stress corrosion cracking of stainless steel in

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having an effective alkali (E.A.) greater than 30 g/L NaOH. No SCC was ..... steels is usually written as a mixed oxide FeO·Cr2O3 (also expressed as FeCr2O4, ...
Paper Presented at the 2008 TAPPI Engineering Conference, Portland OR, August 2008

STRESS CORROSION CRACKING OF STAINLESS STEEL IN CONTINUOUS DIGESTERS Angela Wensley Angela Wensley Engineering Inc. 15397 Columbia Avenue White Rock, BC, V4B 1K1 Canada [email protected] Aaron Leavitt Andritz Inc. 1115 Northmeadow Parkway Roswell, GA, 30076 USA [email protected] Flávio Paoliello Celulose Nipo-Brasileira S.A. BR 381, Km 172 35196-000 Belo Oriente, MG, Brazil [email protected] ABSTRACT ________________________________________________ Stress corrosion cracking (SCC) of the 316L stainless steel central pipe and the 304L stainless steel clad impregnation zone of a kraft continuous digester at a mill in Brazil prompted an investigation into the environmental conditions that could support the SCC of austenitic stainless steels. U-bends of 304L and 316L stainless steels were exposed in synthetic "weak white liquors" of different hydroxide levels (10 to 25 g/L NaOH) and sulfide content (0 to 40 g/L Na2S) at 165ºC. Four different metallurgical conditions were investigated: annealed, welded, stress relieved, and sensitized. SCC of all specimens was found after 712 hours immersion exposure in weak white liquors having an effective alkali (E.A.) greater than 30 g/L NaOH. No SCC was observed in a weak white liquor of effective alkali (E.A.) equal to 25 g/L NaOH without Na2S. The SCC in all cases was both transgranular and intergranular. SCC initiated earlier (after 190 hours) in the stress relieved and sensitized specimens than in the annealed or welded conditions. The SCC of type 316L was often more severe than was the case for SCC of type 304L stainless steel. ________________________________________________ INTRODUCTION Many continuous digesters have been constructed either partially or completely using clad plate with type 304L or 316L austenitic stainless steel for corrosion protection on the process side. In addition, austenitic stainless steels have been (and continue to be) used extensively for nozzles, screens, liners, top separators, central pipes, bottom scrapers, weld overlays, and other components of continuous digesters. Austenitic stainless steels have much better resistance than carbon steels to corrosion thinning in alkaline pulping liquor environments characteristic of continuous digesters [1] but process-side corrosion and cracking of 304L and 316L austenitic stainless steels have occurred in continuous digesters. Sensitized type 304L stainless steel cladding has experienced intergranular attack [2]. Bottom scrapers made using 304L and also 2205 have experienced severe SCC [3, 4]. SCC has occurred in stainless-capped welds in clad digesters. Central pipes have experienced SCC in both anodically protected digesters with centrally-mounted cathodes and in digesters with no anodic protection. The current state-of-the-art material for continuous digesters is type 2205 duplex stainless steel. Except for the highly-stressed bottom scraper, there have been no significant corrosion or cracking problems with type 2205 duplex stainless steel in continuous digesters [5]. The two continuous digesters in a pulp and paper mill in Brazil that prompted this research into the SCC of stainless steels are both single-vessel systems (no impregnation vessel) pulping eucalyptus using the kraft (alkaline sulfide) pulping processes.

