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Environmental effects on near-neutral pH stress corrosion cracking in pipelines
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Environmental effects on near-neutral pH stress
corrosion cracking in pipelines W. Chen a, R.L. Eadie a, R.L. Sutherby b aDepartment of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada b TransCanada Pipeline Limited, Calgary, Canada
Abstract Stress corrosion crack (SCC) growth mechanisms in pipeline steels exposed to near-neutral pH environments are not well understood, although cracking is generally thought to result from some combination of crack-tip dissolution and hydrogen embrittlement. Many near-neutral pH SCC cracks in the field are seen to have a wide crack crevice and a blunt crack tip, which could result from rapid dissolution of the crack surface. The cracks are thought to have become dormant at that stage. This study was aimed at understanding how electrochemical factors in the environment influence the dissolution and cracking rates in near-neutral pH SCC, with the ultimate goal of identifying factors that influence the tendency of cracks to become dormant. Synthetic aqueous solutions were prepared based on actual environment compositions found at pipeline crack sites. Solutions were sparged with 5% CO2 + N2 gas to simulate anaerobic conditions occurring in the field. Corrosion rates were determined using weight loss coupons. The effect of cathodic current in modifying electrolyte chemistry was also studied. Finally, crack growth rates in two different test solutions were determined and crack morphology examined. Wide cracks did not occur in the solutions that caused a rapid weight loss during corrosion tests. An alternative hypothesis was developed to explain the appearance of blunt cracks.
I. Introduction It is generally acknowledged that near-neutral pH stress corrosion cracking (SCC) [1-4] results from a synergistic interaction between stress, material and environment. One of the prerequisites for crack initiation is that an appropriate environment comes into contact with the pipe surface. For this to occur, the groundwater must penetrate a protective coating on the pipeline. Aqueous electrolyte, which picks up ions from the soil, is essential for cracking to occur, but the influence of the electrolyte on the cracking process is still unknown. To date, several site selection models based on soil type and drainage conditions have been developed to help predict the location of SCC, and they have been reasonably successful. (Note: Such models are currently proprietary and cannot be found in the literature.) It has been suggested that both soil chemistry and
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Crack Growth in Pipeline Steels Under Cyclic Loading
groundwater flow influence the cracking process, but it is not clear which parameters are conducive to SCC. According to a statistical analysis of SCC crack data from the field, more than 95% of cracks have a blunt tip at a crack depth of ~1 mm or less and are dormant [5]. Although hypotheses have been proposed for explaining dormancy [6,7], the critical factors remain unclear. Frequently in experimental studies, crack growth can only be detected under aggressive loading conditions, which do not represent real loading conditions in the field (where SCC failures were found) [4,8-21]. Clearly, it may be critical in understanding field failures to determine the factors that enhance cracking under less severe loading conditions. The majority of tests has been done with an aqueous solution referred to as NS4 (Table 1), which is a composite of electrolytes found in cracks. Johnson et al. [22] and Beavers et al. [23] have studied the effect of an environment on near-neutral pH SCC parameters. Johnson et al. [22] found that increasing the level of CO2 in the sparging gas lowered the pH of electrolyte, increased the hydrogen permeation rate measured in steel coupons in Devanathan/Stachurski-type cells [24], and increased the rate of cracking advance measured in SCC tests at a loading frequency of 1 x 10-5 Hz and high levels of stress intensity factor (K) and stress intensity factor range (AK = Kmax- Kmin) in NS4 solution. In subsequent tests, Beavers et al. [23] found that the cracking rate also varied considerably with electrolyte composition and that most electrolytes tested gave more rapid crack advance in SCC tests than the NS4 solution. They reported that high crack growth rates correlated with low corrosion rates as measured by ir in potentiodynamic polarization tests, but did not find a correlation with hydrogen permeation as measured in coupon tests. On the other hand, using a series of six soil electrolytes Jack et al. [ 10] found that cracking in slow strain rate tensile (SSRT) tests correlated with hydrogen concentration. The highest level of hydrogen determined by the Devanathan/Stachurski method was found not to occur at low pH but rather at a high level of bicarbonates. Consequently, Jack et al. [ 10] concluded that bicarbonates rather than H § must be the source of hydrogen. A possible reaction producing dissolved hydrogen would be Fe + HCO3- ~ e- + H (dissolved) + FeCO3. The current study was initiated to determine whether distinct chemical environments can increase corrosion and crack growth rates, and secondly to identify the specific conditions that enhance environmentally-dependent crack growth and thereby help us to understand crack dormancy effects. Because corrosion is a long-term phenomenon, we decided to conduct long-term weight loss tests rather than measure short-term corrosion currents as was done elsewhere [22,23].
2. Experimental methods X-65 pipeline steel with the following composition (wt.%) was used in this study: 0.03 C, 1.49 Mn, 0.008 S, 0.25 Si, 0.27 Cu, 0.014 Sn, 0.12 Ni, 0.203 Mo, 0.015 AI, 0.08 Cr, 0.065 Nb, 0.015 Ti, 0.0124 N, and balance Fe. The steel was characterized as highly susceptible to near-neutral pH SCC [6,8,25]. Plate coupons for corrosion weight loss tests and compact toughness (C-T) specimens (formerly termed "compact tension specimens") for crack growth measurements were machined from the steel.
