Corrosion fatigue and nearneutral ph stress corrosion. Buried pipelines provide the most efficient and safest ways for oilgas longdistance transmission. Near neutral ph stress corrosion cracking nnphscc has been an integrity risk for oil and gas pipelines for more than two decades. Tests were performed by slow strain rate testing at 90 c and at 60 c. The test may not be relevant to stresscorrosion cracking in polythionic acid or caustic environments. The slow strain rate test showed ductile and brittle fracture feature at low and high applied potentials, respectively. Stress corrosion cracking behaviour of api 5l x52 linepipe steel in near neutral and high ph solution is mixcontrolled by anodic dissolution and hydrogen based mechanism. Scc results from the formation of cracks due to various factors in combination with the environment surrounding the pipeline that together reduces the pressurecarrying capability of the pipe. The severity of four different soil environments toward the development of near neutral ph stress corrosion cracking scc of pipeline steel was evaluated using slow strainrate testing ssrt. Characteristics and historical information on both forms of cracking are discussed.
Investigating the mechanism of stress corrosion cracking. It was found that there is a critical potential range, i. An overview of nearneutral ph stress corrosion cracking. What is stress corrosion cracking and why does it occur. Environmentally induced cracking of api grade steel in near. Copper rods were tested for signs of stress corrosion cracking in salt solutions with different sulfide concentrations. Stress corrosion cracking of x70 pipeline steel in near neutral ph solution subjected to constant load and cyclic load testing. Weixing chen, an overview of nearneutral ph stress corrosion. These soils were collected from pipeline sites where near neutral ph scc has been observed. The mechanism of scc changes with a change in applied cathodic potential, so that at low and high applied cathodic potentials, anodic dissolution and hydrogen embrittlement. An overview of nearneutral ph stress corrosion cracking in.
Mechanisms of highph and nearneutralph scc of underground. What causes stress corrosion cracking in pipelines. Stress corrosion cracking of x80 pipeline steel in nearneutral ph. In this article, the impact of plastic prestrain on nearneutral ph stress corrosion cracking scc susceptibility of welded x70 steel was investigated with a slow strain rate tensile ssrt test. This mode of attack is termed stress corrosion cracking scc. This paper is aimed at providing an overview of stress corrosion cracking scc of pipeline steels exposed to near neutral ph environments and corresponding integrity strategies being developed based on the understanding of the cracking mechanisms determined. The results showed that scc intensity increased at high cathodic protection potential. The most common environmental exposure condition responsible for scc of stainless steels is the presence of chlorides. Although no stainless steel grade is totally immune.
Scc is highly chemically specific in that certain alloys are likely to undergo scc only when exposed to a small. Investigating a mechanism for transgranular stress corrosion. Environmental aspects of nearneutral ph stress corrosion cracking. Luo, relationship between yield strength and nearneutral ph stress corrosion cracking resistance of pipeline steelsan effect of microstructure, corrosion, 2006, 622, p 129140. This paper provides an overview of mechanisms for highph and near neutral ph stress corrosion cracking of underground pipelines. The ph of the test solutions were kept near neutral by addition of 10 mm phosphate buffer of ph 7. To clarify the role of dissolved oxygen in nearneutral ph stress corrosion cracking scc, slow strain rate test ssrt, dynamic polarization technique, and electrochemical impedance spectroscopy eis were used to investigate the relationship comprehensively between scc susceptibility and the electrochemical corrosion reactions occurring at the steelsolution interface in diluted, varying n 2purged, nearneutral ph solutions. The mechanism of stress corrosion cracking scc involves a very complicated sequence of corrosion and fracture. Stress corrosion cracking scc in nearneutral ph environment remains a major concern for high pressure pipelines transporting oil and gas. Hydrogen in stress corrosion cracking of x70 pipeline steels. Extensive field investigations have shown that nnphscc is characterized by wide transgranular cracks, and usually occurs in dilute near neutral ph environments with a ph around 6.
