The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence

Acta Materialia 55 (2007) 29–42 www.actamat-journals.com

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence G. Van Boven

a,1

, W. Chen

a,*

, R. Rogge

b

a b

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada T6G 2G6 National Research Council of Canada, Steacie Institute for Molecular Sciences, Chalk River, Ont., Canada Received 4 January 2006; received in revised form 10 August 2006; accepted 12 August 2006 Available online 2 November 2006

Abstract In this investigation, tensile test specimens were fabricated with increasing levels of compressive and tensile residual stress on the surface and through the thickness of the specimen. These residual stresses were then measured by neutron diffraction at multiple points along the length and through the depth of the specimens. The specimens were then exposed to a neutral pH aqueous soil environment in combination with an applied cyclic stress for various lengths of time in order to initiate and propagate stress corrosion cracking (SCC). The formation of micro-pitting was found to occur preferentially in areas where the tensile residual stresses were the highest (approximately 300 MPa), while SCC initiation occurred with a 71% normalized frequency in areas where the surface residual stress was in the range 150–200 MPa. The difference between residual stress levels occurring at SCC locations versus pitting locations resulted from both the change of residual stress during cyclic stress application during SCC testing and the residual stress gradient in the depth direction.  2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Residual stresses; Neutron diffraction; Ferritic steels; Corrosion; Stress corrosion cracking

1. Introduction Stress corrosion cracking (SCC) can be defined as the interaction of a tensile stress and an aqueous environment acting on a susceptible metallic surface to initiate and propagate cracks. The formation of SCC occurs below the yield strength of the bulk material and typically below the design stress and fatigue limit of an engineered structure. Since the first discovery of SCC on the exterior surface of a buried high-pressure natural gas transmission pipeline in 1965, SCC has continued to make a significant contribution to the number of leaks and ruptures experienced by the massive North American and world infrastructure of buried commodity pipelines [1]. *

Corresponding author. Tel.: +1 780 492 7706; fax: +1 780 492 2881. E-mail address: [email protected] (W. Chen). 1 Formerly graduate student, currently with Duke Energy Gas Transmission, Vancouver, BC, Canada.

Two forms of SCC can exist on buried mild steel pipelines. The first discovered form of SCC propagates intergranularly and is associated with a concentrated alkaline electrolyte in contact with the steel surface [1]. The mechanism of crack growth is reasonably well understood, and control of this type of cracking is possible by reducing the pipe temperature and controlling the pipe electrochemical potential range. A second form of SCC was discovered in Canada in the early 1980s [2,3]. This form of SCC propagates transgranularly and is associated with a dilute, neutral pH electrolyte in contact with the steel surface. It is this form of SCC, designated transgranular SCC (tSCC), which will be the focus of this study. Currently the exact mechanism of tSCC, although theorized, is not well defined or completely supported by the scientific studies to date. Current mechanistic theories are typically a derivation of past theories of cracking on systems other than pipelines that exhibit similar crack characteristics. Some theorized mechanistic features of tSCC

1359-6454/$30.00  2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.08.037

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include the following: (i) a role for hydrogen in enhancing crack tip dissolution [4,5]; (ii) discontinuous propagation of the crack from a location beyond the crack tip back to the crack tip [4,6,7]; and (iii) a possible synergistic growth by fatigue and corrosion [4,8–10]. Although stress is a necessary parameter for tSCC, little research has focused on defining the role of stresses in crack initiation and growth. The stress exerted on the high-pressure gas transmission pipeline steels is complex in nature. It is a combination of stresses related to the internal operating pressure, residual stress from pipe fabrication and pipeline construction, and possible external stresses. In a recent investigation, Beavers et al. [11] found that residual stresses at tSCC colonies were substantially higher than those measured in adjacent tSCC free material. In their study, a valid empirical correlation between residual stress and the incidence of tSCC was found. However, little research has been carried out to understand the mechanistic basis of this correlation. Residual stress is defined as a tensile or compressive force that exists in the bulk of a material without application of an external load. Based on the length scale, residual stresses are often categorized into three types [12]. Type I is the macro-scale residual stresses. Macro-scale residual stresses vary continuously over large distances of at least several grain diameters. Typical sources of Type I residual stresses in pipeline steels may include the bending of steel plate during pipe forming, differential cooling through the wall thickness and along the surface during rolling, and localized plastic deformation during handling. Type II is the micro-scale residual stresses, which vary over the grain scale. Type II residual stresses in pipeline steel are related to the presence of banded microstructures, texture on the surface, and regions with different microstructures such as pearlite colonies versus ferritic grains. Type III residual stress involves stresses at the atomic scale. These residual stresses are caused by chemical segregations at grain boundaries and small coherent phases in microalloyed steels.

For plastically deformable materials, the residual and applied stresses can be summed until the yield strength is reached. For a component under low amplitude and high cycle fatigue, the addition of a residual stress would have the largest effect on the fatigue life of the component by either raising or lowering the mean stress experienced over a fatigue cycle, which in turn may accelerate or delay the onset of plastic deformation [13]. Considerable advantage can be gained by engineering a compressive in-plane stress in the near surface region. In strain-controlled low cycle fatigue, however, the residual stresses can be quickly relaxed by the plastic strains, and would therefore have the least effect on the fatigue life of the component. This study will explore the relationship between the Type I residual stresses described above and their effect on the initiation and growth of tSCC on pipeline steels in a neutral pH environment. 2. Material and experimental methods 2.1. Test material and design of test specimens The material used in these tests was an API 5LX-65 (Grade 448 MPa) pipeline steel removed from service after a nearby near-neutral pH SCC rupture. The chemical composition of the steel is reported in Ref. [6]. The yield strength and the ultimate tensile strength were determined to be 480 and 607 MPa, respectively. Flat tensile specimens with a large residual stress gradient in both the longitudinal and through thickness directions of the test specimen were designed through various steps as illustrated in Fig. 1 and described below. Four steel plates with a ‘‘roof’’-like shape were machined from the pipeline diameter in such a way to maximize the resultant plate thickness (Fig. 1a). Two plates had a roof angle of 165.6 while the remaining two plates had an angle of 171.0. The plates had a thickness of 6 mm and total length of 128 mm (Fig. 1b). The roof plates were then subjected to three-point bending until a center axis of the plate

Fig. 1. Illustration showing various steps for preparing a flat specimen with varied residual stress from a pipe section. (a) Orientation of the residual stress test plate when removed from the pipeline and (b) the residual stress test plate dimensions before three point bending. (c) Sectioned view of the angled plate after three-point bending and removal of surface layer to produce a flat plate. (d) Orientation of the tensile specimen with respect to residual stress plate.