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The synthetic near-neutral pH soil solutions were designed on the basis of compositions of electrolytes extracted with distilled water from soils removed from near-neutral pH SCC sites [26]. Compositions of the commonly-used NS4 solution and the occasionally-used NOVA Trapped Water (NOVATW) are given in Table 1. As observed by Chen et al. [26], the pH of solutions extracted from different soils depends primarily on the concentration of bicarbonates (in the near-neutral pH range carbonates (CO32-) are converted primarily to bicarbonates (HCO3-)) with pH dropping with the level of HCO3-. Because it was observed that cracking tendency in SSRT tests tended to correlate with pH (and only secondarily with the concentration of CI-) [26], we developed a series of electrolytes with different pH levels by systematically varying the content of calcium carbonate (CaCO3). The electrolyte series used is listed in Table 2. Solutions were prepared using deionised water and sparged with 5% CO2 before and during tests. For some solutions containing higher contents of CaCO3 and also magnesium carbonate (MgCO3), a prepared solution was firstly sparged with 100% CO2 and then stirred using a magnetic stirrer to aid steel dissolution. Subsequently the solutions were sparged with 5% CO2 + N2 until pH was stabilized prior to testing.
Table 1 Compositions ofNS4, NOVATW and the range observed in solutions extracted from sites where near-neutral pH SCC had been detected Ionic species
Ca2+ Mg2+ K+ Na+ CO32- + HCO3CISO42-
Ionic concentration (mg/1) Soil extracts Range observed in six soils 9.2-194 2.7-205 <1.0-7.94 8.0-464 29.9-283 2.5-18.8 3.1-2160
Synthetic NS4 . 49.5 12.9 64.0 132.2 350.8 145.5 51.1
NOVATW 100 102 7.9 119.6 1110.2 7.13 19.3
The corrosion resistance of X-65 steel in various synthetic solutions was determined using weight loss coupons of ~ 15 x 15 x 1 mm suspended in test solution. All solutions were sparged with 5% CO2 + N2 gas prior to and during the steel exposure in order to achieve near-neutral pH levels. C-T specimens were machined from the X-65 pipe with the crack plane normal to the circumferential (hoop) direction. Each specimen was polished to produce a scratchfree surface prior to tests. Specimens were then pre-fatigued in air to produce a sharp crack tip from the machined notch in accordance with ASTM E647. The pre-fatigued crack length on both sides of the surface was 2-3 mm long. The difference in crack length on both sides of each specimen was less than 0.2 mm. After pre-fatigue cracking, a specimen was cyclically loaded in near-neutral pH synthetic soil solutions. A triangle waveform was used with a loading frequency of 0.005 Hz and a stress ratio (R) of 0.6. Maximum and minimum stresses were controlled to provide Km~x-~ 35 MPa~/m and AK from 10 to 20 MPa~/m. Tests were performed on a
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Crack Growth in Pipeline Steels Under Cyclic Loading
pneumatic cyclic loading horizontal frame. The C-T specimen was pin-hole loaded and sealed in the test cell with the test solution filled above the root of the machined notch. Crack growth was monitored by a potential drop method [27]. A constant current of 10 A was applied across the crack crevice, and the change of potential with time caused by crack growth was related to the crack length. Test temperature was maintained at 30 + 0.1 ~ The measured potential was converted to actual crack length by measuring the crack length on the fractured specimen. For the method used, the conversion factor was found to be 25.48 mm/mV (an average value of several tests). 3. Results
3.1. Characteristics of the synthetic solutions prepared Table 2 lists four synthetic solutions used in this study. They differed only in their CaCO3 content, which varied from 0.0061 g/l in C1 solution to 0.4845 g/l in C4 solution. The pH of solutions in equilibrium with 5% CO2 + N2 gas mixture varied from 5.9 to 7.2. pH values showed an expected increase with the total level of CO32-.
Table 2 Synthetic solutions used in this study Composition (~1) .... MgSOa'7H20 CaCI2 KC1 NaHCO3 CaCO3 pH (purged with 5% CO2 + N2)
Cl 0.0274 0.0255 0.0035 0.0195 0.0061 5.89
C2 0.0274 0.0255 0.0035 0.0195 0.0606 6.29
C3 0.0274 0.0255 0.0035 0.0195 0.2422 6.83
C4 0.0274 0.0255 0.0035 0.0195 0.4845 7..19
3.2. Corrosion characteristics of the CX series electrolytes Fig. 1 shows the weight loss of steel coupons exposed to various solutions for various periods of time. There was little difference in weight losses among the solutions during the first five days. The difference, however, became significant after longer times of exposure. The highest weight loss (corrosion rate) occurred in the steel coupon exposed to C 1 solution, which has the lowest pH. Plotting weight loss vs. pH showed that NS4 solutions and NOVATW fell along the same weight-loss trend line. Fig. 1 shows also that the corrosion rate (slope of the weight loss-time curve) was very high at the beginning, but slowed with time. This is particularly significant for synthetic solutions with higher pH levels. For example, the average dissolution rate in the C4 solution was calculated to be ~3.2 x 10-5 mm/s in the first day of exposure, but became ~2.8 x 10-7 mm/s atter 16 days. In contrast to very high pH solutions, the low pH solutions showed high corrosion rates even at extended exposure time.
215
Volume 2: Prediction, Industrial Developments and Evaluation 3.0E-5 c1
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(b) Fig. 1. Weight loss of steel coupons exposed to various electrolytes with 5% C O 2 + N2 gas for different times: (a) weight loss as a function of test time, and (b) weight loss after being tested for 24 days plotted vs. pH.
3.3. Crack growth measurements
Crack growth rates were evaluated in C2 solution and NOVATW. The C2 solution had a pH of 6.29, while NOVATW had a pH of 7.11. Fig. 2 shows crack length increments with increasing test time for two C-T specimens being tested under the same
216
Crack Growth in Pipeline Steels Under Cyclic Loading
cyclic loading conditions in different environments. The crack length increment in the C2 solution was about three times higher than that in NOVATW in both tests.