However, due to changes in loading or cyclic loading there is crack tip strain resulting in breakage of oxide film. Cyclic tests of stresscorrosion cracks in steel gas pipelines with. The application of strainbased design for pipelines requires comprehensive understanding of the postyield mechanical behavior of materials. They found that ac interference changed the high ph scc mode from intergranular to transgranular 15 and the near neutral ph scc susceptibility was enhanced by ac current 16. Transgranular stress corrosion cracking tgscc of pipeline steels in near neutral environments was first reported in the mid1980s. Wet h 2 s cracking can occur in susceptible steels exposed to aqueous environments containing hydrogen sulfide. Investigating a mechanism for transgranular stress. The ph was maintained nearneutral in the lab tests by continuous purging of the test solution with co2n2. It has been found that low ph scc occurs in environments with a low concentration of. Investigating the mechanism of stress corrosion cracking in. We have completed the public inquiry into stress corrosion cracking on canadian oil and gas pipelines in. Metallographic examination of these cracks show an extensive dissolution of the crack walls, where the cracks were generally filled with predominantly iron carbonate containing corrosion product. Near neutral ph stress corrosion cracking nnphscc, which occurs when ground water penetrates under the pipe coating, causes longitudinal cracks to develop on the surface of pipelines.
The tensile stress may be residual stress resulting from heat treatment. Stakeholder communications stress corrosion fact sheet. Mar 21, 2001 this paper provides an overview of mechanisms for highph and near neutral ph stress corrosion cracking of underground pipelines. Combined effect of alternating current interference and. Proceedings of the 2018 12th international pipeline conference. In order to identify the mechanism contributes in stress corrosion. Some factors affecting initiation of stress corrosion cracking of an x80 pipe steel in near neutral ph environment.
Standard practice for evaluating stresscorrosioncracking. Microstructural effect on nearneutral ph stress corrosion cracking resistance of pipeline steels article pdf available in journal of materials science 4115. Us70249b1 method for detecting near neutrallow ph stress. Highph scc failures of underground pipelines have occurred in a wide.
Cepa recommended practices for managing near neutral ph. Although it has traditionally been termed stress corrosion cracking, crack growth has never been observed under a static loading condition. Stress corrosion cracking of x70 pipeline steel in near. A method for predicting the location of stress corrosion cracking in a steel gas pipeline in which inline stress corrosion cracking smart tool data, external low level metal loss data and soil characterization data are compiled to predict the location of stress corrosion cracking in a steel gas pipeline segment.
However, stress corrosion cracking scc has been recognized as one of potential threats to the safe operation of pipelines. Microstructural effect on nearneutral ph stress corrosion cracking. Department of chemical and materials engineering, university of alberta, edmonton, canada. Scc in pipelines is a type of environmentally assisted cracking. Managaing nearneutral ph scc 4 of 158 3rd edition, may 2015 term definition collapse limit plastic collapse limit the maximum stress, strain or load which may be applied prior to onset of plastic collapse. The effect of hydrogen on the stress corrosion cracking scc for x70a and x70b pipeline steels in near neutral ph environments was studied by monotonic tensile tests on slow strain rate tests ssrt machine and cyclic tests on ssrt machine and fatigue machine.
Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. Microstructural effect on nearneutral ph stress corrosion. Proximity of nearneutral ph scc hydrostatic test failures to compressor. Transgranular stress corrosion cracking of highpressure pipelines in contact with solutions of near neutral ph r. Effect of co2 on nearneutral ph stress corrosion cracking. Mar 22, 2018 du and coworkers investigated the influence of ac on the stress corrosion cracking scc behavior of pipeline steels in high ph and near neutral ph solutions. An overview of nearneutral ph stress corrosion cracking in pipelines and mitigation strategies for its initiation and growth. It is a form of hydrogenrelated cracking and can have two distinct morphologies.
Stress corrosion cracking of pipeline steel in nearneutral. Near neutral ph stress corrosion cracking nonclassic type a near neutral ph scc environment appears to be a dilute groundwater containing dissolved co 2. Some factors affecting initiation of stress corrosion. These recommended practices represent an overview of the methods used by cepa member companies to. Two types of pipeline scc modes have been identified based on the environment developed under disbonded coating, i.