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was straight when no force was applied. Next, the straightened plates were machined to remove the peaks and valleys formed from the deformation process, resulting in a flat plate with an uniform thickness of 3.4 mm (Fig. 1c). Finally, three identical tensile specimens with dimensions of 32 mm in gauge length, 3.4 mm in thickness, and 7 mm in width were cut from each steel plate using electrical discharge machining (EDM). Table 1 lists the prepared tensile specimens. The tensile specimens were identified using two digit numbers. The first number indicated the plate from which the tensile specimen originated; the second number differentiated tensile specimen originating from the equivalent plate. The tensile specimens were polished to a 400-grit finish before residual stress measurements were performed. After completion of the measurements and prior to testing, a mirror-finish polish was applied to both specimen surfaces. 2.2. Residual stresses measurement by neutron diffraction Residual stress measurements were performed using a neutron spectrometer located at Chalk River Laboratory, Ontario, Canada. Only one specimen from each plate was chosen for residual stress measurements due to the lengthy measurement time and the expense of the facility involved. As each specimen from a plate was cut identically using a precision EDM method, the measurements for one specimen were thought to adequately characterize the remaining specimens cut from the same plate. An additional specimen which had been previously cyclical loaded was also measured. The principle of residual stress measurement by neutron diffraction can be found in Refs. [14,15]. A jig was designed and built to support the tensile specimen and provide the correct mounts for the two-coordinate table upon which the tensile specimen was mounted. The jig also applied a small tensile stress of approximately 8 MPa in order to keep the specimen flat for ease of measurement alignment in the spectrometer. The unit volume measured for these tensile specimens was 1 mm3. The symmetry of the unit volume can be altered by changing the slit dimensions of the incident beam. In this

31

case, the depth direction was minimized to be 0.7 mm, while the surface width was the maximum length direction for the measured unit volume. As a result, for these tensile specimens measuring approximately 3.4 mm in thickness, five depth measurements were adequate to quantify the residual stress in the depth direction from the front surface to the back surface of the tensile specimen. The five depth measurements were performed at seven equally spaced points along the gauge length, in increments of 5 mm, for a total gauge length measurement of 30 mm. Each strain measurement of an unit volume of material was made in three spatial directions in order to derive the gauge length stress. This multi-spatial measuring resulted in a total of 105 strain measurements per tensile specimen, with each measurement needing approximately 20 min to accumulate the necessary number of neutron counts to produce a statistically valid Gaussian profile to achieve a standard deviation of ±100 micro-strain, which translates to approximately ±20 MPa for the steel. The final output from the residual stress measurements is a residual stress versus location plot for each group of identical tensile specimens. 2.3. SCC testing The tensile specimen was tested in a sealed environmental cell made of Plexiglas. The test solution used was C2, the composition of which is reported elsewhere [10,16], and which was designed based on that of soil electrolytes extracted from the surface of operating pipelines in areas where tSCC was found to be present. The test solution was purged with a 5% carbon dioxide and 95% nitrogen gas mixture before and during the test to exclude oxygen and to maintain a pH of approximately 6.3. A cyclic load pattern was applied during SCC testing (Fig. 2). The maximum applied stress was 448 MPa or 100% of specified minimum yield strength (SMYS) and the R ratio was 0.8. The test duration was 2631 h, or approximately 4 months, in which 517 cycles were applied. The loading rate was 3.4 · 10 8 s 1 while the unloading rate was a magnitude faster than the loading rate at

Table 1 List of tensile specimens and testing performed Plate ID

Roof angle ()

Specimen ID

Experimental procedure

1

165.6

11 12 13

Destructively tested for stress–strain curve Not used – machined outside of specification Neutron diffraction analysis and corrosion test under cyclic loading for 2631 h

2

165.6

21 22 23

Neutron diffraction analysis and corrosion test under cyclic loading for 2631 h Corrosion test under cyclic loading for 2631 h Corrosion test under modified slow strain rate loading for 12 days

3

171.0

31 32 33

Corrosion test under cyclic loading for 2631 h Neutron diffraction analysis and corrosion test under modified slow strain rate loading for 12 days Neutron diffraction analysis and corrosion test under modified slow strain rate loading for 12 days

4

171.0

41 42 43

Neutron diffraction analysis and corrosion test under Modified slow strain rate loading for 12 days Corrosion test under Modified slow strain rate loading for 12 days Corrosion test under Modified slow strain rate loading for 12 days

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and consecutively grinding, polishing and inspecting each half of the tensile specimen from the inner depth face to the outer depth face in increments of 0.5 mm. The surfaces were then inspected by digital optical microscopy or, in the case where the cracks were very small, by scanning electron microscopy. The deepest point of the crack was recorded and the location on the tensile specimen was measured relative to the gauge centerline. An average crack growth rate was calculated by dividing the depth of a crack at the deepest point by the total testing time.

500 2 hour 450

SMYS

400

Stress (MPa)

350 Strain rate = 3.4 x 10-8 / S

300 250

Strain rate = -4.7 x 10-7 / S

200 150 100

Strain rate = 3.4 x 10-8 / second

50

3. Results

0 0

20000

40000

60000

80000

100000

120000

140000

3.1. Residual stress analysis of test specimens

Time (seconds)

Fig. 2. Load pattern used in the cyclic loading experiment.