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Fig. 3 shows crack tip morphologies in C-T specimens after their corrosion exposure (as in Fig. 1). The crack tip was extremely sharp when a specimen was tested in the C2 solution (Fig. 3(a)), but relatively blunt when a specimen was tested in NOVATW (Fig.
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3(b)). In both cases, transgranular crack growth was observed and was consistent with typical near-neutral pH SCC [1-4]. 4. Discussion
Crack growth in the C2 solution and NOVATW differed in terms of crack growth rates and crack tip morphologies. A wide crack crevice is otten observed in near-neutral pH stress corrosion cracks in the field. This characteristic is generally believed to arise from corrosive attack, as passivation is not present in near-neutral pH environments. Higher corrosion rates result in wider crack crevices.
(a)
(b) Fig. 3. SEM image showing the crack tip morphologies in C-T specimens after cracking in (a) C2 solution and (b) NOVATW.
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Crack Growth in Pipeline Steels Under Cyclic Loading
The results of this study appear in consistent with the literature. The wider crack crevice was observed in specimens exposed to NOVATW, which actually has a lower long-term corrosion rate. Although the corrosion rate is substantially higher in the C2 solution than in NOVATW, the crack crevices were too f'me to be observed at many locations. The latter indicates that only small crack widening occurred on crack crevices in the C2 solution. A clear answer to the above discrepancy seems to be impossible using the limited results of this study. However, a number of factors that may be responsible for the discrepancy are analyzed below.
4.1. Effect of dissolution rate The fact that a more dilute solution showed a higher long-term corrosion rate suggests that a surface film that was formed in the more concentrated ("aggressive") solution reduced the corrosion rate with time, while such a film was absent or much weaker in the more dilute (less "aggressive") solution. The higher crack growth rate in the C2 solution might be related to the higher dissolution capacity of this solution. Because the crack tip is highly stressed/deformed, the process of dissolution could be limited to the area around the crack tip. Galvanic coupling between the crack tip and crack walls might prevent the latter from further dissolution. In addition, it is necessary to remember that due to crack opening during the application of a load, mixture of solutions inside and outside the crack can occur.
4.2. Effect of hydrogen Although hydrogen-permeation behaviour was not examined in this study, it has been shown elsewhere [ 10] that solutions with a higher concentration of HCO3- tend to produce more diffusible hydrogen even at very high pH levels. However, the presence of dissolved hydrogen (at equilibrium in Devanthan/Stachurski cells) is not a likely explanation for observed SCC because the above trend is wrong. This statement agrees with that of Beavers et al. [23], who wrote that the level of dissolved hydrogen does not correlate with cracking rate. This does not mean, however, that dissolved hydrogen does not contribute to rapid cracking. Atomic hydrogen tends to segregate to the triaxial tensile zone ahead of the crack tip that initiates the appearance of microcracks atter the critical hydrogen concentration is achieved. The critical hydrogen concentration seems plausible in the C2 solution, in which the crack was very sharp and could significantly increase stresses ahead of the crack tip. For the specimen tested in NOVATW, the rounded crack tip might mean that the hydrogen concentration ahead of the crack tip was below the critical value required for microcracks formation even though the dissolved hydrogen tended to be highly concentrated. Thus the sharpness of the crack tip might be the critical factor that invokes the hydrogen embrittlement mechanism rather than the concentration of dissolved hydrogen.
4.3. Crack tip blunting by room temperature creep deformation Arguments presented in Sections 4.1 and 4.2 also suggest that the wide crack crevice observed in NOVATW was not related to the process of dissolution at the crack crevice. A possible explanation for SCC morphology in the NOVATW solution might be related to creep deformation at the crack tip. It has been shown [28,29] that pipeline
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steels are susceptible to creep deformation at room temperature, which can produce several percent strain under a constant stress above the yield strength. At the crack tip, a portion of an applied load cycle can be well above the yield strength. Therefore, it is possible that time-dependent plastic deformation occurred. Because in our study the same type of loading waveform was applied to specimens tested in two different solutions, the intensity of creep deformation should depend on the time period over which the metal ahead of the crack tip experienced plastic deformation. Because the crack grew slower in NOVATW, accumulated creep time should be longer. Consequently, more creep deformation should be produced at the crack tip when testing in NOVATW. Plastic deformation would blunt the crack tip, which in turn would reduce the stress intensity ahead of the crack and slow the crack growth rate. This was confirmed when a test was conducted under the same cyclic loading condition in air. In this latter test, no crack growth was observed, but the crack tip was blunted significantly. 5. Conclusions
9 Synthetic soil solutions showed significant differences in corrosion rate after extended test time. Higher corrosion rate was observed in the more dilute solution with lower pH. 9 Two distinct types of crack tip morphology were observed: A sharp crack tip was produced when a specimen was exposed to the more dilute solution with lower pH and the higher corrosion rate. A blunt crack tip was produced when a specimen was exposed to the more concentrated solution with higher pH and the lower corrosion rate. Crock growth rates in the former solution were much higher than in the latter solution. 9 Observed differences in crack tip morphologies and growth rates were due to different corrosion rates, and also to the combination of hydrogen embrittlement and possible creep deformation at crack tips. References
[1] R.N. Parkins, A review of stress corrosion cracking of high pressure gas pipelines, CORROSION/2000, NACE International, Houston, TX, 2000, paper no. 00363. [2] B. Delanty, J. O'Beime, Oil Gas J. 90(24) (1992) 39-44. [3] R.N. Parkins, W.K. Blanchard Jr., and B.S. Delanty, Corrosion 50 (1994) 394. [4] R.N. Parkins, Investigations relating to environment sensitive fracture in the TransCanada pipeline system, Report to TCPL, 1988. [5] R.R. Fessler, K. Krist, Research challenges regarding stress-corrosion cracking of pipelines, CORROSION/2000, NACE International, Houston, 2000, paper no. 00370. [6] R. Chu, W. Chen, S. Wang, F. King, T. R. Jack, R.R. Fessler, Corrosion, in press. [7] J.A. Beavers, J.T. Johnson, R.L. Sutherby, Materials factors influencing the initiation of near-neutral pH SCC on underground pipelines, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 979. [8] Y.Z. Wang, R.W. Revie, M.T. Shehata, Early stages of stress corrosion crack development of X-65 pipeline steel in near-neutral pH solution, in: L. Collins (Ed.), Materials for Resource Recovery and Transport, Metallurgical Society of CIM, Montreal, 1998, p. 71. [9] B.N. Leis, J.A. Colwell, Initiation of stress-corrosion cracking on gas transmission piping, in: W.A. Van Der Sluys, R.S. Piascik, R. Zawierucha (Eds.), Effects of the Environment on Initiation of Crack Growth, ASTM STP 1298, ASTM, Philadelphia, 1997, p. 34.
