IPC MECHANICAL FACTORS AFFECTING STRESS CORROSION CRACK GROWTH RATES IN BURIED PIPELINES. Copyright 2000 by ASME

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1 IPC MECHANICAL FACTORS AFFECTING STRESS CORROSION CRACK GROWTH RATES IN BURIED PIPELINES S.B. Lambert University of Waterloo J. A. Beavers CC Technologies B. Delanty R. Sutherby A. Plumtree TransCanada PipeLines Ltd. TransCanada PipeLines Ltd. University of Waterloo ABSTRACT Over the past several years, investigations have been carried out into the rate of crack growth in pipeline steels in simulated, near-neutral ph, groundwater environment (NS4 solution). Pre-cracked specimens were subject to constant amplitude loading under various frequencies, maximum loads and /{-ratios (minimum/maximum load). Test times varied from about 20 to 400 days. Transgranular crack features, similar to those found in service, have been observed. The extent of crack growth was monitored using either electrical potential drop or detailed metaliographic examinations at two laboratories. The resulting crack growth rates from both labs are consistent with a superposition model based on a summation of fatigue (Paris Law) and static (SCC) crack growth rates. Differences between the results at the two laboratories are discussed. INTRODUCTION Stress Corrosion Cracking (SCC) has been observed on the soil side of buried, natural gas pipelines since the early 1960's in the USA (Parkins, 1974) and more recently, since 1985, in Canada (Justice and Mackenzie, 1988). Over the past 10 years, two distinct forms of SCC have been identified based on the electrolyte thought responsible (high versus near-neutral ph) and the cracking morphology (inter- versus trans-granular). However, both types share several important general characteristics. SCC in pipelines must pass through several phases. In the first phase, the coating applied to the pipeline during installation is degraded and a suitable electrolyte comes into contact with the pipe surface. The second phase involves the initiation and growth of individual cracks to form colonies. In the third phase, these cracks may continue to grow and coalesce. In the final phase, a dominant crack may reach a critical size for rapid growth to failure, producing either a leak or rupture. Most colonies will not complete all of these phases, and the relative timing depends on a number of factors including the pipe material, the stress history, environment, and the distribution of cracks. Cracking occurs in colonies of from several to several hundred individual surface cracks. These colonies may extend from several millimetres to several meters in length and breadth. Cracks are generally oriented longitudinally, perpendicular to the tensile hoop stress. Most cracks are very small (of the order of a few millimetres in length), and remain small throughout the life of the pipeline. One of the key objectives of research into pipeline SCC is to determine which colonies will eventually fail, and at what rate. Research can be divided into field studies, laboratory measurements, and model development. Field studies are used to determine the extent and distribution of SCC in particular lines, and to relate these to environmental and loading conditions. In addition, electrolyte samples are taken and conditions monitored for use in the lab. Laboratory studies are used to assess the relationship between environmental factors, mechanical effects, and crack growth rates. The information gained from these studies can be brought together through modeling efforts. In the present work, laboratory crack growth rate measurements conducted at two different laboratories, in nearneutral ph electrolyte, under a range of mechanical conditions are presented and compared. Two different specimen configurations were used: edge notch bending and compact tension. The use of different specimens permits the investigation of crack growth in the two orientations of primary interest: through the depth and along the surface. Both sets of data are presented using a superposition model. Copyright 2000 by ASME

