chemistry of 316L. The right SEM micrograph represents the pipe on the thinner side of the weld. The left SEM micrograph is near the rupture on the thicker side. Both sides show intergranular corrosion, a sign of caustic attack.

Figure 6: Catastrophic rupture of an 80% solids recirculation line from the concentrator to the flash tank

thinner 316Lthicker 316L

Figure 7: The stainless steel thinned to 0.010” before failing. Note the step at the butt weld indicating poor fit-up and incomplete penetration.

Figure 8: The ruptured pipe matches the chemistry of 316L. The right SEM micrograph displaying some intergranular attack is from an area next to the perforation. The pipe on the other side of the weld matches 304L. The left SEM micrograph shows that this side is also corroding.

316L 304L

Figure 9: Cracked long radius bends. The ID is coated with a thick, uniform deposit.

Figure 10: An EDX scan shows the deposit contains sodium, oxygen and sulfur. Chlorides are not present at detectable levels.

Figure 11: Extensive ID cracking was observed once the deposits were removed. There was no thinning of the pipe.

Figure 12: Cold working during bending created raised bands of metal. This photograph shows a crack running across one of these bands.

RRaaiisseedd bbaanndd

Figure 13: Many crack patterns were observed. This is an example of star cracking that initiated at what appeared to be punch marks.

Figure 14: Metallography shows branched stress corrosion cracking. The cracking is primarily transgranular. Parallel deformation bands within austenite grains indicate severe cold work.

50X 500X

Figure 15: The 316L spiral enhancer bar inside the concentrator tube has cracked. The tube material is 304L.

Figure 16: External cracks of a concentrator tube follow a diagonal line where the enhancer rested against the tube ID.

Figure 17: Fine stress corrosion cracking is seen on the tube OD

Figure 18: Stress corrosion cracking initiates from the OD.

46X

Figure 19: Concentrator tubes also corrode. Here is an example of fine pitting.

Figure 20: The entire surface has thinned leaving a raised diagonal line where the enhancer rested against the ID, offering some protection against corrosion.

Figure 21: A metallographic cross section shows the ledge that formed under the enhancer.

Figure 22: An SEM micrograph of the corroded tube shows intergranular corrosion.

92X

Figure 23: 2205 duplex stainless steel tubes have etched welds.

Figure 24: The wear plate of a 304L liquor tank thinned and perforated due to liquor impingement.

Figure 25: The replacement wear plate made out of 2205 is also thinning as seen by the discoloration where the liquor hits the tank wall.

Corrosion of Stainless Steel in High Solids Black LiquorMargaret Gorog

TAPPI Engineering Conference October 2007

Background

Published experimental results•NaOH causes grain boundary corrosion.

•Chromium improves corrosion resistance.

•No gain with molybdenum.

•304L is more resistant than 316L

•Releases of sulfurous gases increase the risk of SCC

316L stainless steel piping, ¼” thick, leaked after 9 years of service. The weld is intact.

73% Suction Reducer

Notice the intergranular etching.

Thinning is caused by corrosion.

73% Suction Reducer

50X

200X

Comparing stainless steel compositions

reducer to piper weld

thin pipe

thick pipe weld

73% Suction Reducer

316L equivalents thinned

corrodedsome attack

73% Suction Reducer

Chemistry thick reducer to thinned thickElement reducer side pipe weld pipe pipe weldCarbon, C% 0.027 0.022 0.002 0.031Manganese, Mn% 1.94 1.81 1.8 1.21Sulfur, S% 0.01 0.016 0.006 0.013Silicon, Si% 0.43 0.47 0.47 0.58Nickel, Ni% 8.23 11 10.5 10.4Chromium, Cr% 18.1 17.9 17 19.1Phosphorus, P% 0.023 0.019 0.027 0.017Copper, Cu% 0.23 0.11 0.27 0.11Molybdenum, Mo% 0.33 2.13 2.36 0.93closest match 304L 316L 316L 304