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[ 10] T.R. Jack, M.J. Wilmott, W. Chen, SCC susceptibility of X-70 steel in contact with various neutral pH soil solutions, in: Proc. EPRG/PRCI 12th Biennial Joint Technical Meeting on Line Pipe Research, Houston, PRCI, 1999, paper 10. [11] Y.-Z. Wang, R.W. Revie, R.N. Parkins, Mechanistic aspects of stress corrosion crack initiation and early propagation, CORROSION/99, NACE International, Houston, TX, 1999, paper no. 143. [12] J.A. Beavers, C.L. Durr, S.S. Shademan, Mechanistic studies of near-neutral pH SCC on underground pipelines, in: L. Collins (Ed.), Materials for Resource Recovery and Transport, Metallurgical Society of CIM, Montreal, 1998, p. 31. [13] D. He, W. Chen, J. Luo, F. King, T. Jack, K. Krist, Effect of surface scratch roughness and orientation on the development of SCC of line pipe steel in near neutral pH environment, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 997. [14] S.-H. Wang, W. Chen, T. Jack, F. King, R.R. Fessler, K. Krist, Role of prior cyclic loading in the initiation of stress-corrosion cracks on pipeline steels exposed to near-neutral pH environment, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 1005. [15] X.-Y. Zhang, S.B. Lambert, R. Sutherby, A. Plumtree, Corrosion 55 (1999) 297. [ 16] T.M. Ahmed, S.B. Lambert, R. Sutherby, A. Plumtree, Corrosion 53(1997) 581. [17] S.B. Lambert J.A. Beavers, D. Delanty, R. Sutherby, A. Plumtree, Mechanical factors affecting stress corrosion crack growth rates in buried pipelines, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 961. [18] M.J. Wilmott, R.L. Sutherby, The role of pressure and pressure fluctuations in the growth of SCC in line pipe steels, in: Proc. 1998 International Pipeline Conference, vol. 1, ASME, New York, 1998, p. 409. [19] M.P.H. Brongers, J.A. Beavers, C.E. Jaske, B.S. Delanty, Influence of line-pipe steel metallurgy on ductile tearing of stress-corrosion cracks during simulated hydrostatic testing, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 743. [20] W. Zheng, F.A. MacLeod, R.W. Revie, W.R. Tyson, G. Shen, D. Kiff, M. Skaff, E.-W. Wong, Recent progress in the study of transgranular SCC in pipeline steels, in: T. Magnin (Ed.), Corrosion Deformation Interactions (CDI'96), EFC 21, The Institute of Materials, London, 1997, p. 282. [21] T. Kushida, K. Nose, H. Asahi, M. Kimura, Y. Yamane, S. Endo, H. Kawano, Effects of metallurgical factors and test conditions on near-neutral pH SCC of pipeline steel, CORROSION/2001, NACE International, Houston, 2001, paper no. 01213. [22] J.T. Johnson, C.L. Durr, J.A. Beavers, Effect of 02 and CO2 on near-neutral pH stress corrosion cracking propagation, CORROSION/2000, NACE International, Houston, 2000, paper no. 00356. [23] J.A. Beavers, C.L. Durr, B.S. Delanty, D.M. Owen and R.L. Sutherby, Near neutral pH SCC: crack propagation in susceptible soil environments, CORROSION/2001, NACE International, Houston, 2001, paper no. 01217. [24] M.A.V. Devanathan, Z. Stachurski, J. Electrochem. Soc. 111 (1964) 619. [25] W. Chen, S.-H. Wang, R. Chu, F. King, T.R. Jack, R.R. Fessler, Metall. Mater. Trans. A, 24A (2003) 2601. [26] W. Chen, F. King, T. Jack, M. Wilmott, Metall. Mater. Trans. A, 33A (2002) 1429. [27] W. Chen, R. Sutherby, Environmental effect of crack growth rate of pipeline steel in nearneutral pH soil environments, in: Proc. 5th International Pipeline Conference (IPC2004), vol. 1, ASME, New York, 2004, p. 123. [28] S. Wang, W. Chen, Mater. Sci. Eng. A325 (2002) 144. [29] S. Wang, Y.G. Zhang, W. Chen, J. Mater. Sci. 36 (2001) 1931.