Line 1 Digester The Line 1 continuous digester was built in 1977 to the Swedish pressure vessel Code, with the top eight sections being of 304L stainless-clad construction. It was post-weld heat treated approximately 2 years after it entered service. No significant corrosion problems were encountered until after a change in 2000 from conventional cooking to a proprietary cooking process. The new cooking process involved the use of higher-concentration white liquor in the top of the digester and recirculation of the extraction liquor to above the cooking screens. In 2002, rapid corrosion thinning of the carbon steel shell and blank plates was observed in and above the cooking zone. The carbon steel corrosion problems were addressed by the application of type 312 stainless steel weld overlay and the installation of an anodic protection system. This digester has never been acid washed. In 2002, the type 316L stainless steel central pipe in the Line 1 digester failed due to SCC. A replacement type 316L stainless steel central pipe had extensive SCC by 2004 (Figure 1). In both cases the failures occurred in the upper digester. It was necessary to replace the central pipe again, this time using type 2205 duplex stainless steel. In 2003, extensive SCC of the stainless steel cladding was found in the Line 1 digester (Figure 2). Much of the SCC was in the cladding adjacent to the circumferential weld at the transition between the stainless-clad top section and the bare carbon steel section below. SCC had also occurred in the carbon steel to a depth of 10 mm all around the circumference of the digester at the bottom edge of the stainless-clad section. Over the next 2 years much of the SCC was removed from the cladding followed by stainless steel weld overlay repairs. The anodic protection system was extended to offer protection to the stainless-clad impregnation zone where the environment was estimated to contain 25 g/l NaOH. Line 2 Digester The Line 2 continuous digester was built in 1994 to the ASME Code, using 316L stainless-clad plate for the entire shell. It was partially heat treated. During the heat treatment of the digester the shell spent 50 hours at temperatures in excess of 400ºC, a treatment that promotes sensitization of stainless steel having more than 0.015% carbon content. The stainless steel clad plate had a maximum carbon content of 0.015%. Samples of cladding removed for metallurgical examination confirmed that there was no bulk sensitization although there was a "carbide affected zone" in the stainless steel where carbon had migrated across the carbon steel-stainless steel interface during thermal processing. Following a change in cooking process in 2006, a peculiar blue color was observed on stainless steel surfaces inside the Line 2 digester at locations ranging from the top separator to the bottom rings (Figures 3 and 4). No appreciable corrosion or SCC problems were encountered in the stainless steel cladding or the central pipe. An anodic protection system was installed in 2007. This digester has never been acid washed. LITERATURE REVIEW Conventional iso-corrosion diagrams for stainless steel in NaOH show different ranges for SCC of 304L and 316L stainless steels (Figures 5 and 6). One iso-corrosion diagram from a stainless steel supplier [6] suggests that SCC should not occur at NaOH concentrations characteristic of white liquor (that typically contains 100 g/L NaOH) yet another iso-corrosion diagram [7] suggests that SCC may be possible at NaOH concentrations characteristic of white liquor. Both these iso-corrosion diagrams predict no SCC of 304L and 316L stainless steels at NaOH concentrations less than 25 g/L and the temperatures below 170ºC as is typical in a continuous digester. Crowe [8] determined that the SCC of type 316L stainless steel in 3.35 M (134 g/L) NaOH at 92ºC was dependent on the applied potential. He found that SCC was associated with the transitions from active to passive and from passive to transpassive corrosion behavior. SCC did not normally occur for 316L in 3.35 M NaOH at 92ºC. Audouard [9] exposed four-point bent beam specimens of austenitic and duplex stainless steels in white liquor (123 g/L NaOH + 48 g/L Na2S + 10 g/L NaCl) at 150ºC. Type 304L stainless steel experienced cracking in 100 hours; type 316L stainless steel experienced cracking in 500 hours; the duplex stainless steels did not crack after 1000 hours. Audouard [10] later tested constant load specimens of austenitic and duplex stainless steels in white liquor containing 125 g/L NaOH and 50 g/L Na2S. He found the threshold temperature for SCC of type 316L stainless

steel was approximately 140ºC. He also found that chloride ions in the liquor had no effect on SCC. He found no SCC of duplex stainless steels in white liquor at 170ºC. Honda [11] produced SCC in U-bends of types 304L, 316L, and 317L austenitic stainless steels with 720 hours exposure in an alkaline sulfide solution of 5% NaOH + 2% Na2S at 150ºC and 200ºC, but not at 100ºC. Testing in 5% NaOH without Na2S gave no SCC at 200ºC. Kivisäkk [12] performed slow strain rate testing (SSRT) of type 304L and 2304 stainless steels in white liquor containing 2.30 M (92 g/L) NaOH and 0.70 M (55 g/L) Na2S (plus lesser amounts of Na2CO3, Na2S2O3, Na2SO4, and NaCl). Type 304L was sensitive to SCC at 170º but not at 130ºC. Type 2304 duplex stainless steel was not sensitive to SCC. Troselius [13] exposed U-bend specimens of 304L and 316L austenitic stainless steels and 2205, 2304, and 2507 duplex stainless steels at 150ºC for 329 hours in various solutions of NaOH, Na2S, and NaCl. He found: - no SCC of any specimen in the 150 g/L NaOH solution, - both types 316L and 304L experienced SCC in 300 g/L NaOH, - types 316L, 304L, 2205, and 2507 had SCC in 108 g/L NaOH + 60 g/L Na2S (E.A. approximately the same as for 150 g/L NaOH solution); only type 2304 duplex stainless steel did not crack in 108 g/L NaOH + 60 g/L Na2S, - no SCC of the duplex stainless steels in 150 g/L NaOH + 16.5 g/L Cl, - type 316L cracked but type 304L did not crack in 150 g/L NaOH + 16.5 g/L Cl. Leinonen [14] produced SCC of welded type 2205 duplex stainless steel at 170ºC in simulated cooking liquor that contained 180 g/L Na2S and 94 g/L NaOH Leinonen [15] also tested welded austenitic and duplex stainless steels in simulated cooking liquors including a "black" liquor containing 150 g Na2S/kg H2O at 170ºC; a white liquor containing 55 g Na2S/kg H2O + 100 g NaOH/kg H2O at 130ºC, 170ºC, and 210ºC, and green liquor containing 55 g Na2S/kg H2O + 20 g NaOH/kg H2O at 170ºC. He found that: - type 304L cracked in all the liquors at 130ºC, 170ºC, and 210ºC, - type 2205 cracked in all liquors at 170ºC and 210ºC, and - type 2304 cracked only in white liquor at 210ºC. Singh et al [16] did SSRT of type 304L stainless steel in pure Na2S solutions (1M, 3.9M, and saturated) at temperatures up 100ºC and found no SCC. SCC occurred in 350 g/L NaOH + 265 g/L Na2S at 150ºC. Singh et al [17] performed SSRT of stainless steels in simulated white liquor of 120 g/L NaOH + 50 g/L Na2S at 170ºC. Type 304 experienced severe SCC, while types 2205 and 2304 duplex stainless steels had slight SCC susceptibility. None of these investigations explored the possibility of SCC of stainless steels in alkaline pulping liquors having NaOH concentrations comparable with those that actually exist in continuous digesters. EXPERIMENTAL We carried out U-bend exposures of 304L and 316L stainless steels in synthetic "weak white liquors" at 165ºC. The test temperature was a typical cooking temperature in a continuous digester. We also performed three potentiostatic polarization tests in weak white liquor. Materials Corrosion coupons of types 304L and 316L stainless steels were obtained from Metal Samples Inc. The coupons were 127 mm x 29 mm x 1.4 mm in dimension, pre-polished to a 600 grit finish and pre-drilled. The compositions of the stainless steels are given in Table 1. Four metallurgical conditions were investigated: AN: annealed (the as-received coupons). W: welded (autogenous welds with no filler metal). SR: stress relieved (heated at 600ºC for 3 hours; air-cooled). SE: sensitized (heated at 600ºC for 24 hours; air-cooled).