The residual stress in the axial direction was calculated and plotted versus both distance along the gauge length and the depth direction. The axial direction is also equivalent to the hoop direction of the original pipeline material from which the residual stress plates and ultimately the tensile specimens were derived. Fig. 3a illustrates the distribution of residual stress along the gauge length in specimen 13 from plate 1. As the thickness of the specimen was 3.4 mm, the 0.4 and 3.0 mm data lines represent the level of residual stress of approximately 0.4 mm to the surface while the 1.7 mm data line reflects the residual stress at the center of the specimen. The data points plotted in Fig. 3 provide the residual stress measurement center point in the gauge length

4.7 · 10 7 s 1. At the maximum applied load, a static hold time of 2 h was applied. Some specimens were also loaded under more aggressive conditions up to 580 MPa at a rate of 2.1 · 10 8 s 1, and then cyclically loaded between 580 and 448 MPa with an R ratio of 0.77 for 12 days. 2.4. Examination for SCC At the end of the test the specimens were dried and their surfaces inspected optically for signs of SCC and pitting. Examination of the detected SCC was performed by cross-sectioning the tensile specimen down the gauge length

250

a

b

200

200

Residual stress (MPa)

Residual stress (MPa)

300

100

0

0.4 mm 1.05mm 1.7mm 2.35 mm 3.0 mm

-100

-200

150 0.4 1.2 2.0 2.8 3.6

100 50 0

mm mm mm mm mm

-50 -100 -150 -200

-300 -15

-10

-5

0

5

10

-250

15

-15

-10

-5

Distance from center of gauge length (mm)

10

15

d

200

100 0.4 mm 1.05mm 1.7mm 2.35 mm 3.0 mm

0

-100

200

100 0

-100

0.4 mm 1.05mm 1.7mm 2.35 mm 3.0 mm

-200

-200 -300 -15

5

300

c Residual stress (MPa)

Residual stress (MPa)

300

0

Distance from center of gauge length(mm)

-10

-5

0

5

10

Distance from center of gauge length (mm)

15

-300 -15

-10

-5

0

5

10

15

Distance from center of gauge length (mm)

Fig. 3. Axial (gauge length) residual stresses measured on tensile specimen 13 (a), specimen 21 (b), specimen 33 (c), and specimen 41 (d).

G. Van Boven et al. / Acta Materialia 55 (2007) 29–42

3.2. SCC testing under long-term cyclic loading Four specimens from three different plates were cyclically loaded for 2631 h under SCC test conditions. To minimize the effect of broad scale galvanic corrosion (accelerated corrosion caused by spontaneous current between regions with tensile residual stresses and those with compressive residual stresses) occurring between edges and the opposing side of the tensile surface, the two sides and the opposing side of the specimen were coated with an epoxy coating, leaving only one surface exposed to the testing environment (Fig. 1a).

3.2.1. Corrosion pits The observed pitting was non-random and occurred in clusters on the surface. It was evident that galvanic action had occurred over substantial distances of the specimen face. The opposing surface and sides of the specimen had been sealed with an insulating coating and therefore did not participate in the galvanic couple. Approximately 70 corrosion pits were found. Fig. 4 illustrates the distribution of pits as a function of residual stresses. A peak in corrosion pitting was observed at approximately 250 MPa, and pits were also occasionally found in the region with a compressive residual stress primarily present in specimen 33. It should be noted that the pitting distribution illustrated in Fig. 4 was not normalized by the exposed surface area at a particular stress range. Three of four exposed surfaces (specimens 13, 21, and 22) had residual stresses that primarily fell between 100 and 200 MPa, while at only one location on the surface of specimen 13 there was a residual stress of approximately 300 MPa. Therefore, the true peak occurrence of pitting should be located at the highest residual stress. This suggests that areas of high tensile residual stress are anodic with respect to areas with lower residual stress levels. In specimen 33, surface stresses were primarily compressive and pitting occurrence was low, even if it was normalized by area when compared with the surface with tensile residual stresses. A typical sectional image of a pit is illustrated in Fig. 5a. These pits are generally deep, up to 200 lm, which is approximately 6% of the sample thickness. The pits are also wide in the lateral direction and possess a very small depth to length ratio. 3.2.2. Micro-cracks All the specimens were sectioned to allow for inspection for micro-cracking. Fig. 5 illustrates some typical ‘‘microcracks’’ that were observed. These cracks generally initiated from pits on the surface. However, not all pits were

18 16

Number of Corrosion Pits

direction and in the depth direction of the 1 mm3 volume of measured material. This volume of material is referred to the ‘‘strain pixel’’. In the depth direction, the strain pixel extends 0.35 mm to each side of the center point. For example, at a measured depth of 0.4 mm the strain pixel extends in depth from 0.05 mm (surface) to 0.75 mm. Similarly, the center point along the gauge length represents a length extending 0.5 mm on each side of the indicated distance. Finally, the measured width of the strain pixel was 1.43 mm, which was centered in the middle of the 7 mm gauge width. The surface measurements of the tensile specimen corresponding to the roof top (Fig. 1) consistently indicate a tensile residual stress in the range of 200–300 MPa. In contrast, the residual stress on the surface corresponding to the inner roof surface was compressive only at the approximate center of gauge section, but was tensile towards the two ends. The distribution of residual stress was also not symmetrical with respect to the gauge center. These unexpected characteristics were related to the grinding after the roof-shaped specimen was straightened. As is illustrated in Fig. 1c, the removal of material from very surface may have altered the level and the distribution of residual stresses. The residual stress distribution may also be a result of non-symmetric bending and work-hardening during the flattening of the specimens. Fig. 3b illustrates the distribution of residual stress in specimen 21 from plate 2, which was prepared by the same way as plate 1 (Table 1). The distribution of residual stress both in the length direction and the depth direction is generally consistent between specimens 13 and 21 except that the residual stress at the surface in specimen 21 is a little lower. Fig. 3c and d illustrates the distribution of residual stresses in specimen 33 and 41, which were machined from plates 3 and 4, respectively. Both the plates have the same roof angle, which was slightly larger than that of plates 1 and 2. The general trend of distribution in Figs. 5 and 6 is similar, that is, the surface of the specimen is generally compressive while it is tensile below the surface; however, the absolute level of residual stress is larger in specimen 33 than in specimen 41. Again, this difference is related to variations in the grinding process after the roof-shaped plates were straightened.