Environmental effects on near-neutral pH stress
corrosion cracking in pipelines W. Chen a, R.L. Eadie a, R.L. Sutherby b aDepartment of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada b TransCanada Pipeline Limited, Calgary, Canada
Abstract Stress corrosion crack (SCC) growth mechanisms in pipeline steels exposed to near-neutral pH environments are not well understood, although cracking is generally thought to result from some combination of crack-tip dissolution and hydrogen embrittlement. Many near-neutral pH SCC cracks in the field are seen to have a wide crack crevice and a blunt crack tip, which could result from rapid dissolution of the crack surface. The cracks are thought to have become dormant at that stage. This study was aimed at understanding how electrochemical factors in the environment influence the dissolution and cracking rates in near-neutral pH SCC, with the ultimate goal of identifying factors that influence the tendency of cracks to become dormant. Synthetic aqueous solutions were prepared based on actual environment compositions found at pipeline crack sites. Solutions were sparged with 5% CO2 + N2 gas to simulate anaerobic conditions occurring in the field. Corrosion rates were determined using weight loss coupons. The effect of cathodic current in modifying electrolyte chemistry was also studied. Finally, crack growth rates in two different test solutions were determined and crack morphology examined. Wide cracks did not occur in the solutions that caused a rapid weight loss during corrosion tests. An alternative hypothesis was developed to explain the appearance of blunt cracks.
I. Introduction It is generally acknowledged that near-neutral pH stress corrosion cracking (SCC) [1-4] results from a synergistic interaction between stress, material and environment. One of the prerequisites for crack initiation is that an appropriate environment comes into contact with the pipe surface. For this to occur, the groundwater must penetrate a protective coating on the pipeline. Aqueous electrolyte, which picks up ions from the soil, is essential for cracking to occur, but the influence of the electrolyte on the cracking process is still unknown. To date, several site selection models based on soil type and drainage conditions have been developed to help predict the location of SCC, and they have been reasonably successful. (Note: Such models are currently proprietary and cannot be found in the literature.) It has been suggested that both soil chemistry and
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Crack Growth in Pipeline Steels Under Cyclic Loading
groundwater flow influence the cracking process, but it is not clear which parameters are conducive to SCC. According to a statistical analysis of SCC crack data from the field, more than 95% of cracks have a blunt tip at a crack depth of ~1 mm or less and are dormant [5]. Although hypotheses have been proposed for explaining dormancy [6,7], the critical factors remain unclear. Frequently in experimental studies, crack growth can only be detected under aggressive loading conditions, which do not represent real loading conditions in the field (where SCC failures were found) [4,8-21]. Clearly, it may be critical in understanding field failures to determine the factors that enhance cracking under less severe loading conditions. The majority of tests has been done with an aqueous solution referred to as NS4 (Table 1), which is a composite of electrolytes found in cracks. Johnson et al. [22] and Beavers et al. [23] have studied the effect of an environment on near-neutral pH SCC parameters. Johnson et al. [22] found that increasing the level of CO2 in the sparging gas lowered the pH of electrolyte, increased the hydrogen permeation rate measured in steel coupons in Devanathan/Stachurski-type cells [24], and increased the rate of cracking advance measured in SCC tests at a loading frequency of 1 x 10-5 Hz and high levels of stress intensity factor (K) and stress intensity factor range (AK = Kmax- Kmin) in NS4 solution. In subsequent tests, Beavers et al. [23] found that the cracking rate also varied considerably with electrolyte composition and that most electrolytes tested gave more rapid crack advance in SCC tests than the NS4 solution. They reported that high crack growth rates correlated with low corrosion rates as measured by ir in potentiodynamic polarization tests, but did not find a correlation with hydrogen permeation as measured in coupon tests. On the other hand, using a series of six soil electrolytes Jack et al. [ 10] found that cracking in slow strain rate tensile (SSRT) tests correlated with hydrogen concentration. The highest level of hydrogen determined by the Devanathan/Stachurski method was found not to occur at low pH but rather at a high level of bicarbonates. Consequently, Jack et al. [ 10] concluded that bicarbonates rather than H § must be the source of hydrogen. A possible reaction producing dissolved hydrogen would be Fe + HCO3- ~ e- + H (dissolved) + FeCO3. The current study was initiated to determine whether distinct chemical environments can increase corrosion and crack growth rates, and secondly to identify the specific conditions that enhance environmentally-dependent crack growth and thereby help us to understand crack dormancy effects. Because corrosion is a long-term phenomenon, we decided to conduct long-term weight loss tests rather than measure short-term corrosion currents as was done elsewhere [22,23].
2. Experimental methods X-65 pipeline steel with the following composition (wt.%) was used in this study: 0.03 C, 1.49 Mn, 0.008 S, 0.25 Si, 0.27 Cu, 0.014 Sn, 0.12 Ni, 0.203 Mo, 0.015 AI, 0.08 Cr, 0.065 Nb, 0.015 Ti, 0.0124 N, and balance Fe. The steel was characterized as highly susceptible to near-neutral pH SCC [6,8,25]. Plate coupons for corrosion weight loss tests and compact toughness (C-T) specimens (formerly termed "compact tension specimens") for crack growth measurements were machined from the steel.