The stainless steel coupons were purchased in both the AN and W conditions. The welded coupons had autogenous welds (no filler metal). Some of the AN coupons were heat treated in a furnace in a laboratory furnace to produce either the SR or SE conditions. The SR and SE heat treatments were intended to simulate best-case and worst-case scenarios of duration of post weld heat treatment for the carbon steel pressure shell in a digester constructed using clad plate. Table 1. Compositions of the Alloys Tested (%). Alloy

C

Cr

Ni

Mn

Mo

Cu

Si

N

304L

0.025

18.3

8.3

1.62

0.29

0.27

0.34

0.07

304L*

0.022

18.2

8.5

1.65

0.42

0.39

0.45

0.09

316L

0.020

16.5

10.2

1.40

2.12

0.23

0.49

0.02

316L* 0.029 * Welded coupons.

16.9

10.3

1.65

2.17

0.37

0.30

0.04

The U-bends were prepared immediately prior to the testing in the weak white liquor. For the welded coupons, the U-bends were prepared with the flat-ground transverse weld as close as possible to the outside on the apex of the bend. Weak White Liquors The liquors used in the corrosion testing were four "weak" versions of full-strength "standard" synthetic white liquor (Table 2). The standard white liquor contained 100 g/L NaOH and 40 g/L Na2S, and was close in composition to the actual mill white liquor. The NaOH contents in the test liquors were varied from 25 g/L NaOH down to 10 g/L NaOH, encompassing the range of NaOH concentrations that would be encountered in the upper zones of both continuous digesters. The Na2S contents in the test liquors were varied from 40 g/L Na2S down to 0 g/L Na2S to better understand the role of sulfide on the SCC of stainless steels. Table 2. Synthetic White Liquor Compositions (g/L). Constituent

Standard

A

B

C

D

Hydroxide, as NaOH

100

25

10

25

25

Sulfide, as Na2S

40

40

40

10

0

Polysulfide, as Na2Sx

0

0

0

0

0

Thiosulfate, as Na2S2O3

5

5

5

5

5

Sulfate, as Na2SO4

5

5

5

5

5

Carbonate, as Na2CO3

20

20

20

20

20

Chloride, as NaCl

2

2

2

2

2

120.5

45.5

30.5

30.1

25.0

Effective Alkali (E.A.), as g/L NaOH

Table 2 also gives the E.A. of the test liquors, calculated using the following formula: E.A. = g/L NaOH + ½ (g/L Na2S expressed as equivalent NaOH) The E.A. is a representation of the contributions of both the NaOH and the Na2S to the alkalinity of the liquors.