Details of the rupture site

0.140 0.0750.0150.330

0.275 0.0150.365

70% Recirculation Line

SEM examination of the ID reveals intergranular corrosion

Thicker Ruptured


316 Fingerprint

Note the chromium content1.972.312.22Molybdenum9.6711.2212.65Nickel

17.1019.6118.30ChromiumRupturedWeldThicker% Element



Remnants of a catastrophic rupture

Note the step at the butt weld


Intergranular corrosion near the rupture

Corrosion of 304 pipe

316 304


1.841.630.53Molybdenum10.0911.137.92Nickel17.8418.8719.37Chromium

RupturedWeldThicker% Element

Note the chromium content

304 Fingerprint


• 6” 316L piping, 6 years old

• 240 - 270 °F

• Periodically cleaned with 400 °F steam

• 5D bends crack

• Some pump vibration

73%, High Solids Concentrator Piping

A thick deposit coats the ID of the cracked pipe


The deposit contains mainly sodium, oxygen and sulfur.



Wall grooving creates a variation in thickness

Average hardness is Rockwell C33.6

Pipe 1 Pipe 2 Pipe 3 Pipe 4No. (in) (in) (in) (in)1 0.213 0.155 0.259 0.2562 0.202 0.146 0.224 0.1973 0.228 0.165 0.228 0.2094 0.169 0.171 0.196 0.1975 0.166 0.154 0.195 0.1706 0.158 0.175 0.185 0.2037 0.144 0.150 0.175 0.1588 0.147 0.133 0.154 0.1629 0.144 0.139 0.161 0.165

10 0.155 0.130 0.192 0.17411 0.165 0.134 0.185 0.19912 0.169 0.130 0.194 0.180

After cleaning cracking is readily observed

Grooving and raised forming bands indicate severe cold work


Cold work deformation bands

500X

50X

Stress Corrosion Cracking


Examples of enhancer cracking, tube OD cracking, ID pitting, and ID thinning.

70%, Concentrator Tube Corrosion and Cracking

The material is 316L

Metal spirals enhance liquor turbulence

Enhancer Cracking

Stress corrosion cracking

184X

Enhancer Cracking

Diagonal cracking follows the enhancer line

Concentrator Tube Cracking

A close-up shows branched cracking on the OD


SCC initiates from the OD


46X

ID pitting

Concentrator Tube Pitting

A thin section of the tube collapsed upon removal

Concentrator Tube Thinning

A diagonal line formed under the enhancer spiral as tube metal next to it thinned.


Corrosion next to the turbulator bar

The metal under the bar is protected from corrosion.

92X


Intergranular corrosion –caustic attack

304L tubes are corroding from the ID


Weld etching

71%, Tube Element Concentrator

70% Flash Tank

Thinning due to liquor impingement

Original plate – Intergranular corrosion

70% Flash Tank

1000X

Wear plate – Intergranular corrosion

70% Flash Tank

500X

70% Flash Tank

Thinning of 2205 duplex stainless steel

%Solids 70 – 80; Temperature > 230 ºF; All flow related

Summary

Equipment Failure Mode Metal FixProduct liquor flash tank Corrosion 304L 2205BL heater pipe reducer Corr/Cracking 316L 304LRing header piping Corrosion 316L 2205HSC tubes Corr/Cracking 304L 304L/2205HSC spiral enhancers Cracking 316L RemovedElement concentrator tube Corrosion 304L 2205Liquor Heater Corrosion 316L/304L 304L/designHSC valves (cast) Corrosion CF8M CF8MHSC piping Corr/Cracking 316L/304L 304LLiquor Solids Meter Corrosion 316L Alloy 20

Discussion

Stainless steel failures continue to occur in high solids black liquor service

Every failure has occurred in a region of high velocity or turbulence.

Sodium hydroxide concentration in the liquor is consistently high.

Stress corrosion cracking is also an issue.

Discussion continued

316L is not recommended for HSBL service

304L is suitable for applications that don’t involve high velocities, splashing and turbulence

2205, 2304, 2101 duplex stainless steels have improved resistance but are more expensive and less available.

Consider a corrosion allowance for 304L and even 2205.

Conclusions

Stainless steel corrosion occurs in black liquor with as low as 70% solids.

The failure mechanism is flow assisted corrosion.

Reducing stresses and deposition will improve resistance to stress corrosion cracking.

Conclusions Continued

At least 18% chromium is required for corrosion resistance.

304L is sufficient for most applications.

Duplex stainless steels are more resistant though not immune to corrosion in high flow conditions.

Questions?

316 stainless failure

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