33

14 12 10 8 6 4 2 0 -400

-300

-200

-100

0

100

200

300

400

Surface Residual Stresses, MPa

Fig. 4. Number of corrosion pits versus residual stress level observed on the four tensile specimens after 4 months of testing in the cyclic loading experiment.

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Fig. 5. Typical cracks formed in tensile specimen 21 during a 4 months cyclic loading test in an aqueous environment: (a) 14.1 mm from the gauge length center; (b) 4.3 mm from the gauge length center; (c) 5.0 mm from the gauge length center; and (d) 3.4 from the gauge length center.

associated with a micro-crack. It should be noted that the micro-crack illustrated in Fig. 5 is not a crack from fracture mechanics point of view; rather, it developed into a deep pit or a crack-like defect. These crack-like defects, however, will be called micro-cracks in the proceeding sections for the reasons described in Section 4.3. The crack exhibits a cone (Fig. 5b) or balloon-like (Fig. 5c) morphology. In some cases, the balloon appeared in the middle of the crack, and the crack crevice became finer towards the tip (Fig. 5d). A total of 19 micro-cracks were found on the sectioned surfaces of four tested specimens (Table 1). Fig. 6 illustrates the depth and the location of each micro-crack found in the various specimens, together with the distribution of residual stress on the surface of these specimens where cracks were developed. Fig. 6a and b illustrates relatively constant tensile residual stresses on both surfaces of the specimen. However, microcracks were observed primarily within a distance of 5 mm from the gauge center. This suggests that the level of residual stresses is not the only factor controlling the occurrence of

cracking. In addition, micro-cracks did not necessarily occur at the location with the largest residual stresses, although that is the case for the occurrence of pitting. Fig. 6c illustrates the distribution of residual stresses on the tensile surface in specimen 31 and the corresponding locations of micro-cracks found on the surface. The surface was primarily compressive, and no cracks were observed at locations other than at the end of the gauge length, where the residual stress is slightly tensile. However, the absolute value of the residual stress at this location is much lower than at the micro-crack locations that were found in Fig. 6a and b. This again suggests that the absolute value of residual stresses measured prior to SCC testing may not be the only factor controlling the development of tSCC. Fig. 7 illustrates the frequency of cracking occurrence as a function of surface residual stress for all four tested specimens. This figure illustrates that the highest occurrence of cracking occurs at residual stresses, which range from 150 to 200 MPa. No micro-cracks were found in the region

300

600

6

4

2

400 -10

-5

0

5

10

15

Distance from center of gauge length (mm)

Crack depth (μm) Residual stress (MPa)

8

0 -400

-300

Max Residual Stress

800

10

Min Residual Stress

200

-15

-200

-100

0

100

200

300

400

Surface Residual Stresses, MPa

Fig. 7. Measured surface residual stress versus frequency of cracking of four tensile specimens after a 4 months cyclic loading experiment.

200

100

800 600 400 200 -15

-10

-5

0

5

10

15

Distance from center of gauge length (mm)

Crack depth (μm) Residual stress (MPa)

35

12

Number of Crack Occurrences

Crack depth (μm) Residual stress (MPa)

G. Van Boven et al. / Acta Materialia 55 (2007) 29–42

0

Table 1 for details). Some pitting occurred on the surface of these aggressively loaded specimens, although these pits were approximately an order of magnitude shallower than those cracks detected on tensile specimens from the specimens tested under milder cyclic load and longer duration. Ten cracks in total were detected on six aggressively loaded tensile specimens. These 10 cracks were also an order of magnitude shallower than those detected in the longer duration cyclic loading experiments. A typical sectional morphology of these 10 cracks is illustrated in Fig. 8. These 10 cracks ranged from 8.0 to 293 lm in depth, with the average crack depth being approximately 35 lm. Aside from the shorter length, the cracks formed from

-100 -200 -300

200

100 -15

-10

-5

0

5

10

15

Distance from center of gauge length (mm)

Fig. 6. Distribution of axial residual stress as measured on the surface of various specimens prior to corrosion exposure and the depth and the location of cracks found on the surface after corrosion exposure for 4 months under cyclic loading: (a) specimen 13; (b) specimens 21 and 22; and (c) specimen 31.

which had compressive residual stresses, or in the region which had the highest tensile residual stress. 3.3. Accelerated SCC test under cyclic loading A total of five tensile specimens were subject to 12 days of cyclic loading under more aggressive conditions (see

Fig. 8. SEM image showing a micro-crack in specimen 43 after being exposed to corrosion for 12 days under an aggressive cyclic loading.

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G. Van Boven et al. / Acta Materialia 55 (2007) 29–42

Crack depth (μm) Residual stress (MPa)

aggressively loading were distinguished from cracks created by long-term cyclic loading by their sharper crack tips, inclined direction of growth and minimal corrosion damage of the crack wall. The location and the depth of the micro-cracks detected along the gauge length in samples 41–43 are plotted in Fig. 9, together with the distribution of residual stress. As illustrated in Fig. 9, micro-cracks are present primarily in a region around the 5 mm position. The highest occurrence of cracking, as illustrated in Fig. 10, is within a residual stress range of 150–200 MPa. This range is consistent with that illustrated in Fig. 7 for the long-term cyclic testing. The aggressively loaded specimens had only their edges coated and sealed, leaving both opposing gauge surfaces exposed. Despite both surfaces being exposed to electrolyte, all cracks originated on the surface of the specimen containing tensile residual stresses.