SCC Test Procedure For each test, eight U-bends (304L-AN, -W, -SR, -SE and 316L-AN, -W, -SR, -SE) were mounted on an assembly (Figure 7) that fit inside in a laboratory autoclave of type 2205 duplex stainless steel construction. The U-bends were fully immersed in approximately 1.75 L of weak white liquor. Figure 8 shows the apparatus during a test. No attempt was made to eliminate oxygen from the sealed autoclaves. No agitation was done. A constant temperature of 165ºC was maintained using external heating mantels. The initial tests were done using an exposure time of 190 hours. Most of the tests were done using an exposure time of 712 hours. After each test was over the U-bends were removed, cleaned, weighed (to give the corrosion rate), and examined for cracking under a microscope. Two conditions were used to describe the cracking observed on the U-bends: "SCC" or "Severe SCC." In "SCC" specimens, cracks were observed at or near the apex of the U-bend but they did not penetrate through the thickness of the specimens. In "Severe SCC" specimens, the U-bend was broken into two pieces. Potentiostatic Polarization Test Procedure We did three potentiostatic polarization tests with type 304L or type 316L stainless steel cylindrical specimens immersed for 5 days in weak white liquor "A" at 165ºC. Polarization was done at -100, +100, and +150 millivolts with respect to a molybdenum (Mo) reference electrode (mV vs Mo). In each test, duplicate specimens were exposed at their own open-circuit (free) corrosion potential. After testing, some specimens were cleaned and weighed to determine their corrosion rates, while other specimens were not cleaned so that the surface films could be further examined using a scanning electron microscope (SEM) and by X-ray photoelectron spectroscopy (XPS). RESULTS U-bend Test Results The results for all of the U-bend tests are given in Tables 3 through 6. Table 3. Results of the Testing of 304L and 316L U-bends in Weak White Liquor "A" at 165ºC. Test 1

2

NaOH (g/L) 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25

Na2S (g/L) 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40

Time (hours) 190 190 190 190 190 190 190 190 712 712 712 712 712 712 712 712

U-bend 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE

Corrosion Rate (mpy) 1.1 1.4 1.7 1.3 2.3 2.0 2.4 2.1 0.7 0.9 0.9 0.9 1.8 1.7 1.9 1.6

Results No SCC No SCC No SCC No SCC No SCC No SCC SCC SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC

Figures 9 through 16 are views of the severe SCC experienced at the apex of the U-bends by the 304L and 316L specimens after test 2 where they were exposed for 712 hours in weak white liquor "A". These views are typical of the severe SCC observed in the other tests. The U-bend tests in weak white liquor "A" showed that more than 190 hours was required for all specimens to experience SCC. The only SCC observed after 190 hours was in the 316L specimens that had been exposed at temperatures in the sensitizing range. The corrosion rates of the 304L specimens were lower than those of the 316L specimens. Table 4. Results of the Testing of 304L and 316L U-bends in Weak White Liquor "B" at 165ºC. Test 3

4

NaOH (g/L) 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

Na2S (g/L) 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40

Time (hours) 190 190 190 190 190 190 190 190 712 712 712 712 712 712 712 712

U-bend 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE

Corrosion Rate (mpy) 0.7 0.7 1.0 0.7 0.7 0.6 0.7 0.7 0.5 0.6 0.6 0.6 1.1 1.0 1.1 1.1

Results No SCC No SCC No SCC No SCC No SCC No SCC No SCC No SCC SCC SCC SCC SCC Severe SCC Severe SCC Severe SCC Severe SCC

These tests showed that at least 712 hours of exposure was required to produce SCC in all specimens, so this exposure time was selected for the remainder of the U-bend tests. The 316L specimens were all severely cracked, suggesting that the propensity for SCC of the 316L was slightly greater than that for the 304L. Corrosion rates were also lower than in liquor "A" as may be expected for the lower NaOH content in liquor "B". Again, the corrosion rates of the 304L specimens were typically lower than those of the 316L specimens. Table 5. Results of the Testing of 304L and 316L U-bends in Weak White Liquor "C" at 165ºC. Test 5

NaOH (g/L) 25 25 25 25 25 25 25 25

Na2S (g/L) 10 10 10 10 10 10 10 10

Time (hours) 710 710 710 710 710 710 710 710

U-bend 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE

Corrosion Rate (mpy) 0.8 0.7 0.7 0.9 1.7 1.7 1.4 1.5

Results Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC Severe SCC

Keeping the hydroxide content at 25 g/L NaOH while lowering the sulfide content to 10 g/L Na2S did not significantly affect the occurrence of SCC and all the U-bends were broken in a similar manner to those tested for 712 hours in weak white liquor "A". Corrosion rates were also lower than in liquor "A" as may be expected for the lower Na2S content in liquor "C". Again, the corrosion rates of the 304L specimens were lower than those of the 316L specimens. Table 6. Results of the Testing of 304L and 316L U-bends in Weak White Liquor "D" at 165ºC. Test 6

