Stress Corrosion Cracking Of Pipelines Research Articles

The synergistic effect of parallel magnetic field and sulfate-reducing bacteria on stress corrosion cracking of buried X80 pipeline steel

In this work, the synergistic effect of parallel magnetic field and sulfate-reducing bacteria (SRB) on stress corrosion cracking (SCC) of X80 pipeline steel in buried soil environments was investigated for the first time by electrochemical experiments, slow strain rate tensile tests (SSRT) and surface characterization. The results showed that the parallel magnetic field suppressed electrochemical corrosion in a sterile environment; the SCC susceptibility decreased with increasing magnetic field strength (0–100 mT). The magnetic field and SRB synergistically promoted the formation of a dense film layer on the surface of the specimen in the inoculated SRB environment. The dense film layer suppresses the local corrosion of the metal and therefore the SCC susceptibility is reduced. The suppression effect no longer increased significantly when the magnetic field strength reached 75 mT. The synergy between the parallel magnetic field and the SRB was responsible for the reduced susceptibility of the SCC. This work contributed to a more in-depth study of the mechanisms of pipeline SCC and the development of pipeline SCC prevention technologies.

Stress corrosion cracking in pipelines

Stress corrosion is viewed as cracking of a pipe metal in cathodic protection regions of gas transmission pipelines, which are simultaneously affected by various unrelated, at first glance, factors. Stress corrosion is otherwise referred to as stress corrosion cracking or SCC. Stress corrosion cracking of pipelines is the main cause of emergencies in gas transmission pipelines. Currently, this type of corrosion is widespread and very dangerous. The reliability and safety of modern pipelines is of great national importance. Despite being significant, multiple efforts to address this problem have not yet identified the causes and algorithms for handling stress corrosion.

Providing resistance to sulfide stress corrosion cracking of pipelines welded joints by selection of welding parameters

Sulfide stress cracking (SSC) is one of the most dangerous types of pipelines destruction. Thermal impact of the welding process drives to heterogeneity of the microstructure and properties of the metal, which can lead to cracking of pipeline welded joints. Resistance to SSC of welded joints is determined by the thermal cycle of welding and cooling rate in the temperature range of austenite transformation. Due to performed studies based on simulation of welding heating the recommended range of cooling rates of 10–30 ° C/s was established, in which the resistance to SSC of welded joints is ensured. To calculate the cooling rates in coarse grained heat affected zone (CGHAZ) finite-element models of heat distribution were developed for longitudinal multi-electrode submerged arc welding (SAW) and multi-pass girth welding of pipes. Using the developed welding models, it was found that in order to achieve the cooling rate in CGHAZ it is necessary to reduce heat input up to 15-30% during multi-electrode SAW process of longitudinal welds of pipes . For multi-pass girth welding it is necessary to preheat the edges to be welded up to 100-300 °C depending on type of welding (GMAW or SMAW) and pipe wall thickness.