2 EXPERIMETNAL PROGRAM Figure 1: Crack orientation in pipe. Figure 1 illustrates an isolated SCC crack on the surface of a pipeline segment. Also shown are the designations for metallographic orientation as a consequence of pipeline manufacture: L - longitudinal, T - transverse, and S - short transverse. The crack is aligned with the longitudinal (L) direction and grows in the S-direction through the depth and the L-direction along the surface, driven by the hoop stress (the normal stress aligned with the T-direction). Figure 2 illustrates the specimen configurations and orientations used herein: Figure 2(a) illustrates the edge crack specimen used at Waterloo, and Figure 2(b) illustrates the compact tension specimen configuration used at CC Technologies. All testing was performed using a near-neutral ph electrolyte, designated NS4, under free corrosion potentials. NS4 has the following chemical composition: g/1 KC1, g/1 NaHCOj, g/1 CaCL 2-2H 2 0 and g/1 MgS0 4-7H 2 0. It was among several electrolytes isolated in the field, which were associated with near-neutral SCC, and has since become the standard for this testing. When first mixed, this electrolyte has a ph of about 8. A 5% C0 2, balance nitrogen, mixture was bubbled continuously through the electrolyte throughout testing to reduce the ph to the range All testing was conducted under anaerobic conditions. Prior to the commencement of the test, the electrolyte and enveloping environmental chamber are actively purged with pure nitrogen gas for about 24 hours to purge the system of oxygen. Then, about 1 hour before the test, the purge gas is switched to 5% C0 2, balance Nitrogen. The flow rate of this mixture is adjusted to ensure that the ph of the electrolyte is in the desired range throughout the test. if F f 1 Edge Notch Bending Specimens Edge notch bending specimens were used in the research program at Waterloo, Figure 2(a). Specimens were prepared from rings of NPS 20 pipe, 6.35 mm wall thickness, which had been exposed to service conditions for about 17 years, prior to being removed. The material was an API 5L X-65 pipeline steel. Three separate edge cracks were introduced into each specimen using the fine slitting saw. The cracks were sharpened in fatigue and grown to a total depth of about 3 mm, about one-half of the wall thickness. Cracks were in the T-S orientation - in a plane perpendicular to the transverse (hoop) direction and growing in the short-transverse (S) or thickness direction. Fatigue loading was applied to ensure that the maximum load never exceeded the maximum load experienced in testing. This load varied for each crack due to the cantilevered nature of the applied bending loading, Figure 2(a). Crack 1, nearest the built-in end, experienced the maximum load, with a slight reduction for the others. The final depth of each crack was selected to obtain the desired maximum stress intensity factor. Nominally, the stress intensity factors were 38, 36 or 34 MPaVm for cracks 1 to 3, respectively. Crack growth during prefatigue was monitored using electrical potential drop (Yee and Lambert, 1997). However, crack growth measurement during SCC testing was monitored instead using metallographic examinations. Photos were taken of the crack tips prior to each test and micro-hardness indentations were made near the crack tips to serve as a reference for post-mortem analyses. Using this technique, crack growth as small as 5 pm was detectable by comparing before and after photographs. Compact Tension Specimens Compact tension specimens were used in the research program conducted at CC Technologies (Beavers, 2000). These specimens were machined from two different API 5L X- 65 pipeline steels. Neither of these materials was exactly the same as was used for the bending specimens. The specimens were machined according to ASTM E 813, except that the thickness was only 54 the pipe wall thickness, being limited due to the curvature. The CT specimens were fatigue precracked to approximately 4 mm depth using a servo-hydraulic machine. The cracks were in the T-L orientation - in a plane perpendicular to the transverse (hoop) direction and growing in the longitudinal direction. A load shedding technique (intermittently decreasing the load as the crack increases in length) was utilized in order to minimize the plastic zone size during precracking. In order to meet crack straightness requirements, side-grooves were machined in each specimen following precracking to a total of 20% of the specimen thickness (10% on each side). (a) Edge notch bending (b) Compact tension Figure 2: Schematic illustration of specimen