NaOH (g/L) 25 25 25 25 25 25 25 25

Na2S (g/L) 0 0 0 0 0 0 0 0

Time (hours) 710 710 710 710 710 710 710 710

U-bend 304L-AN 304L-W 304L-SR 304L-SE 316L-AN 316L-W 316L-SR 316L-SE

Corrosion Rate (mpy) 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.3

Results No SCC No SCC No SCC No SCC No SCC No SCC No SCC No SCC

Weak white liquor "D" contained the same 25 g/L NaOH concentration as in liquors "A" and "C" but had no Na2S. No SCC was observed in weak white liquor "D". The corrosion rates in liquor "D" were significantly lower than those observed in the other liquors, and 304L had only slightly lower corrosion rates than did 316L. Color of the U-bends The U-bends exposed in weak white liquors "A," "B," and "C" had a faint purple-blue colour before they were acid cleaned. Figure 17 is typical of the appearance of the specimens before cleaning. Most of the color was removed by immersion in nitric acid at 60ºC. Figures 9 through 16 are all of cleaned U-bends. The specimens exposed in weak white liquor "D" (without Na2S) did not have any appreciable color. Metallographic Examination Selected U-bends were cross sectioned, polished, and etched to show the path of the cracks through the stainless steel microstructure. In general, there were no distinguishing features that could be used to identify either the alloy or the heat treatment. All the cracking was clearly characteristic of SCC, having a branched and irregular appearance. Figures 18 through 21 are cross sections of the 304L and 316L specimens in the AN condition that were exposed for 712 hours in weak white liquor "B". There were myriad cracks on the surface of the specimens. The cracks were both transgranular and intergranular. Figures 22 through 25 are cross sections of the 304L and 316L specimens in the W condition that were exposed for 712 hours in weak white liquor "B". There were myriad cracks on the surface of the specimens that showed no preference for either the welds or the heat affected zones. The cracks were both transgranular and intergranular. Figures 26 and 27 are cross sections of the 304L specimen in the SR condition that was exposed for 712 hours in weak white liquor "A". The cracks were both transgranular and intergranular. Figures 28 and 29 are cross sections of the 316L specimen in the SE condition that was exposed for 712 hours in weak white liquor "B". The cracks were both transgranular and intergranular.

Potentiostatic Polarization Results Figures 30 through 32 are views of the cylindrical specimens of 304L and 316L after exposure in weak white liquor "A" for approximately 5 days at 165ºC. The specimens whose potential was not controlled potentiostatically had open-circuit potentials that floated between approximately 0 mV and +40 mV vs Mo. The open-circuit specimens developed a blue color that was difficult to remove by cleaning. The specimens held at -100, +100, and +150 mV vs Mo were not appreciably colored. The weight loss corrosion rates for the 316L specimens are summarized in Table 7. Table 7. Corrosion Rates for 316L in Weak White Liquor "A" at 165ºC. Test P1 P1 P2 P2 P3 P3

Potential , mV vs Mo -100 Open-circuit +100 Open-circuit +150 Open-circuit

Corrosion Rate, mm/y

Corrosion Rate, mpy

0.044 0.052 0.035 0.020 0.042 0.044

1.8 2.0 1.4 0.8 1.6 1.8

The corrosion rates were difficult to measure after such short exposure times and have an error of at least ± 100%. There were no significant differences in corrosion rates among any of the specimens regardless of the potential. Analysis of the Blue Film Examination of the cylindrical specimens in an SEM revealed an indistinct surface film at the limit of the instrument capability (Figure 33). X-ray energy spectroscopy gave only the elemental composition of the underlying metallic substrate (Figure 34). XPS (Figure 35), however, identified the blue film as "mixed oxides of iron and chromium." This means that the blue film is simply a product of passivation of the stainless steel. The passive film on stainless steels is usually written as a mixed oxide FeO·Cr2O3 (also expressed as FeCr2O4, an iron-chromium-oxide spinel). The blue is an interference color that is a consequence of light passing through the passive film. DISCUSSION This work has extended the known range for SCC to occur in 304L and 316L austenitic stainless steels in alkaline pulping liquors to significantly lower concentrations of NaOH (as low as 10 g/L NaOH). Figure 36 is a temperature versus E.A. plot of this work and previously published SCC testing results in alkaline-sulfide environments. The data points from this work are plotted as circles. The data are not completely consistent, but this is likely due to the differences in experimental technique (for example, exposure times for U-bends) amongst the different researchers. The role of sulfide in caustic SCC is not yet clear. We found that SCC did not occur in weak white liquor without Na2S, as had been observed by Honda [10] in 5% NaOH at 200ºC and by Troselius [12] in 150 g/L NaOH at 150ºC. This suggests that sulfide may be an SCC agent in its own right, although Singh et al [15] found no SCC in solutions of Na2S without NaOH (their testing was at the significantly lower temperature of 100ºC). Sulfide contributes to the E.A. and makes the solutions more caustic than if one simply considers the NaOH concentration by itself. This work suggests that there is a minimum value of E.A. between 25 g/L as NaOH and 30.1 g/L as NaOH required to support caustic SCC. Sulfide ions may have a detrimental effect on the passive film that forms on stainless steels, with the result that cracks form and grow more readily on highly stressed surfaces in alkaline-sulfide solutions than they do in alkaline solutions without sulfide. Sulfide also increases the corrosion rate of the stainless steels, suggesting that sulfide ions play a direct role in the anodic dissolution that occurs at the crack tips during SCC. The higher corrosion rate of type 316L stainless steel compared with type 304L stainless steel in alkaline pulping liquors is a consequence of its