The first type is commonly referred to as hydrogen induced cracking hic and can occur where little or no applied or residual tensile stress exists. Stress corrosion cracking initiation under the disbonded. Cepa recommended practices for managaing near neutral ph scc 1 of 158 3rd edition, may 2015 executive summary this 3rd stedition of the cepa recommended practices for stress corrosion cracking builds on the 1 and. Stress corrosion cracking of x80 pipeline steel in nearneutral ph environment under constant load tests with and without preload. This cracking occurs only when the steel is sensitized as defined by the acid copper sulfate test. The source of the co 2 is typically the decay of organic matter and geochemical reactions in the soil. Pipeline stress corrosion cracking scc corrosion doctors. Environmental aspects of nearneutral ph stress corrosion.
Stress corrosion testing of copper in near neutral sulfide. A thin oxide layer is formed in the concentrated carbonatebicarbonate environment, which around the crack surfaces provides protection. Fullscale stress corrosion crack growth testing of an x70 spiral. Results showed that scc susceptibility is inversely. This paper investigates mechanistically stress corrosion cracking scc of an x70 pipeline steel that is under cathodic protection cp in a nearneutral ph solution. Transgranular stress corrosion cracking of highpressure. Nearneutral ph stress corrosion cracking susceptibility of. Nearneutral ph scc this type of transgranular cracking mode of scc was. No matter how well these pipelines are designed, constructed and protected, once in place they are subjected to environmental abuse, external damage, coating disbondments, inherent mill defects, soil movementsinstability and third party damage. Stress corrosion cracking scc of pipeline steels in nearneutral ph environments has remained a significant integrity risk for oil and gas pipelines. Cepa recommended practices for managaing nearneutral ph scc 1 of 158 3rd edition, may 2015 executive summary this 3rd stedition of the cepa recommended practices for stress corrosion cracking builds on the 1 and 2nd editions published in 1999 and 2007, respectively.
When stainless steels are fully immersed, it is rare to see chloride stress corrosion cracking at temperatures below 60 c 150 f. Growth of surfacetype stress corrosion cracks in near. Colony an area of stress corrosion cracks occurring in groups of a few to thousands of cracks within a relatively. It has been shown that, for some materials, fine pitting is needed to start the activity. Mechanistic aspect of nearneutral ph stress corrosion. The combination of tensile stress and a specific corrosive environment can crack stainless steels.
Abstract nearneutralph scc has been one of the major remaining. Stress corrosion cracking behaviour of api5lx52 steel under cathodic protection in nearneutral and high ph conditions was studied using slow strain rate test method and electrochemical measurements. Stress corrosion cracking scc tests were conducted on american petroleum institute api grade x70 line pipe steel using smooth. Further progress is then a process alternating between corrosion and mechanical cracking until the material fails completely. All existing crack growth models were developed based on the results obtained from tests either under constant load for the case of scc or under constant stress. One was to determine if the compliance technique could be used to measure nearneutral ph stress corrosion cracking scc in a dilute carbonatebicarbonate environment like that observed under the coating disbondments in the field when this kind of cracking is encountered.
Effect of dissolved oxygen on stress corrosion cracking of. The severity of four different soil environments toward the development of nearneutral ph stress corrosion cracking scc of pipeline steel was evaluated using slow strainrate testing ssrt. Such cracks grow over time and can ultimately lead to pipeline failure. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Stress corrosion cracking of x70 pipeline steel in near neutral ph. Initiation of stress corrosion cracks in x80 and x100 pipe. Tests for stress corrosion stress corrosion cracking is a timedependent process in which a metallurgically susceptible material fractures prematurely due to the synergistic interaction of a corrosive environment and sustained tensile stress at the metal surface. Stresscorrosion cracking of sensitized type 304 stainless. As such, the newly discovered form of scc is equally referred to as transgranular scc or near neutral ph scc.
938 1144 1233 1250 419 1292 578 886 373 1568 1524 560 1291 790 846 1262 1583 1416 53 1333 590 1558 501 245 1333 699 1336 343 500 1433 345 1143 315 931 164 1219 52 1445 967 1071 1143 1110 1113 981 1265 131 194