200 150 100 50

60 40 20 0 -15

-10

-5

0

5

10

15

Distance from center of gauge length (mm)

Visual differences existed between surface areas with compressive residual stresses and tensile residual stresses. Surface areas with tensile residual stresses were darker and exhibited a striated appearance. Conversely, surface areas with compressive residual stresses were covered with lighter, more uniform deposits, suggesting that differences exist in the electrochemical potential of the two surfaces. The frequency of crack initiation and the level of residual stress from both the short-term accelerated test and the long-term cyclic load experiment is illustrated in Fig. 11a. The cracking data from specimen 31 were not included as the exposed surface of this specimen was primarily compressive and only two short cracks were observed at the end of gauge length. In this figure, the number of cracks is normalized for the area of the tensile specimen at a particular residual stress level. Normalization removes the bias created by the variable specimen area that may exist at a particular stress level. The normalized figure conclusively illustrates that the occurrence of SCC is the greatest in the residual stress range of 150–200 MPa. The normalized frequency of occurrence of pitting is illustrated in Fig. 11b, which, similar to Fig. 11a, has excluded the pitting data from specimen 31. Differences exist between the pitting occurrence and the cracking occurrence when comparing the levels of residual stress. One difference is that the peak occurrence of pitting is located at the highest residual stress, while the peak occurrence of cracking is both well below the highest residual stress and non-existent at the highest residual stress level. A second difference is that at the low residual stress levels, pits were observed at almost every level of residual stress (Fig. 4), but cracks were only initiated from pits located within a narrow range of residual stresses.

Fig. 9. Distribution of axial residual stress as measured on the surface of specimen 41 prior to corrosion exposure and the depth and the location of cracks found on the surface of specimens 41, 42, and 43 after corrosion exposure for 12 days under an aggressive cyclic loading.

Cracking Occurrences

Normalized frequency of cracking occurrence

10

8

0 -400

-300

-200

-100

0

100

200

300

Normalized frequency of pitting occurrence

2

3 2 1

Pitting Occurrences Max Residual Stress

4

4

0

6 Min Residual Stress

Number of Crack Occurrences

5

400

Surface Residual Stresses, MPa

Fig. 10. Measured surface residual stress versus frequency of cracking on six tensile specimens tested in the 12 days modified slow strain rate experiment.

20

10

0 0

100

200

300

Range of surface residual stresses, MPa

Fig. 11. Measured surface residual stress versus normalized frequency of cracking for all cracks detected.

G. Van Boven et al. / Acta Materialia 55 (2007) 29–42

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3.4. Accelerated SCC test on specimens with modified residual stresses Although residual stresses are primarily induced during pipe fabrication, SCC cracks are usually developed only after the protective coatings are damaged and ground water is allowed to contact the pipe surface. It is recognized that pipeline load cycling prior to coating damage may alter the residual stresses formed during fabrication. Sample 32 was cyclically loaded in air to model the loading a pipeline may experience prior to steel exposure to an SCC environment. The level of cyclic stresses and the number of cycles applied to the specimen were equivalent to approximately 20 years of typical pipeline operation. (Approximately 8000 cycles at a maximum stress of 100% of specified minimum yield strength and varied minimum stress/maximum stress ratios ranging from 0.5 to 0.9 [17].) Fig. 12 illustrates the residual stresses in sample 32 measured after cyclic conditioning. When compared with Fig. 3c, which represents the residual stress distribution prior to cyclic conditioning, a change in residual stress caused by cyclic conditioning is evident. Generally, the pre-cyclic conditioning reduced both the maximum absolute residual stresses and the sharp variation of residual stresses in the specimen. For a given position, however, the level of residual stress can either increase or decrease depending on the initial stress value. Fig. 13 provides a comparison of residual stress on the tensile surface before and after cyclic conditioning. In this figure, cyclic conditioning has increased the residual stresses at the surface. As illustrated in Fig. 6c, long-term SCC exposure under cyclic loading has resulted in the formation of micro-cracks at the end of gauge length, which has a slightly tensile residual stress. In the short-term aggressive testing, no cracks were observed in specimen 33, which should have the same residual stress distribution as that of specimen 31, as it was machined from the same plate. This discrepancy in cracking may be related to the relatively short

Fig. 13. A comparison of axial residual stress along gauge length as measured before and after cyclic conditioning in air.

period of SCC exposure for specimen 33, and also to the lower potential for growth at this gauge position, as the cracks observed in Fig. 6c were the shortest when compared with those produced in other specimens subjected to the same testing conditions. Specimen 32 was cyclically conditioned in air and exposed for 12 days under accelerated SCC growth conditions. For this specimen, only one relatively long crack was detected, as illustrated in Fig. 14. This crack was located at +13.2 mm from center of gauge length. This position coincides with the location of the highest tensile residual stress, which was produced by cyclic conditioning (Fig. 13). The length of this crack is approximately four times longer than the longest cracks observed in the specimens under the same SCC exposure but without cyclic conditioning. This large difference in crack length may suggest that the crack observed in specimen 32 was caused by a different mechanism, e.g. fatigue cracking. This is a likely explanation, as a high stress amplitude exists at this position.

300

Residual stress (MPa)

200

100

0

-100

-200

-300 -15

0.4 mm 1.05mm 1.7mm 2.35 mm 3.0 mm -10

-5 0 5 10 Distance from center of gauge length (mm)

15

Fig. 12. Axial (gauge length) residual stresses measured on the preconditioned tensile specimen 32.

Fig. 14. Electron micrograph of cracking in pre-conditioned tensile specimen 32 after being exposed to corrosion for 12 days under an aggressive cyclic loading.

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4. Discussion 4.1. Representation of surface residual stresses measured by neutron diffraction The current investigation intends to correlate Type I residual stresses measured by neutron diffraction with events of crack initiation and the growth of pipeline steel in a near-neutral pH environment. The neutron diffraction measurements provide stress values for the bulk material. Residual stress measurements by X-ray diffraction may yield better spatial resolution along the surface. However, this was not performed in the current investigation due to the large specimen size, which prevented in situ X-ray measurements. Type I is the macro-scale residual stresses, which vary continuously over large distances of at least several grain diameters. Typical causes of Type I residual stresses in pipeline steels may include, for example, bending of steel plate during pipe forming, differential cooling through the wall thickness and along the surface caused by rolling, as well as localized plastic deformation. However, Type I residuals stresses occur over a large area, and abrupt changes, which can occur with Type II and III residual stresses, are not typically expected with this stress type. Therefore, the neutron diffraction measurements performed in this study should yield a good representation of the changes in Type I residuals stresses in the thickness direction and in the length direction of the specimen. 4.2. Selective initiation of SCC cracks From the results presented above, it is evident that the pitting initiation location and the SCC initiation location were influenced by different components of residual stress. It is understandable that crack initiation did not occur on the surface with compressive stresses. However, it is not clear why cracks were not initiated at the location having the highest residual stress. It is also unclear why cracks were initiated only from pits located within a certain residual stress range. Further, micro-cracks were observed only at discrete locations, even though the residual stresses at the surface were nearly constant. A number of factors related to these observations are discussed below. 4.2.1. Effect of plastic deformation As indicated in Section 2, the roof plates had a thickness of 6 mm before being flattened to produce straightened plates. However, approximately 1.3 mm of the surface on each side of the plates was removed by low stress grinding before the plates were EDM machined into specimens. This machining would minimize localized damage caused by the loading rod contact point at the center of gauge length. The observed cracks also appeared to be unrelated the location of these contact points. The variation of Type I residual stress is related to the different amount of macro-scale plastic deformation pro-

