1、 Item No. 24221 NACE International Publication 35103 This Technical Committee Report has been prepared By NACE International Task Group 040* on Stress Corrosion Cracking of Underground Pipelines External Stress Corrosion Cracking of Underground Pipelines October 2003, NACE International This NACE In
2、ternational technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products, processes, or procedures not incl
3、uded in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for
4、 infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of thi
5、s report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relatio
6、n to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Users of this NACE report are also responsi
7、ble for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this report. CAUTIONARY NOTICE: The user is ca
8、utioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may receive current information on all NA
9、CE International publications by contacting the NACE Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200). Foreword The purpose of this technical committee report is to provide useful information on stress corrosion cracking (SCC) for engineer
10、s, designers, consultants, and others involved in the design, maintenance, and rehabilitation of underground petroleum (including gas, crude oil, and refinery products) pipelines. This technical committee report contains informa-tion obtained from a survey of the open literature on the subject. This
11、 NACE technical committee report was prepared by Task Group (TG) 040 on Stress Corrosion Cracking of Underground Pipelines. TG 040 is administered by Specific Technology Group (STG) 35 on Pipelines, Tanks, and Well Casings. This report is published by NACE International under the auspices of STG 35.
12、 NACE acknowledges the National Energy Board of Canada (NEB)(1)for granting per-mission to cite the NEB report MH-2-95, “Stress Corrosion Cracking on Canadian Oil and Gas Pipelines,”1and to use parts of the document in the preparation of this report. Introduction SCC is one form of environmentally a
13、ssisted cracking (EAC). EAC is a generic term that describes all types of cracking in materials in which the environment and stress act together to reduce the strength or load-carrying capacity of the material. Other forms of EAC include hydrogen embrittlement, sulfide stress cracking, and corrosion
14、 fatigue. EAC is an ongoing integrity concern for many industries including oil and gas, nuclear power, and chemical process. It affects most common construction materials including car-bon steels, stainless steels, and copper-based alloys. _ *Chairman John A. Beavers, CC Technologies, Dublin, OH. (
15、1)National Energy Board (NEB), 444 Seventh Avenue SW, Calgary, AB T2P 0X8 Canada. NACE International 2 The first reported incident of external SCC on natural gas pipelines occurred in the mid-1960s, and numerous failures have occurred since that time.2SCC failures have also been reported on liquid p
16、ipelines, and SCC continues to be an integrity concern. It is now recognized that there are two forms of external SCC on underground pipelines: high-pH SCC (also referred to as classical SCC) and near-neutral-pH SCC (also referred to as low-pH SCC). A characteristic of both forms of SCC is the devel
17、opment of colonies of up to thousands of longitudinal surface cracks in the body of the pipe that link up to form long, shallow flaws. In some cases, growth and interlinking of the stress corrosion cracks produce flaws that are of sufficient size to cause leaks or ruptures of pipelines. The high-pH
18、form of SCC is intergranular, and there is usu-ally little evidence of general corrosion associated with the cracking. A concentrated carbonate-bicarbonate (CO3-HCO3) solution was identified as the most probable envi-ronment responsible for this form of cracking.3The near-neutral-pH form of SCC is t
19、ransgranular and is associated with corrosion of the crack faces, and in some cases with corrosion of the external surface of the pipe as well. This form of cracking occurs in near-neutral-pH (6 one year). In the case of the FBE coating, conduction of the CP current was associated with the formation
20、 of coating blisters containing a high-pH (12) electrolyte. Similar behavior would be expected with other coatings that are water-permeable. This pH is higher than the pH range for high-pH cracking, such that this form of cracking is unlikely to occur even if the potential range were appropriate for
21、 cracking. The shielding behavior of tape and asphalt/coal tar coatings is consistent with the prevalence of near-neu-tral-pH SCC with these types of coatings. The relationship between surface condition of a line-pipe steel and SCC has been the subject of several previous PRCI laboratory research pr
22、ograms19and has been sum-marized by Beavers.9The research indicates that grit-blasted surfaces are more resistant to high-pH SCC initia-tion than mill-scaled surfaces, primarily because grit blasting imparts a compressive residual stress in the pipe surface. Clean grit-blasted surfaces are also read
23、ily polarized in the presence of CP such that the potential is not likely to remain in the cracking range for long periods of time. The majority of single-layer FBE coatings are applied in coating mills over grit-blasted surfaces, whereas the older coatings were frequently applied over the ditch on
24、mill-scaled surfaces. While the above research was performed on initiation of high-pH SCC, it is possible that the beneficial effects of grit blasting extend to near-neutral-pH SCC initiation as well. The 1992 PRCI research study extended this prior labora-tory work to coatings applied in actual mil
25、ls.10This work confirmed the fundamental findings of the previous research in that a high-quality grit-blasting surface treatment was found to be beneficial. However, a much higher level of grit blasting than that previously reported was found to be necessary to improve the cracking resistance. This
26、 behav-ior was attributed to the extensive energy required to clean and grit blast a mill-scaled surface of a pipe that is covered with a thick corrosion product layer from yard storage. For-tunately, the white or near-white surface finishes necessary for FBE coating were found to be highly resistan
27、t to high-pH SCC. On the other hand, the lower-quality grit blast that is commonly used with plant-applied coal-tar coatings actually decreased SCC resistance over that found with a mill-scaled surface. In summary, field experience and related research support the contention that single-layer FBE co