3 Crack growth during the test was monitored using a direct current (DC) potential drop technique. A steady current of 20 1 E-01 1.E E-03 1.E-04 1.E-05 3 «CT c/d ± CT c/d i o Bend -1 c/d Bend - 40 c/d \ O Bend c/d Bend c/d Superposition Model 1 cycle / 2 days 1 cycle/day C>0 40 cycles / day higher order polynomial prior to calculating crack velocities. The crack length as a function of time was determined from the electrical potential drop data using the Johnson (1965) equation. RESULTS Figure 3 presents some of the results from the present research, plotted in terms of cyclic crack growth rates (mm/cycle) versus the range in stress intensity factor (MPaVm). The edge notch specimen results were obtained by Zhang et al. (1999), using cyclic frequencies from 1 to 5000 cycles per day ( to Hz) and i?-ratios (minimum over maximum load) from 0.5 to Average cyclic crack growth rates were obtained by dividing the total crack growth, which occurred during the test, for each crack, by the total number of cycles experienced by the specimen. Much scatter is evident. However, the results were reasonably well described by a model which assumes a linear superposition of fatigue and SCC (Wei and Landes, 1969) as follows: 1.E cycles / day da dn Total da IN +- 1 da fatigue f dt see (1) 1.E E cycles / day ak [MPa?m] Figure 3: Crack growth rates for edge notch bending and compact tension (CT) specimens. amperes was passed through the specimen and the voltage drop across the crack was monitored and correlated to crack growth. A data acquisition system recorded date, time, elapsed time, potential drop, load, load-line displacement, and corrosion potential of the specimen every 15 minutes during the test. After completion of the test, the specimen was broken open and the fatigue precrack and environmental crack lengths were measured with an optical microscope and calibrated eyepiece, using the nine-point measurement method described in ASTM E 813. Fracture surfaces of some specimens were examined using a scanning electron microscope to assess crack morphology. Analysis of the potential drop crack data involved averaging every 5 data points from the original data acquisition files in order to reduce the amount of noise in the data and enable better curve fitting. Because of the inherent noise in the electrical potential drop data, the data were fitted to a third or where the fatigue component is given by the Paris equation: da_ dn fatigue = CAK" (2) Here, da/dn is measured in m/cycle, AK is measured in MPaVm,/is in cycles/s and da/dt is in m/s. The lines in Figure 3, denoted 'superposition model', correspond to the following data for an i?-ratio of 0.5: fatigue constants C = 2.92x10-^3 and m , and an assumed constant SCC crack growth rate of 2.94x10~12 m/s. This crack growth rate was chosen to provide the best fit of the model to this data set. Alternatively, one could say that the model was used to estimate the SCC crack growth rate from crack growth measurements on cyclic specimens. Note that, at the right, for higher AK's, the model lines converge to the fatigue (Paris) line, since the SCC component is insignificant. At lower AK's, the lines tend to a constant crack growth rate which is independent of AK, and corresponds to the SCC growth rate divided by the frequency. In this model, the SCC component is represented as a single constant value, independent of applied maximum stress intensity factor. There was insufficient data to observe whether the SCC component exhibited threshold behaviour or showed any dependence on K max. The observed SCC crack growth rate was about an order of magnitude higher that Faradaic corrosion rates, and somewhat less than maximum average crack growth rates observed late in life for cracks in the field.