lower chromium content. This may be sufficient to explain why the caustic SCC of type 316L stainless steel was more severe than that of type 304L stainless steel. SCC Risk of Stainless Steel in Continuous Digesters For SCC to occur, three prerequisites must be met: (1) the material must be susceptible to SCC, (2) the environment must contain an SCC agent, and (3) high tensile stresses must be present. Types 304L and 316L stainless steels are well known to be susceptible to SCC in many pulp and paper mill environments. Digester liquor environments contain caustic and sulfide which are agents for SCC. High residual tensile stresses are known to result from forming and welding operations. While caustic SCC of austenitic stainless steels is well known in full-strength white liquors, this study has shown that 304L and 316L stainless steels have susceptibility to SCC at NaOH and Na2S levels that can actually exist in continuous digesters. There have been numerous cases reported of SCC occurring in 304L and 316L stainless steels in continuous digesters. SCC of the stainless steel cladding and SCC of stainless steel weld caps have occurred in digesters that were post weld heat treated, which had evidently impaired the resistance to SCC. Examples of components with high forming and welding stresses that have experienced SCC failures in continuous digesters include central pipes and bottom scrapers. It is interesting to speculate as to why there are not more reports of SCC of stainless steels in continuous digesters. It may be that most continuous digester liquors have an E.A. below 25 g/L NaOH. Analyses of extraction liquors [1] confirm that the E.A. in most continuous digesters is typically below 25 g/L NaOH. Newer cooking processes that result in higher NaOH concentrations may put stainless steels at greater risk for SCC. U-bend specimens are by definition at the yield stress. The stress state in many stainless steel parts may be lower than the yield stress. Stainless steel welds have residual stresses at or near the yield stress but typically contain some ferrite phase that makes them behave more like duplex stainless steels. The U-bends tested in this study were fully austenitic and the autogenous welds in the 304L-W and 316L-W specimens had only small amounts of ferrite (1.6% and 1.4%, respectively) which made them essentially fully austenitic. Metallurgical condition is also a factor in SCC. This work showed that SCC initiated sooner when 304L and 316L that had been exposed to temperatures in the sensitizing range. Exposure of L-grade stainless steel to temperatures in the sensitizing range was found to be sufficient to impair SCC resistance even through the time at temperature may not have been long enough to form a complete network of chromium carbides at the grain boundaries in the microstructure. Protection against SCC Some means of protection of austenitic stainless steels in digesters may be essential to prevent caustic SCC failures in continuous digesters where the E.A. of the liquor may exceed 25 g/L NaOH. Protection may involve changing to a material that is more resistant to SCC, or it can be accomplished by other means such as anodic protection, stainless steel weld overlay, or thermal spray coating. Audouard [9, 10], Kivisäkk [12], Troselius [13], Leinonen [14, 15], and Singh [17] have demonstrated that 2205, 2304, and 2507 duplex stainless steels (typically containing 40% to 50% ferrite in the microstructure) are much more resistant to SCC in alkaline liquors than are 304L and 316L austenitic stainless steels (100% austenite in the microstructure). The current material of choice for new digesters is 2205 duplex stainless steel [5]. The central pipe in the Line 1 digester was replaced using type 2205 duplex stainless steel. Anodic protection is effective against potential-dependent corrosion phenomena. Caustic SCC occurs at the activepassive transition and at the transpassive transition [8]. For stainless steel continuous digesters the corrosion potential is normally just above the active-passive transition, where there may be a risk for SCC. Anodic protection is accomplished by shifting the corrosion potential from the active-passive transition well up into the passive zone where SCC does not occur. This shift is easily accomplished in stainless-clad digesters, but can also be done for carbon steel digesters or digesters that are only partially stainless-clad [18]. Both the Line 1 digester and the Line 2 digester are now anodically protected.