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The synthetic near-neutral pH soil solutions were designed on the basis of compositions of electrolytes extracted with distilled water from soils removed from near-neutral pH SCC sites [26]. Compositions of the commonly-used NS4 solution and the occasionally-used NOVA Trapped Water (NOVATW) are given in Table 1. As observed by Chen et al. [26], the pH of solutions extracted from different soils depends primarily on the concentration of bicarbonates (in the near-neutral pH range carbonates (CO32-) are converted primarily to bicarbonates (HCO3-)) with pH dropping with the level of HCO3-. Because it was observed that cracking tendency in SSRT tests tended to correlate with pH (and only secondarily with the concentration of CI-) [26], we developed a series of electrolytes with different pH levels by systematically varying the content of calcium carbonate (CaCO3). The electrolyte series used is listed in Table 2. Solutions were prepared using deionised water and sparged with 5% CO2 before and during tests. For some solutions containing higher contents of CaCO3 and also magnesium carbonate (MgCO3), a prepared solution was firstly sparged with 100% CO2 and then stirred using a magnetic stirrer to aid steel dissolution. Subsequently the solutions were sparged with 5% CO2 + N2 until pH was stabilized prior to testing.
Table 1 Compositions ofNS4, NOVATW and the range observed in solutions extracted from sites where near-neutral pH SCC had been detected Ionic species
Ca2+ Mg2+ K+ Na+ CO32- + HCO3CISO42-
Ionic concentration (mg/1) Soil extracts Range observed in six soils 9.2-194 2.7-205 <1.0-7.94 8.0-464 29.9-283 2.5-18.8 3.1-2160
Synthetic NS4 . 49.5 12.9 64.0 132.2 350.8 145.5 51.1
NOVATW 100 102 7.9 119.6 1110.2 7.13 19.3
The corrosion resistance of X-65 steel in various synthetic solutions was determined using weight loss coupons of ~ 15 x 15 x 1 mm suspended in test solution. All solutions were sparged with 5% CO2 + N2 gas prior to and during the steel exposure in order to achieve near-neutral pH levels. C-T specimens were machined from the X-65 pipe with the crack plane normal to the circumferential (hoop) direction. Each specimen was polished to produce a scratchfree surface prior to tests. Specimens were then pre-fatigued in air to produce a sharp crack tip from the machined notch in accordance with ASTM E647. The pre-fatigued crack length on both sides of the surface was 2-3 mm long. The difference in crack length on both sides of each specimen was less than 0.2 mm. After pre-fatigue cracking, a specimen was cyclically loaded in near-neutral pH synthetic soil solutions. A triangle waveform was used with a loading frequency of 0.005 Hz and a stress ratio (R) of 0.6. Maximum and minimum stresses were controlled to provide Km~x-~ 35 MPa~/m and AK from 10 to 20 MPa~/m. Tests were performed on a
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Crack Growth in Pipeline Steels Under Cyclic Loading
pneumatic cyclic loading horizontal frame. The C-T specimen was pin-hole loaded and sealed in the test cell with the test solution filled above the root of the machined notch. Crack growth was monitored by a potential drop method [27]. A constant current of 10 A was applied across the crack crevice, and the change of potential with time caused by crack growth was related to the crack length. Test temperature was maintained at 30 + 0.1 ~ The measured potential was converted to actual crack length by measuring the crack length on the fractured specimen. For the method used, the conversion factor was found to be 25.48 mm/mV (an average value of several tests). 3. Results
3.1. Characteristics of the synthetic solutions prepared Table 2 lists four synthetic solutions used in this study. They differed only in their CaCO3 content, which varied from 0.0061 g/l in C1 solution to 0.4845 g/l in C4 solution. The pH of solutions in equilibrium with 5% CO2 + N2 gas mixture varied from 5.9 to 7.2. pH values showed an expected increase with the total level of CO32-.
Table 2 Synthetic solutions used in this study Composition (~1) .... MgSOa'7H20 CaCI2 KC1 NaHCO3 CaCO3 pH (purged with 5% CO2 + N2)
Cl 0.0274 0.0255 0.0035 0.0195 0.0061 5.89
C2 0.0274 0.0255 0.0035 0.0195 0.0606 6.29
C3 0.0274 0.0255 0.0035 0.0195 0.2422 6.83
C4 0.0274 0.0255 0.0035 0.0195 0.4845 7..19
3.2. Corrosion characteristics of the CX series electrolytes Fig. 1 shows the weight loss of steel coupons exposed to various solutions for various periods of time. There was little difference in weight losses among the solutions during the first five days. The difference, however, became significant after longer times of exposure. The highest weight loss (corrosion rate) occurred in the steel coupon exposed to C 1 solution, which has the lowest pH. Plotting weight loss vs. pH showed that NS4 solutions and NOVATW fell along the same weight-loss trend line. Fig. 1 shows also that the corrosion rate (slope of the weight loss-time curve) was very high at the beginning, but slowed with time. This is particularly significant for synthetic solutions with higher pH levels. For example, the average dissolution rate in the C4 solution was calculated to be ~3.2 x 10-5 mm/s in the first day of exposure, but became ~2.8 x 10-7 mm/s atter 16 days. In contrast to very high pH solutions, the low pH solutions showed high corrosion rates even at extended exposure time.
215
Volume 2: Prediction, Industrial Developments and Evaluation 3.0E-5 c1
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(b) Fig. 1. Weight loss of steel coupons exposed to various electrolytes with 5% C O 2 + N2 gas for different times: (a) weight loss as a function of test time, and (b) weight loss after being tested for 24 days plotted vs. pH.
3.3. Crack growth measurements
Crack growth rates were evaluated in C2 solution and NOVATW. The C2 solution had a pH of 6.29, while NOVATW had a pH of 7.11. Fig. 2 shows crack length increments with increasing test time for two C-T specimens being tested under the same
216
Crack Growth in Pipeline Steels Under Cyclic Loading
cyclic loading conditions in different environments. The crack length increment in the C2 solution was about three times higher than that in NOVATW in both tests.
._ 200
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(b) Fig. 2. Crack length increments as a function of testing time: (a) Test 1 and (b) Test 2.