duced during bending. The pipeline steel is itself in a deformed state prior to bending. This investigation intends to correlate Type I residual stresses caused by long-range differences in plastic deformation with the occurrences of pitting and cracking. Although the yield strength of the as-received pipe was found to be 480 MPa, the yield strength of the tensile specimen prepared after various steps on a 171-degree plate (as illustrated in Fig. 1) was determined to be approximately 575 MPa. The peak cyclic stress applied during long-term SCC exposure was 448 MPa. This suggests that instantaneous macroscopic plastic deformation cannot be induced by the peak cyclic stress. However, microscopic plastic strain may exist at locations with a residual stress higher than 127 MPa. Larger residual stress levels allow for greater amounts of plastic strains to accumulate. The accumulation of plastic strain may explain why more pits were observed at locations where tensile residual stress and plastic deformation is higher. However, it is unclear why microcracks were observed at discrete locations even though the residual stresses at the surface were nearly constant. Straightening of the roof-like plates may produce nonuniform plastic deformation along the gauge length. The plastic strain may start at the ‘‘hinge’’ (roof tip), but will move toward the edge of the plate due to strain hardening with an increase of bending displacement. In the presence of the bending force, tensile force will act at the inner surface, while compressive force will be present at the top surface. The stress state, however, will be reversed upon the removal of the bending force, that is, tensile residual force at the top surface, compressive residual force at the inner surface. This is generally consistent with the residual stress measurements shown in Fig. 3, which were made after the removal of 1.3 mm of surface layer. For specimens conducted under accelerated loading cycles, both the roof top and inner surfaces were exposed to the testing environments (Fig. 9). Despite both surfaces being deformed during bending, cracks originated only on the roof top surface that was deformed by compression stress during bending, but had a tensile residual stress during corrosion exposure (Figs. 3d and 9). This suggests that the nature of residual stresses, rather than the plastic strain, determines the pitting and cracking observed in the current investigation. It should be noted that a similar analysis could not be applied to specimens tested under long-term cyclic loading, as the inner roof surface and two side surfaces were coated with an epoxy coating. 4.2.2. Effect of microstructure The surface microstructure of the test specimens was composed of a cross-section of material from the original pipe wall thickness. This cross-sectional representation of pipe wall material resulted from the differential amounts of grinding involved in fabrication (Fig. 1a). As such, it can be debated that the selective initiation of SCC cracks found in this investigation may be related to the possibility of non-uniform microstructures along the gauge length

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(Fig. 1a). It was also determined that this pipeline steel contains a banded structure in the center region of the wall [6]. A banded structure is prone to the initiation of pitting due to a similar galvanic effect [18]. However, the initiation of cracks was observed at almost all possible locations along the specimen gauge length and no SCC was found on the compressive surface. The poor correlation between location and SCC indicates that microstructure heterogeneity did not play a dominant role in the development of the initiated SCC. 4.2.3. Effect of galvanic corrosion Galvanic corrosion, in the form of a stress cell (regions with different stresses), can occur along the specimen gauge length when subjected to long-term cyclic loading in an aqueous environment. The higher stress and plastic strain locations become anodic to the lower stress locations. The long-term testing supports the concept of the ability of differential stress/strain-induced galvanic cells to cause pitting and eventual cracking by the observation that all the specimens in these tests formed micro-pits and SCC in areas with medium to high levels of residual stress. Conversely, little SCC occurred at lower levels of residual stress, despite the fact that a low level of residual stress in one specimen could correspond to a medium-to-high level in another specimen. This observation suggests it is the stress/strain differential that is important in causing micro-pitting and eventual SCC, rather than the absolute value of the stress. In the accelerated tests, a stress cell can be formed along the gauge length, but also between the two opposing test surfaces (the two edge surfaces were masked). As seen in Fig. 3, the difference in residual stresses between the two opposing specimen surfaces is much larger than that along the gauge length, and therefore a stress cell between the two opposing surfaces was more likely to dominate. However, the crack initiation site was not observed at the position at which the difference in residual stress between two opposing was the largest. This suggests that a simple galvanic corrosion mechanism in the form of stress cell cannot completely explain the selective initiation as observed in the current studies. 4.2.4. Pitting formation and SCC initiation In a typical pipeline steel neutral pH condition, pitting can occur due to galvanic corrosion at metallurgical discontinuities such as grain boundaries [18], inclusions [19], or phase interfaces [18–20]. However, in typical field conditions pitting usually has large depth to lateral length ratios, which is quite different from the observations in this experimental study. Both the field and experimental pitting likely have a similar initial mechanism of pitting, which is related to corrosion at metallurgical discontinuities. However, the large stress cells formed in this experiment likely influence later pit growth by driving very large anodic–cathodic reactions with consequent high rates of dissolution, which is atypical of field pitting.