Stainless steel weld overlay is not a direct protective measure for SCC of stainless steels, although locations where cracks have been ground out of stainless steel cladding or stainless-clad welds may require overlay to restore them. Overlay is normally applied to protect bare carbon steel digester shells from undergoing rapid corrosion thinning [19]. Austenitic stainless steel weld overlays contain residual welding stresses that are at or near the yield point. Fully austenitic stainless steel weld overlays may be at risk for SCC. Fortunately, most type 309L austenitic stainless steel weld overlays contain some ferrite in the microstructure, and may be considered to be a form of duplex stainless steel. Stainless steel weld overlays made using type 312 duplex stainless steel are likely to be resistant to SCC. The Line 1 digester was partially overlaid using type 312 duplex stainless steel. Thermal spray coatings provide a barrier between a corrosion- or SCC-susceptible material and the environment. Thermal spray coatings have been successfully used for SCC protection of the transitions between stainless-clad and bare carbon steel sections of the digester shells [20]. CONCLUSIONS 1.

Types 304L and 316L stainless steels are susceptible to SCC at 165ºC in weak white liquors containing (25 g/L NaOH and 40 g/L Na2S), (10 g/L NaOH and 40 g/L Na2S), and (25 g/L NaOH and 10 g/L Na2S).

2.

Types 304L and 316L stainless steels are not susceptible to SCC at 165ºC in weak white liquor containing 25 g/L NaOH and 0 g/L Na2S.

3.

SCC of 304L and 316L stainless steels in weak white liquors at 165ºC occurs when the E.A. is greater than 25~30 g/L NaOH.

4.

Exposure to sensitizing temperatures promotes SCC of 304L and 316L stainless steels in weak white liquors at 165ºC.

5.

Type 316L is slightly more susceptible than type 304L stainless steel to SCC in weak white liquors at 165ºC.

6.

The blue film is an interference color that is a characteristic of the passive film that forms naturally in weak white liquors.

REFERENCES 1.

Wensley, A., Corrosion of Batch and Continuous Digesters, Proceedings of the International. Symposium on Corrosion in the Pulp and Paper Industry, Ottawa pp. 27-36 (1998). 2. Wensley, A., Intergranular Attack of Stainless Steels in Kraft Digester Liquors, Paper No. 465, NACE Corrosion Conference (1996). 3. Wensley, A., Corrosion and Cracking of Bottom Scrapers in Continuous Digesters, Paper No. 05199, NACE Corrosion Conference (2005). 4. Gorog, M., Digester Outlet Device Scraper Arm Cracking, Paper 26-3, TAPPI Engineering Conference (2006). 5. Wensley, A., Experience with Duplex Stainless Steel Kraft Digesters, Paper No. 04249, NACE Corrosion Conference (2004). 6. Corrosion Handbook, 9th Edition, Outokumpu Stainless AB, Avesta, Sweden (2004). 7. LaQue, F.L., and Copson, H.R., Sodium Hydroxide Advisor, ChemCor 6 MTI (1992). 8. Crowe, D.C., and Tromans, D., Caustic Cracking of Stainless Steel, Canadian Metallurgical Quarterly, Vol. 23 No. 1, pp. 99-106 (1984) 9. Audouard, J.-P., A New Special Austenitic-Ferritic Stainless Steel for Kraft Pulp Batch Digesters, Proc. 4th International Symposium on Corrosion in the Pulp and Paper Industry, Stockholm (1983). 10. Audouard, J.-P., Dupoiron, F., and Jobard, D., Stainless Steels for Kraft Digesters in New Pollution Free Mills, Proc. 6th International Symposium on Corrosion in the Pulp and Paper Industry, Helsinki (1989). 11. Honda, M., et al, Stress Corrosion Cracking of Stainless Alloys in Alkaline-Sulfide Solutions, Corrosion, Vol. 48(10), pp. 822-829 (1992). 12. Kivisäkk, U., Stress Corrosion Cracking of Carbon Steel and Stainless Steel in White Liquor, Proc. 8th International Symposium on Corrosion in the Pulp and Paper Industry, Stockholm (1995).

13. Troselius, L., Corrosion in Evaporator Plants and Heavy Black Liquor Storage Tanks, Paper ISC0410, Proc. of the 11th International Symposium on Corrosion in the Pulp & Paper Industry, Charleston (2004). 14. Leinonen, H., Corrosion Resistance of Duplex Stainless Steel and its Welds in Modern Kraft Cooking, Paper ISC0407, Proc. 11th International Symposium on Corrosion in the Pulp & Paper Industry, Charleston (2004). 15. Leinonen, H. and Pohjanne, P., Stress Corrosion Cracking Susceptibility of Duplex Stainless Steels and Their Welds in Simulated Cooking Environments, Paper No. 06254, NACE Corrosion Conference (2006). 16. Singh, P.M., Ige, O., and Mahmood, J., Stress Corrosion Cracking of 304L Stainless Steel in Sodium SulfideContaining Caustic Solutions, Paper No. 03518, NACE Corrosion Conference (2003). 17. Singh, PM., Mahmood, J., and Conde, P., Stress Corrosion Cracking and Corrosion Susceptibility of Duplex Stainless Steels in Caustic Solutions, Paper No. 05196, NACE Corrosion Conference (2005). 18. Wensley, A., Anodic Protection against Corrosion and Cracking of Digester Vessels, Electronic Proceedings of the TAPPI Eng. Conf. (2003). 19. Wensley, A., Weld Overlay for Corrosion Protection of Continuous Digesters, Electronic Proceedings of the TAPPI Eng. Conf. (2002). 20. Wensley, A., Thermal Spray Coatings for Corrosion Protection of Continuous Digester and Flash Tanks, Electronic Proceedings of the TAPPI Eng. Conf. (2001).