Stress Generation Accompanying Intergranular Corrosion of X70 Steel

Stress corrosion cracking (SCC) is a primary degradation mode in steel oil and gas pipelines (1). Pre-emptive detection and control of SCC of pipeline steel in high-pH soils is difficult owing to a prolonged incubation stage of precursory intergranular corrosion (IGC). In high-pH environments, pipeline steel exhibits greatest susceptibility to SCC within a narrow potential range between the active dissolution peak potential and the passivation potential, which may suggest an important role for oxide formation (2,3). In the present work, IGC is characterized by electrochemical and microscopic measurements together with in situ stress monitoring. The latter method is well-suited to detect stress generated by internal grain boundary oxidation (4). Low-carbon ferritic steel samples (X70 type) were cut from a pipeline specimen. After polishing, the specimens were mounted in electrochemical cells with the front surface exposed to solution (aqueous 1 M NaHCO3at pH 8.0-8.2) and the back surface utilized for in situ stress monitoring by curvature interferometry. Prior to each constant potential corrosion experiment, the potential was held at -0.978 V vs. Ag/AgCl to cathodically reduce the native oxide layer. Corrosion morphology was examined with scanning electron microscopy (SEM) and profilometry. The internal IGC morphology was exposed at various depths by shallow-angle polishing. Fig. 1 shows the current transients at three potentials between the active current peak potential and the passivation potential. At the two highest potentials, the current density increased to maxima after initial transients associated to passive film formation, and thereafter slowly decayed. Stress measurements at the two highest potentials showed steady compressive stress generation. At -0.575 V, compressive stress increased for about 10 min and then diminished. SEM revealed that localized grain boundary attack initiated during the current rise. During the current decays at -0.521 V and -0.478 V, a micron-thick granular corrosion product accumulated on the steel surface (A in Fig. 2). This external corrosion product is likely an iron oxy-carbonate precipitate (5), with the current decay due to its mass transport resistance. Examination of the internal IGC morphology at these potentials revealed interconnected voids along grain boundaries and a primarily oxide corrosion product coating the attacked steel grains (B-D). At the greatest depths within the IGC layer (D), the oxide completely filled the corroded grain boundaries. In contrast, no oxidation on grain boundaries was detected at -0.575 V. Figs. 1 and 2 demonstrate that the potential dependence of compressive stress follows that of grain boundary oxide formation. This correspondence is evidence that the measured stress is due to steel oxidation along grain boundaries. Compressive stress generation would be localized close to the depth of maximum IGC penetration, where oxide completely fills the grain boundary (D in Fig. 2). The constant rate of stress generation associated with grain boundary oxide formation is permits IGC penetration into steel. We suggest that grain boundary oxide advance may be assisted by tensile stress in the grain boundary ahead of the oxide front, which accompanies compressive oxide stress. Such tensile stress could enhance the driving force for oxygen ion migration to the uncorroded grain boundary, so as to promote self-sustaining oxide penetration (6). ACKNOWLEDGMENT Financial support was provided by U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration under Competitive Academic Agreement Program No. DTPH5614HCAP03 and DTPH5614HCAP01. REFERENCES 1. Y. F. Cheng, Stress Corrosion Cracking of Pipelines, Wiley, 2013. 2. R. D. Armstrong and A. C. Coates, Corros. Sci., 16, 423 (1976). 3. R. N. Parkins and S. Zhou, Corros. Sci., 39, 175 (1997). 4. Ö. Ö. Çapraz, P. Shrotriya, K. R. Hebert, J. Electrochem. Soc., 160, D501 (2013). 5. D. H. Davies and G. H. Burstein, Corrosion, 36, 416 (1980). 6. H. Gao, L. Zhang, W. D. Nix, C. V. Thompson, and E. Arzt, Acta Mater., 47, 2865 (1999). Figure 1

An Overview of Near-Neutral pH Stress Corrosion Cracking in Pipelines and Mitigation Strategies for Its Initiation and Growth

Stress corrosion cracking (SCC) of pipeline steels in near-neutral pH environments has remained a significant integrity risk for oil and gas pipelines. Although it has traditionally been termed “stress corrosion cracking,” crack growth has never been observed under a static loading condition. It was determined later that the cracking is driven by corrosion-fatigue mechanisms with some uniqueness. First, the loading frequencies typically vary over a wide range from 10−1 Hz to 10−6 Hz, which is usually beyond the scope of most fatigue investigations. Second, the rate of corrosion is typically well below 0.1 mm/y at which a premature failure solely by corrosion would occur much longer than that actually found in the field. Third, hydrogen, a by-product of corrosion, can be generated to a level at which hydrogen embrittlement may occur only under special conditions. Fourth, pipelines are operated under variable pressure fluctuations that may lead to enhanced crack growth resulting from load-interactions effec...

Hydrogen Evolution and Absorption in an API X100 Line Pipe Steel Exposed to Near-Neutral pH Solutions

Hydrogen evolution kinetics and hydrogen absorption in an API X100 line pipe steel exposed to near-neutral pH solutions is studied in this work. At the corrosion potential, dissolved CO2 in the solution accelerates the hydrogen evolution kinetics through the carbonic acid discharge reaction. At potentials more negative than the corrosion potential, hydrogen evolution due to bicarbonate discharge dominates the other cathodic reactions occurring at the ocp. The hydrogen diffusivity in the API X100 steel is found to be approximately 4.4×10−7cm2/s. A measurable amount of diffusible hydrogen of approximately 1 appm is detected in the steel at the freely corroding condition using an electrochemical hydrogen permeation technique. Higher concentration of diffusible hydrogen is observed at more negative applied potentials as well as at higher CO2 partial pressures. The results in this paper support the attribution of stress corrosion cracking of pipelines in near-neutral pH trapped water environments to hydrogen embrittlement.