Fig. 3 shows crack tip morphologies in C-T specimens after their corrosion exposure (as in Fig. 1). The crack tip was extremely sharp when a specimen was tested in the C2 solution (Fig. 3(a)), but relatively blunt when a specimen was tested in NOVATW (Fig.
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3(b)). In both cases, transgranular crack growth was observed and was consistent with typical near-neutral pH SCC [1-4]. 4. Discussion
Crack growth in the C2 solution and NOVATW differed in terms of crack growth rates and crack tip morphologies. A wide crack crevice is otten observed in near-neutral pH stress corrosion cracks in the field. This characteristic is generally believed to arise from corrosive attack, as passivation is not present in near-neutral pH environments. Higher corrosion rates result in wider crack crevices.
(a)
(b) Fig. 3. SEM image showing the crack tip morphologies in C-T specimens after cracking in (a) C2 solution and (b) NOVATW.
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Crack Growth in Pipeline Steels Under Cyclic Loading
The results of this study appear in consistent with the literature. The wider crack crevice was observed in specimens exposed to NOVATW, which actually has a lower long-term corrosion rate. Although the corrosion rate is substantially higher in the C2 solution than in NOVATW, the crack crevices were too f'me to be observed at many locations. The latter indicates that only small crack widening occurred on crack crevices in the C2 solution. A clear answer to the above discrepancy seems to be impossible using the limited results of this study. However, a number of factors that may be responsible for the discrepancy are analyzed below.
4.1. Effect of dissolution rate The fact that a more dilute solution showed a higher long-term corrosion rate suggests that a surface film that was formed in the more concentrated ("aggressive") solution reduced the corrosion rate with time, while such a film was absent or much weaker in the more dilute (less "aggressive") solution. The higher crack growth rate in the C2 solution might be related to the higher dissolution capacity of this solution. Because the crack tip is highly stressed/deformed, the process of dissolution could be limited to the area around the crack tip. Galvanic coupling between the crack tip and crack walls might prevent the latter from further dissolution. In addition, it is necessary to remember that due to crack opening during the application of a load, mixture of solutions inside and outside the crack can occur.
4.2. Effect of hydrogen Although hydrogen-permeation behaviour was not examined in this study, it has been shown elsewhere [ 10] that solutions with a higher concentration of HCO3- tend to produce more diffusible hydrogen even at very high pH levels. However, the presence of dissolved hydrogen (at equilibrium in Devanthan/Stachurski cells) is not a likely explanation for observed SCC because the above trend is wrong. This statement agrees with that of Beavers et al. [23], who wrote that the level of dissolved hydrogen does not correlate with cracking rate. This does not mean, however, that dissolved hydrogen does not contribute to rapid cracking. Atomic hydrogen tends to segregate to the triaxial tensile zone ahead of the crack tip that initiates the appearance of microcracks atter the critical hydrogen concentration is achieved. The critical hydrogen concentration seems plausible in the C2 solution, in which the crack was very sharp and could significantly increase stresses ahead of the crack tip. For the specimen tested in NOVATW, the rounded crack tip might mean that the hydrogen concentration ahead of the crack tip was below the critical value required for microcracks formation even though the dissolved hydrogen tended to be highly concentrated. Thus the sharpness of the crack tip might be the critical factor that invokes the hydrogen embrittlement mechanism rather than the concentration of dissolved hydrogen.
4.3. Crack tip blunting by room temperature creep deformation Arguments presented in Sections 4.1 and 4.2 also suggest that the wide crack crevice observed in NOVATW was not related to the process of dissolution at the crack crevice. A possible explanation for SCC morphology in the NOVATW solution might be related to creep deformation at the crack tip. It has been shown [28,29] that pipeline
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219
steels are susceptible to creep deformation at room temperature, which can produce several percent strain under a constant stress above the yield strength. At the crack tip, a portion of an applied load cycle can be well above the yield strength. Therefore, it is possible that time-dependent plastic deformation occurred. Because in our study the same type of loading waveform was applied to specimens tested in two different solutions, the intensity of creep deformation should depend on the time period over which the metal ahead of the crack tip experienced plastic deformation. Because the crack grew slower in NOVATW, accumulated creep time should be longer. Consequently, more creep deformation should be produced at the crack tip when testing in NOVATW. Plastic deformation would blunt the crack tip, which in turn would reduce the stress intensity ahead of the crack and slow the crack growth rate. This was confirmed when a test was conducted under the same cyclic loading condition in air. In this latter test, no crack growth was observed, but the crack tip was blunted significantly. 5. Conclusions
9 Synthetic soil solutions showed significant differences in corrosion rate after extended test time. Higher corrosion rate was observed in the more dilute solution with lower pH. 9 Two distinct types of crack tip morphology were observed: A sharp crack tip was produced when a specimen was exposed to the more dilute solution with lower pH and the higher corrosion rate. A blunt crack tip was produced when a specimen was exposed to the more concentrated solution with higher pH and the lower corrosion rate. Crock growth rates in the former solution were much higher than in the latter solution. 9 Observed differences in crack tip morphologies and growth rates were due to different corrosion rates, and also to the combination of hydrogen embrittlement and possible creep deformation at crack tips. References
[1] R.N. Parkins, A review of stress corrosion cracking of high pressure gas pipelines, CORROSION/2000, NACE International, Houston, TX, 2000, paper no. 00363. [2] B. Delanty, J. O'Beime, Oil Gas J. 90(24) (1992) 39-44. [3] R.N. Parkins, W.K. Blanchard Jr., and B.S. Delanty, Corrosion 50 (1994) 394. [4] R.N. Parkins, Investigations relating to environment sensitive fracture in the TransCanada pipeline system, Report to TCPL, 1988. [5] R.R. Fessler, K. Krist, Research challenges regarding stress-corrosion cracking of pipelines, CORROSION/2000, NACE International, Houston, 2000, paper no. 00370. [6] R. Chu, W. Chen, S. Wang, F. King, T. R. Jack, R.R. Fessler, Corrosion, in press. [7] J.A. Beavers, J.T. Johnson, R.L. Sutherby, Materials factors influencing the initiation of near-neutral pH SCC on underground pipelines, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 979. [8] Y.Z. Wang, R.W. Revie, M.T. Shehata, Early stages of stress corrosion crack development of X-65 pipeline steel in near-neutral pH solution, in: L. Collins (Ed.), Materials for Resource Recovery and Transport, Metallurgical Society of CIM, Montreal, 1998, p. 71. [9] B.N. Leis, J.A. Colwell, Initiation of stress-corrosion cracking on gas transmission piping, in: W.A. Van Der Sluys, R.S. Piascik, R. Zawierucha (Eds.), Effects of the Environment on Initiation of Crack Growth, ASTM STP 1298, ASTM, Philadelphia, 1997, p. 34.