28、atings provide resist-ance to both forms of SCC. This class of coatings generally has good disbonding resistance, and when disbondment does occur, CP current can flow to the pipe surface to pro-vide protection. Within blisters, the coating holds the electrolyte at the pipe surface, allowing for the
29、development of benign elevated-pH solutions. The high-quality grit-blasted surface preparation required for FBE coatings is also highly resistant to SCC because of the imparted com-pressive residual stresses. Grit blasting the surface provides an added bonusthe clean surface readily polarizes in the
30、 presence of CP such that the potential is not likely to remain in the cracking range (for high-pH SCC) for long periods of time. It is probable that other high-performance coatings that are applied with grit-blasted surfaces and have similar electrical properties would provide as much resistance to
31、 SCC as do single-layer FBE coatings. Soil In 1973, Wenk described results of analyses of soil and water extracts (from the soil) taken from high-pH SCC loca-tions.2While supporting data were not provided, it was stated that SCC had occurred in a wide variety of soils, covering a range in color, tex
32、ture, and pH. No single char-acteristic was found to be common to all of the soil samples. Similarly, the compositions of the water extracts did not show any more consistency than did the physical descrip-tions of the soils, according to Wenk. On several occa-sions, small quantities of electrolytes
33、were found beneath disbonded coatings near locations at which stress corrosion cracks were detected. The principal components of the electrolytes were sodium carbonate and bicarbonate. Sod-ium-bicarbonate crystals were also found on pipe surfaces near some SCC colonies.2Based on the presence of the
34、sodium-based carbonates and bicarbonates, it is likely that these were high-pH SCC sites. Therefore, it is not surpris-ing that these results are not consistent with the results of the TCPL studies performed in the 1980s and 1990s, when near-neutral-pH SCC was found. Mercer described the results of
35、a field study conducted by British Gas Corporation in 1979.20Soil data from both the UK and U.S. were collected and analyzed. As in the study by Wenk, detailed information on the soil analyses was not provided, but it was concluded that soil chemistry had no obvious direct influence on high-pH SCC.
36、The moisture content of the soil, the ability of the soil to cause coating damage, and localized variation in the level of CP were the primary soil-related factors that were identified. Delanty and OBeirne7,21reported on the results of more than 450 investigative excavations performed on TCPLs syste
37、m in the mid- to late 1980s. In the tape-coated portions of the system, near-neutral-pH SCC was found in all of the various types of terrains and soils (e.g., muskeg, clay, silt, sand, and bedrock) present on the system. There was no apparent difference in the soil chemistry for the SCC and non-SCC
38、sites. However, the SCC was predominantly located in imperfectly to poorly drained soils in which anae-robic and seasonally reducing environmental conditions were present. In the same system, near-neutral-pH SCC was found in the asphalt-coated portions of the system, predominantly (83%) in extremely
39、 dry terrains consisting of either sandy soils or a mixture of sand and bedrock. There was inadequate CP in NACE International 6 these locations, based on pipe-to-soil potential measure-ments or pH measurements of electrolytes found beneath disbonded coatings. The remainder of the SCC sites on the a
40、sphalt-coated portions of the system had localized areas of inadequate CP, based on pH measurements of electrolytes. Delanty and Marr developed an SCC severity-rating model for near-neutral-pH SCC for the tape-coated portions of TCPLs system in eastern Canada.5,22The predictors in that model were so
41、il type, drainage, and topography. The soil classifications were based on method of deposition. The most aggressive soil types were lacustrine (formed by deposits in lakes), followed by organics over glaciofluvial (formed by deposits in streams fed by melting glaciers), and organics over lacustrine.
42、 The prevalence of SCC in glacio-fluvial soils was about 13% of that in lacustrine soils, and about 17% of that in soils with organics over glaciofluvial or lacustrine. Very poorly or poorly drained soils were found to be the most aggressive, while level-depressed soil was found to be the most aggre
43、ssive topography. The SCC model did not contain parameters associated with soil chemistry because the results of previous geochemical pro-jects were inconclusive. As described above, neither the early field studies con-ducted on high-pH SCC nor the later field studies conducted on near-neutral-pH SC
44、C detected a correlation between the occurrence of SCC and soil chemistry. On the other hand, high-pH SCC was not reported where the extensive field study of near-neutral-pH SCC was performed in Northern Ontario,7,21suggesting that the soil conditions were not con-ducive to this form of cracking. Fu
45、rthermore, no near-neu-tral or high-pH SCC was found in Northern Ontario where elevated-pH electrolytes were detected, possibly because the soil conditions could not support the development of concentrated carbonate-bicarbonate solutions, even when the CP conditions were conducive to such developmen
46、t. These observations suggest that a further analysis of field soil data might provide insight into the role of soil/ground-water chemistry on the occurrence of SCC.23Cathodic Protection Cathodic protection (CP) is closely related to the high-pH cracking process. The CP current collecting on the pip
47、e surface at disbondments, in conjunction with dissolved CO2in the groundwater, generates the high-pH SCC environ-ment. CP can also place the pipe-to-soil potential in the potential range for cracking. Parkins,24Fessler,25and Beavers et al.26examined the effect of CP on SCC in high-pH cracking envir
48、onments. This research indicates the potential range for cracking generally lies between the native potential of underground pipelines and the potential associated with adequate protection (-850 mV CSE). For example, the center of the potential range in the high-pH cracking environment at 75C (167F)
49、 is -722 mV CSE.24Because the rate of generation of the cracking environment is related to the CP current, it is likely that seasonal fluctu-ations in the CP system are associated with the cracking process. The potent cracking environment might be gener-ated during portions of the year when CP levels are high, while cracking might occur when adequate protection is lost, such as in the summer months when the soil dries out. The effect of CP on cracking in near-neutral-pH cracking environments has been studied by Parkins and others using slow-strain-rate tests