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SCC can be initiated from the bottom of a pit by either a dissolution process or a mechanical process. With the dissolution mechanism, SCC generally requires a dissolution rate at least 10 times higher in the depth direction than in the lateral direction. With the mechanical SCC initiation process, micro-cracks are initiated at the weakest link sites in the hydrostatic zone ahead of a notch tip (the bottom of a pit in the present situation) [21]. The micro-crack is formed due to a sharp crack tip, which then propagates back to the bottom of the pit, thereby changing the pit into a sharp crack. The weakest link locations can be a metallurgical discontinuity with low bond strength or a region with high hydrogen segregation. Due to the large curvature at the bottom of the pits, the stress concentration factor would be relatively low and the formation of a micro-crack ahead of the bottom of a pit may be less likely. Although this crack initiation has been observed in deep blunted cracks from the field, no microcracks were observed to be associated with any deep pits found in this investigation. Instead, a focused dissolution in the depth direction was often observed at the bottom of a surface pit (Fig. 5a). Although cracks observed are all initiated from shallow pits, many pits were not associated with a crack despite being located in a very high residual stress regime. The frequency of pitting occurrence did not correlate well with the frequency of cracking occurrence when compared with the absolute magnitude of the residual stresses present. Such discrepancies seem hard to explain, but may be related to the change of residual stress induced by cyclic loading during SCC exposure. As shown in Fig. 13, cyclic loading caused a significant change to the residual stresses measured prior to SCC exposure. Cyclic loading during SCC exposure should be able to produce similar changes as well. As a result, the initiation of a pit or crack will depend on the initial residual stress distribution, how quickly the new residual stress state can be established, and the new residual stress state distribution. During accelerated short-term SCC exposure, it is believed that the newly established residual stress state determines whether a pit or crack can be initiated at a given position. This is consistent with the observation that microcracks seen after short-term tests were not located in the region of highest residual stresses as determined prior to SCC exposure (Fig. 9). For this to occur, the new residual stress state by cyclic loading must be established relatively quickly so that a large portion of time of SCC exposure would be spent under the new residual stress state. This seems possible as the peak stress under the accelerated test is much higher (above the yield strength), and under this plastic deformation can take place during the first cyclic loading and new stabilized substructures formed by the plastic deformation can then be developed. In long-term tests, SCC exposure was performed under cyclic loading with a very low frequency and peak stress level. The new residual stress state must be established over a much longer time [22,23]. As a result, both the residual

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stress state prior to SCC exposure and the new residual stress distribution established during SCC exposure must govern the pitting formation and crack initiation. It is evident that the region with high tensile residual stresses prior to SCC exposure should favor pitting formation and micro-crack initiation at the beginning of SCC exposure. However, this favorable condition may gradually disappear with increasing cyclic loading so that pits being developed in these locations may cease to grow, and the initiation of a crack from the pits, which happens in the later stage of pitting, may become less likely. That is to say, those pits observed in the high stress regime should be developed in the early stage of SCC exposure. Conversely, pits observed in the region with low tensile residual stress or compressive residual stress will develop only when a new residual stress state is established and this new residual stress state favors pitting formation. Initiating micro-cracks from these pits may depend on how soon the new stress state can be established at a given pitting location. Direct evidence of this statement may be the observation of micro-cracks on the surface of specimen 33 (Fig. 6c), where the initial residual stresses were only slightly positive. However, cyclic loading has increased the residual stress to a level that is very susceptible to crack initiating, as is illustrated in Fig. 13. Since few micro-cracks were observed in the locations with an initial low tensile or compressive residual stresses, the new residual stress state may have been established at a later stage or the new residual stress state was not strongly conducive to crack initiation. The residual stress in the region where a micro-crack can be initiated from a pit must be in a condition that favors dissolution over a long period of time and a long distance to the surface. To achieve this, the residual stresses that favors pitting formation and crack initiation, which was found to be within the range of 150–200 MPa, must have the least change with cyclic exposure. Qualitatively, this statement seems true, as it is seen from a comparison of Fig. 12 with Fig. 3c that the largest change in residual stresses by cyclic loading occurs at the peak of residual stresses either tensile or compressive, which had an absolute value of approximately 300 MPa before cyclic conditioning. The above discussion provides a conceptual rationale for the observation of the different frequencies of occurrence between pitting and micro-cracking in terms of residual stresses. A quantitative model describing the change of residual stresses by cyclic loading and its effect on pitting formation and/or crack initiation is reported in the second part of this investigation. 4.3. Crack growth mechanism A common feature among cracks observed in this investigation is the extensive lateral dissolution at the crack tip within a certain distance from the surface. Increased dissolution in the lateral direction was observed in the crack from the field, which is still not fully understood, despite various suggestions.

Crack growth under cyclic loading is generally related to the stress intensity factor range as, for example, expressed by the Paris equation. Under a given stress ratio, the growth rate can be related simply to the maximum stress intensity factor at the crack tip. In the current situation, the maximum stress intensity factor may decrease with increasing crack depth because the effective stress that sums the applied stress and the residual stresses decreases with increasing crack depth. These two opposite effects are modeled in the second part of this investigation. However, qualitatively, a gradual decrease in growth rate may occur at certain crack depths, which may be the primary reason for the observation of increased lateral dissolution. First of all, it has been illustrated that room temperature creep deformation is very significant in pipeline steels [22–24], and a lower crack growth rate allows more room temperature creep deformation at the crack tip, producing an increased plastic blunting [10]. This would also enable corrosion attack over a large area at the crack tip, resulting in further blunting by dissolution. A blunted crack tip reduces the stress intensity factor, which would drive the crack growth to an even lower level but cause more extensive lateral dissolution, particularly in the plastic zone ahead of crack tip. The above suggestion explains the observation of cone or balloon-like features, as illustrated in Fig. 3. As a result, the ‘‘pit-like defects’’ illustrated in Fig. 5 were originally micro-cracks which have experienced extensive lateral corrosion. Such a feature has been commonly observed on cracks in pipeline steels from the field. The growth rate of all the cracks observed is plotted in Fig. 15 as a function of measured residual stresses. For long-term tests, the crack growth rate increases linearly on a semi-logarithmic scale with the measured residual stress prior to the SCC exposure. For short-term accelerated tests, the growth rate is generally much lower than that determined from the long-term tests. The growth rate from short-term tests also appeared to contain a threshold 5 Long term cyclic testing

3

Short accelerated testing

2

Average growth rate, mm/s

40

1.0E-7 5 3 2

1.0E-8 5 3

0

100

200

300

Measured residual stress, MPa

Fig. 15. Crack growth rate as a function of residual stresses measured at the surface of the specimen.