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[ 10] T.R. Jack, M.J. Wilmott, W. Chen, SCC susceptibility of X-70 steel in contact with various neutral pH soil solutions, in: Proc. EPRG/PRCI 12th Biennial Joint Technical Meeting on Line Pipe Research, Houston, PRCI, 1999, paper 10. [11] Y.-Z. Wang, R.W. Revie, R.N. Parkins, Mechanistic aspects of stress corrosion crack initiation and early propagation, CORROSION/99, NACE International, Houston, TX, 1999, paper no. 143. [12] J.A. Beavers, C.L. Durr, S.S. Shademan, Mechanistic studies of near-neutral pH SCC on underground pipelines, in: L. Collins (Ed.), Materials for Resource Recovery and Transport, Metallurgical Society of CIM, Montreal, 1998, p. 31. [13] D. He, W. Chen, J. Luo, F. King, T. Jack, K. Krist, Effect of surface scratch roughness and orientation on the development of SCC of line pipe steel in near neutral pH environment, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 997. [14] S.-H. Wang, W. Chen, T. Jack, F. King, R.R. Fessler, K. Krist, Role of prior cyclic loading in the initiation of stress-corrosion cracks on pipeline steels exposed to near-neutral pH environment, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 1005. [15] X.-Y. Zhang, S.B. Lambert, R. Sutherby, A. Plumtree, Corrosion 55 (1999) 297. [ 16] T.M. Ahmed, S.B. Lambert, R. Sutherby, A. Plumtree, Corrosion 53(1997) 581. [17] S.B. Lambert J.A. Beavers, D. Delanty, R. Sutherby, A. Plumtree, Mechanical factors affecting stress corrosion crack growth rates in buried pipelines, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 961. [18] M.J. Wilmott, R.L. Sutherby, The role of pressure and pressure fluctuations in the growth of SCC in line pipe steels, in: Proc. 1998 International Pipeline Conference, vol. 1, ASME, New York, 1998, p. 409. [19] M.P.H. Brongers, J.A. Beavers, C.E. Jaske, B.S. Delanty, Influence of line-pipe steel metallurgy on ductile tearing of stress-corrosion cracks during simulated hydrostatic testing, in: J.R. Ellwood (Ed.), Proc. 2000 International Pipeline Conference, vol. 2, ASME, New York, 2000, p. 743. [20] W. Zheng, F.A. MacLeod, R.W. Revie, W.R. Tyson, G. Shen, D. Kiff, M. Skaff, E.-W. Wong, Recent progress in the study of transgranular SCC in pipeline steels, in: T. Magnin (Ed.), Corrosion Deformation Interactions (CDI'96), EFC 21, The Institute of Materials, London, 1997, p. 282. [21] T. Kushida, K. Nose, H. Asahi, M. Kimura, Y. Yamane, S. Endo, H. Kawano, Effects of metallurgical factors and test conditions on near-neutral pH SCC of pipeline steel, CORROSION/2001, NACE International, Houston, 2001, paper no. 01213. [22] J.T. Johnson, C.L. Durr, J.A. Beavers, Effect of 02 and CO2 on near-neutral pH stress corrosion cracking propagation, CORROSION/2000, NACE International, Houston, 2000, paper no. 00356. [23] J.A. Beavers, C.L. Durr, B.S. Delanty, D.M. Owen and R.L. Sutherby, Near neutral pH SCC: crack propagation in susceptible soil environments, CORROSION/2001, NACE International, Houston, 2001, paper no. 01217. [24] M.A.V. Devanathan, Z. Stachurski, J. Electrochem. Soc. 111 (1964) 619. [25] W. Chen, S.-H. Wang, R. Chu, F. King, T.R. Jack, R.R. Fessler, Metall. Mater. Trans. A, 24A (2003) 2601. [26] W. Chen, F. King, T. Jack, M. Wilmott, Metall. Mater. Trans. A, 33A (2002) 1429. [27] W. Chen, R. Sutherby, Environmental effect of crack growth rate of pipeline steel in nearneutral pH soil environments, in: Proc. 5th International Pipeline Conference (IPC2004), vol. 1, ASME, New York, 2004, p. 123. [28] S. Wang, W. Chen, Mater. Sci. Eng. A325 (2002) 144. [29] S. Wang, Y.G. Zhang, W. Chen, J. Mater. Sci. 36 (2001) 1931.