Quantitive Assessment of Corrosion Probability—A Bayesian Network Approach

Corrosion processes mainly affect the probability of failure, which then leads to consequences, such as, fire, explosion, or environmental damage. This paper focuses on the use of Bayesian network models for assessing the probability of corrosion. The Bayesian network approach incorporates cause- effect relationships of complex systems in the form of conditional probabilities. This method considers both knowledge uncertainties (i.e., modeling uncertainties) and data uncertainties to make more informed decisions. The Bayes theorem allows the model to predict the probability of events from their causes, and, if a particular event is known to have occurred, predict probable causes of that event. Two case studies, the first one involving internal corrosion and the second involving external corrosion of oil and gas pipeline, are presented, along with validation using field measurements. The extension of the approach to predicting stress corrosion cracking of pipelines is discussed.

Mechanistic aspect of stress corrosion cracking of X80 pipeline steel under non-stable cathodic polarization

In this work, the square wave polarization (SWP) technique and slow strain rate tensile test were combined to investigate the influence of cathodic potential fluctuation, i.e. the non-stable cathodic polarization, on the electrochemical and stress corrosion cracking (SCC) behavior of X80 pipeline steel in an acidic soil solution. It was found that the non-stable cathodic polarization accelerated both the anodic dissolution and the cathodic hydrogen evolution, inducing the increase of SCC susceptibility compared with the potentiostatic cathodic polarization. This provides a novel and important concern and recommendation for controlling pipeline SCC under CP fluctuation.

3-D Simulation of Remote Field Eddy Current Detection for Stress Corrosion Cracks in Pipeline

Three-dimensional (3-D)finite element simulation models are conducted to simulate the in-serviceinspection of the axial stress corrosion cracking (SCC) in ferromagneticpipeline using remote field eddy current (RFEC) probes. In this paper, atypical remote field eddy current probe which is internal to and coaxial withthe pipe is firstly introduced. Then a 3-D finite element simulation model isestablished in ANSYS to analyze the detection of RFEC technology. Finally theRFEC signals are observed when changing the geometric parameters, and thedefinite influences of crack size on the amplitude and phase lag of REFCsignals are evaluated. The results of the investigations are beneficial to thedetection and sizing of SCC in pipelines

The stress corrosion cracking behavior of X120 pipeline steel in a simulated acidic soil solution

PurposeThe purpose of this paper is to clarify the influence of the cathodic polarized potential on the stress corrosion cracking (SCC) susceptibility of X120 steel in a simulated acidic soil solution and the different SCC mechanisms at different cathodic polarized potentials.Design/methodology/approachThe SCC behaviors of X120 pipe steels at various potentials were investigated in a simulated acidic soil solution by slow strain rate tensile tests, electrochemical impedance spectroscopy and surface analysis techniques.FindingsThe fracture surface of X120 steel stretched in air showed a ductile fracture. Both elongation and tensile strength of X120 steel in the simulated acidic soil solution decreased compared to that obtained in air. A slight cathodic polarization increased the elongation and tensile strength of X120 steel; therefore, the SCC susceptibility was lower at −0.65 VSCE than that at OCP, anodic dissolution dominates the corrosion process. However, a strong cathodic polarization induced hydrogen embrittlement, decreasing elongation and tensile strength, therefore, the SCC susceptibility was highest at −1.1 VSCE, and hydrogen embrittlement became the dominant process.Originality/valueThe paper provides an essential insight into the mechanism of pipeline SCC for X120 steel in acidic soil environments.

STRESS CORROSION CRACK INITIATION BEHAVIOR FOR THE X70 PIPELINE STEEL BENEATH A DISBONDED COATING

Stress corrosion cracking (SCC) has become one of the main threats for pipelines safe. The crack initiation is the first step for pipeline SCC. The disbonded coating affects the SCC initiation behavior. In the present paper, the effect of cathodic protection potential on SCC initiation for X70 pipeline steel in the NS4 solution beneath a disbonded coating is studied by a cyclic loading test. The SCC initiation situation was observed at the different distances beneath a disbonded coating area by SEM. The results show that the SCC initiation degree lightens with the increase of distance from the disbonded coating area under the condition of -850 mV cathodic protection potential. When the cathodic protection potential increases to -1000 mV, the SCC initiation degree slightly increases with the increase of distance from the disbonded position. But compared with that under the -850 mV, the crack initiation degree under -1000 mV is lower at the corresponding position. It is shown that the coating disbondment reduces the cathodic protection effect, and in order to obtain the same cathodic protection effect the cathodic protection potential should be appropriately moved in an negative way.