4 Also shown on Figure 3 are data from compact tension (CT) specimens. These were obtained under the same environmental conditions and for the same nominal pipeline material (X65), but taken from a different pipe. Again, considerable scatter is noted. This data shows a similar trend to that observed for the bending specimens: at lower loading frequencies, faster crack growth rates are observed. However, the observed crack growth rates are about an order of magnitude higher than for the bending specimens. Note that the fatigue crack growth rates in air were also different by a similar order of magnitude. DISCUSSION There are several possible explanations for the differences in observed crack growth rates for the two specimens. First, the materials were different. However, in the research conducted at CC Technologies, two different API X65 steels were used and similar results were obtained. Second, the crack growth rates were measured differently in each case. For the bending specimens, the results correspond to average crack growth rates over the entire course of the experiment, since only the total crack growth was measured. In contrast, the instantaneous crack growth rates were monitored in the CT specimen tests. This might help to explain the somewhat higher scatter observed in the CT specimens, but would not explain the systematic differences observed. The instantaneous crack growth rates would be expected to be higher in some cases and lower in others. This was not observed. In addition, however, the potential drop results were sensitive to any general corrosion occurring on the side of the specimen. This reduction in cross-sectional area would lead to an increase in potential drop not related to crack growth. This effect is expected to be insignificant for the specimen tested at higher frequency and has been estimated to have a magnitude of about 4 x 10" 9 mm/s for the lower frequency result (0.8 cycles/day). Third, the specimens were designed to measure crack growth in different metallographic directions. Both used through edge cracks, but the bending specimens measured crack growth through the thickness and the compact tension specimens measured crack growth along the surface. In the field, cracks tend to be long and shallow, indicating a faster crack growth rate along the surface than through the depth. This is consistent with the data presented herein. However, there are several factors present in the field situation which are not replicated here. Cracks may grow faster along the surface in the field due to surface, mill scale, effects (the CT specimens used herein were machined to remove the mill scale), surface environmental effects, surface residual stresses, or microcoalescence with newly initiated cracks ahead of the dominant crack tip. Further work is necessary to examine these effects; not just to resolve the differences observed here, but to better understand crack growth in the field. The final possibility is that the different metallographic orientations are significant. Due to the manufacturing process, the grains are elongated in the longitudinal and transverse directions, and flattened in the short transverse direction. This creates more microstructural barriers to crack growth in the thickness direction. In fact, the TS crack orientation is sometimes called an 'arrestor' configuration. This is consistent with the slower crack growth rates in the bending specimens. CONCLUSIONS Crack growth rates have been measured on edge notch bending and compact tension specimens for API X65 pipeline steel immersed in a near-neutral electrolyte under anaerobic and free corrosion conditions. The results could be characterized using a linear superposition of fatigue (cycledependent) and SCC (time-dependent) terms. The SCC crack growth rates for the bending specimens were about 3x10~ 9 mm/s. Those for the compact tension specimen were about an order of magnitude faster. Some of this difference may be related to differences in crack growth measurement. The remaining differences in crack growth rate for the two specimens may be related to the different crack orientation. They are consistent with a faster crack growth rate along the surface, leading to long shallow cracks, observed in the field. More research is necessary to determine whether these differences are significant or merely a function of the scatter anticipated in this type of research. ACKNOWLEDGMENTS This research was funded by TransCanada PipeLines Ltd. Their ongoing efforts to understand this important problem are appreciated. REFERENCES J.A. Beavers, 2000, "Effect of Pressure Fluctuations on SCC Propagation", PRCI Contract PR B.A. Harle, J.A. Beavers and C.E. Jaske, 1994, "Low-pH Stress Corrosion Cracking of Natural Gas Pipeline", Proceedings of NACE Canadian Region Western Conference, Calgary, Alberta, pp H.H. Johnson, 1965, "Calibration of the Electrical Potential Drop Method for Studying Slow Crack Growth", Materials Research and Standards, Vol. 5, p J.T. Justice and J.D. Mackenzie, 1988, "Progress in the Control of Stress Corrosion Cracking in a 914 mm O.D. Gas Transmission Pipeline", Proc. NG-18/EPRG Seventh Biennial Joint Technical Meeting on Pipeline Research, paper #28, American Gas Association. R.N. Parkins, 1974, "The Controlling Parameters in Stress Corrosion Cracking", 5th Symposium on Line Pipe Research, American Gas Association.

5 R.P. Wei and J.D. Landes, 1969, "Correlation Between Sustained-Load and Fatigue Crack Growth in High-Strength Steel", Materials Research and Standards, Vol. 9. R. Yee and S.B. Lambert, 1995, "A Reversing Direct Current Potential Drop System for Detecting and Sizing Fatigue Cracks Along Weld Toes", Journal of Testing and Evaluation, Vol. 23, No. 4, pp X. Y. Zhang, S.B. Lambert, R. Sutherby and A. Plumtree, 1999, "Stress Corrosion Cracking of API X-60 Pipeline Steel in a Neutral Solution", Corrosion, Vol. 55, No. 3, pp