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residual stress level for SCC to occur. Considering that cracks observed from short-term accelerated tests were sharp and often inclined in a direction with maximum shear stress, a fatigue mechanism for crack initiation and initial propagation may exist, as this crack has the appearance of a crystallographic stage I fatigue crack, parallel to a slip plane. This would support the presence of a threshold value of residual stress under a constant amplitude fatigue loading used in the current short-term accelerated testing. The SCC growth rate illustrated in Fig. 15 is quite comparable to the rate determined from the field, which varied quite significantly, but generally is accepted to be approximately 1 · 10 8 mm/s [1]. This similarity in SCC growth rates supports an equivalent growth mechanism in the laboratory as what is occurring in the field. 5. Conclusions 1. The formation of micro-pitting to a depth of up to 200 lm occurred preferentially in areas where the tensile residual stresses were highest (approximately 300 MPa). Little pitting or general corrosion occurred on surfaces with compressive residual stresses. This suggests cathodic and anodic interactions are occurring over several millimeters, where compressively stressed areas become cathodic to areas of tensile stresses. 2. All of the cracks detected from both the long-term tests and the accelerated short-term tests were found at the bottom of micro-pits. The location of the cracks in the micro-pits can be related to the stress intensification caused by both the micro-pit and the superimposed tensile residual stress. Crack initiation occurred with a 71% normalized frequency in areas where the surface residual stress was in the range of 150–200 MPa. No cracking initiated from pits located at surface areas measured to have compressive residual stresses or having the highest tensile residual stresses. 3. The different frequency of occurrence of pitting versus cracking when compared with the absolute residual stress level may be caused by the relaxation of residual stress which occurred as a result of SCC exposure under cyclic stress conditions, as well as the residual stress gradient in the depth direction. 4. Cracks formed during long-term SCC exposure experienced crack tip blunting caused by severe lateral dissolution and creep deformation. The lateral dissolution and creep deformation occurs at discrete depth locations where cracking velocity has dropped below that of the anodic dissolution rate, due to a reduction in tensile residual stresses in the growth direction. 5. The SCC growth rate determined is comparable to that observed on operating pipelines. For long-term tests, the growth rate is found to increase with increasing residual stress, while a threshold residual stress seems necessary for crack initiation during short-term accelerated testing.

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Acknowledgements The authors acknowledge the support of the National Research Council of Canada for the use of the Chalk River facility in conducting residual stress measurements. The authors thank TransCanada Pipelines for financial support, NOVA Research and Technology for facility and financial support, and Prof. R.L. Eadie for his useful discussions. References [1] Parkins RN. A review of stress corrosion cracking of high pressure gas pipelines proceedings of corrosion 2000. Paper 00363. Houston, Texas: NACE International; 2000. [2] Parkins RN. Investigations relating to environment sensitive fracture in the TransCanada Pipeline System Report to TCPL, 1988. [3] Parkins RN, Blanchard Jr WK, Delanty BS. Corrosion 1993;50: 395. [4] Parkins RN, Blanchard WK, Delanty BS. Corrosion 1994;50:394. [5] Qiao L, Mao X. Acta Metall Mater 1995;43:4001. [6] Chen W, King F, Vokes ED. Corrosion 2002;58:267. [7] Lufrano J, Sofronis P. Acta Mater 1998;46:1519. [8] Plumtree A, Lambert SB, Sutherby R. Stress corrosion crack growth of pipeline steels in simulated ground water. In: Corrosion–deformation interaction CDI’96, Nice, France; 1996. [9] Zhang XY, Lambert SB, Sutherby R, Plumtree A. Corrosion 1999;55:297. [10] Chen W, Sutherby RL. Environmental effect of crack growth rate of pipeline steel in near-neutral pH soil environments. In: Proceedings of IPC 2004, international pipeline conference, Calgary, Alberta, Canada; October 4–8, 2004, Paper No. IPC04-0449. [11] Beavers JA, Johnson JT, Sutherby RL. Materials factors influencing the initiation of near-neutral pH SCC on underground pipelines. In: Proceedings of 3th international pipeline conference, vol. 2. Calgary, Canada; October 1–5, 2000. p. 979–88. [12] Withers PJ, Bhadeshia HKH. Mater Sci Tech 2001;17:355. [13] Bouchard PJ. In: Buschow KHJ et al., editors. Encyclopedia of materials science and technology. Oxford: Pergamon; 2001. [14] Holden TM, et al. Application of neutron diffraction to engineering problems. Atomic energy of Canada, Chalk River Ontario. In: Proceedings of the fifth canadian conference on nondestructive testing; October 28, 1984. [15] Holden TM, Roy G. The application of neutron diffraction to the measurement of residual stress and strain. Handbook of measurement of residual stresses, society for experimental mechanics. Lilburn (GA): Fairmont Press; 1996. [16] ChenW, Eadie RL, Sutherby RL. Environmental effects on nearneutral pH stress corrosion cracking in pipelines. In: Second international conference on environment-induced cracking of metals, The Banff Centre, Banff, Alberta, Canada; September 19–23, 2004. [17] Chen W, Wang S-H, Chu R, King F, Jack TR, Fessler RR, et al. Effect of pre-cyclic loading on SCC initiation in an X-65 pipeline steel exposed to near-neutral pH soil environment. Metall Mater Trans 2003;34A:2601–8. [18] Chu R, Chen W, Wang S-H, King F, Jack TR, Fessler RR. Corrosion 2004;60:275. [19] Elboujdaini M, Wang YZ, Revie RW, Parkins RN, Shehata MT. Stress corrosion crack initiation processes: pitting and microcrack coalescence. NACE International Corrosion 2000. Paper No. 00379; 2000. [20] Kushida T, Nose K, Asahi H, Kimura M, Yamane Y, Endo S, Kawano H. Effects of metallurgical factors and test conditions on

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