Figure 1.

SCC failure of the 316L stainless steel central pipe in the Line 1 digester.

Figure 2.

SCC of the type 304L stainless steel cladding at the transition between the stainless-clad shell (above) and bare carbon steel shell (below) in the Line 1 digester.

Figure 3.

Blue color in the 304L stainless steel top separator of the Line 2 digester.

Figure 4.

Blue color on the 316L stainless-clad wall in the wash zone of the Line 2 digester.

Figure 5.

Iso-corrosion diagram for stainless steel in NaOH from a stainless steel manufacturer [6] showing zone of risk of SCC.

Figure 6.

Iso-corrosion diagram for 304 and 316 stainless steels in NaOH, showing wider zone of risk of SCC [7].

Figure 7.

Eight U-bend specimens mounted on a holder prior to installing in the autoclave.

Figure 8.

Autoclaves for U-bend SCC testing.

Figure 9.

U-bend of 304L-AN specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 10.

U-bend of 304L-W specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 11.

U-bend of 304L-SR specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 12.

U-bend of 304L-SE specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 13.

U-bend of 316L-AN specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 14.

U-bend of 316L-W specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 15.

U-bend of 316L-SR specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 16.

U-bend of 316L-SE specimen after 712 hours in weak white liquor "A" containing 25 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 17.

Pre-cleaning appearance of U-bend of 304L-SR specimen after 712 hours in weak white liquor "C" containing 25 g/L NaOH and 10 g/L Na2S at 165ºC.

Figure 18.

Cross section of the U-bend of 304L-AN after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 19.

Mixed transgranular/intergranular SCC of 304L-AN after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 20.

Cross section of the U-bend of 316L-AN after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 21.

Mixed transgranular/intergranular SCC of 316L-AN after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 22.

Cross section of the U-bend of 304L-W after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC. The SCC showed no preference for the weld, heat affected zone, or base metal.

Figure 23.

Mixed transgranular/intergranular SCC of 304L-W after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 24.

Cross section of the U-bend of 316L-W after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 25.

Mixed transgranular/intergranular SCC of 316L-W after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 26.

Cross section of 304L-SR U-bend after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 27.

Mixed transgranular/intergranular SCC of 316L-AN after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 28.

Cross section of the U-bend of 316L-SE after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 29.

Mixed transgranular/intergranular SCC of 316L-SE after 712 hours in weak white liquor "B" containing 10 g/L NaOH and 40 g/L Na2S at 165ºC.

Figure 30.

Specimens of 316L held for 5 days at open-circuit (top) and -100 mV vs Mo (bottom) in weak white liquor "A" at 165ºC.

Figure 31.

Specimens of 316L held for 5 days at open-circuit (top) and +100 mV vs Mo (bottom) in weak white liquor "A" at 165ºC.

Figure 32.

Specimens of 304L and 316L held for 5 days at open-circuit (top and middle) and +150 mV vs Mo (bottom) in weak white liquor "A" at 165ºC.

Figure 33.

SEM image of the blue film on type 316L stainless steel after 5 days at open-circuit in weak white liquor "A" at 165ºC.

Figure 34.

XES spectrum of the blue film on type 316L stainless steel after 5 days at open-circuit in weak whiter liquor "A" at 165ºC. It was not possible to distinguish the film from the underlying stainless steel metal.

Figure 35.

XPS spectrum of the blue film on type 316L stainless steel after 5 days at open-circuit in weak white liquor "A" at 165ºC. The film was identified as "mixed oxides of iron and chromium."

Honda [10], 5% NaOH

TEMPERATURE, ºC

200 25 g/L NaOH

175

Kivisäkk [11]

Wensley

Singh [16] Leinonen [13]

Troselius [12] 150 g/L NaOH

150

Troselius [12]

Honda [10] Audouard [8, 9] Leinonen [13]

125 Singh [15], 1M Na2S

Singh [15], 3.9M Na2S

100 0

25

50

75

100

125

150

175

200

EFFECTIVE ALKALI, g/L NaOH

Figure 36.

Graph showing temperature and effective alkali conditions where SCC of types 304L and 316L stainless steels has occurred (red) or not occurred (blue) in alkaline sulfide media.