Mechanistic aspect of near-neutral pH stress corrosion cracking of pipelines under cathodic polarization

This paper investigates mechanistically stress corrosion cracking (SCC) of an X70 pipeline steel that is under cathodic protection (CP) in a near-neutral pH solution. It was found that there is a critical potential range, i.e., −730 and −920mVSCE, where the steel is in a non-equilibrium electrochemical state, and anodic dissolution (AD) reaction may occur when the steel is polarized cathodically. When the applied potential is more positive than this range, SCC is AD-based; while the applied potential is more negative, SCC of pipelines is under hydrogen embrittlement (HE) mechanism. When the polarization potential is within the range, SCC of the steel is under the combined effect of AD and HE. Therefore, AD may still occur on pipeline steel that is under CP with the potential within this critical range, contributing to the cracking process.

Quantitative characterization by micro-electrochemical measurements of the synergism of hydrogen, stress and dissolution on near-neutral pH stress corrosion cracking of pipelines

Occurrence of stress corrosion cracking of pipelines under a near-neutral pH condition depends on the synergism of stress, hydrogen and anodic dissolution at the crack tip of the steel. In this work, micro-electrochemical techniques, including localized electrochemical impedance spectroscopy and scanning vibrating electrode technique, were used to characterize quantitatively the synergistic effects of hydrogen and stress on local dissolution at crack-tip of a X70 pipeline steel in a near-neutral pH solution. Results demonstrate that, upon hydrogen-charging, the anodic dissolution of the steel is enhanced. The resistance of the deposited corrosion product layer depends on the charging current density. There is a non-uniform dissolution rate on the cracked steel specimen, with a highest dissolution current density measured at crack-tip. For a smooth steel specimen, the synergistic effect factor of hydrogen and stress is equal to 5.4, and the total effect of hydrogen and stress on anodic dissolution of the steel is 7.7. In the presence of a crack, the hydrogen effect factor, stress effect factor and the synergistic effect factor are approximately 4.3, 1.3 and 4.0, respectively. The total effect factor is up to 22.4, which is very close to the 20 times of difference of crack growth rate in pipelines in the presence and absence of the hydrogen involvement recorded in the field.

Microelectrochemical Characterization of the Roles of Hydrogen, Stress and Their Synergism in Pipeline Stress Corrosion Cracking under a Near-Neutral pH Condition

not Available.

Cathodic Protection Effect on the Assessment of SCC Susceptibility of X52 Pipeline Steel

ABSTRACTAssessment of anodic and cathodic potentials on stress corrosion cracking (SCC) of API X52 pipeline steel through slow strain rate tests (SSRT) was studied. The SSRT were carried out in a NS4 solution to simulated dilute ground water that has been found to be associated with SCC of pipelines. SSRT were performed and evaluated in air and in the NS4 solution at room temperature at an extension rate of 1×10-6 in/sec. Tests were performed at controlled electrochemical polarization potentials, both anodic and cathodic (100, 200, 400 mV) versus the open circuit corrosion potential. The results of reduction in area ratio (RAR), time to failure ratio (TFR) and plastic elongation ratio (PER) of the specimens tested in the soil solution indicate that X52 pipeline steel was susceptible to SCC at cathodic potentials. These specimens showed a brittle type of fracture with transgranular appearance. The SCC proceess and mechanism of X52 steel in the NS4 solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. At positive potentials the SCC is based mainly on the anodic dissolution mechanism. When the applied potentials shifted negatively, the SCC on the steel follows mainly hydrogen embrittlement mechanism. This mechanism was confirmed through the internal cracks observed in the specimens.

Stress corrosion cracking of oil and gas pipelines in near neutral pH environment: review of recent research

AbstractSince the discovery of transgranular stress corrosion cracking (SCC) on a Canadian gas transmission line in 1985, much research has been conducted in the past 20 years. Findings of the effects of operating conditions, metallurgical and the environmental factors have been useful in preventing and mitigating failures. Several overviews of this problem can be found in the literature and the purpose of this update is to review the research results produced since the turn of the century. The recent report of SCC under static stressing conditions confirms that the cracking is indeed a true SCC process, although the rate of which is low without dynamic loading. In contrast to the high pH pipeline stress corrosion cracking in the carbonate–bicarbonate solution, this forms of cracking in dilute near neutral environment takes much longer time to initiate. Once initiated, the crack growth rate is highly sensitive to the loading rate of the applied mechanical force.

Analysis of the development of stress corrosion cracking in pipelines of compressor stations

The problem of corrosion cracking in underground gas pipelines has been a focus of attention of gas-transporting enterprises for more than 15 years, but so far there is neither an unambiguous assessment of the main causes of this phenomenon nor acceptable methods for reducing it (except detection of stress corrosion cracking (SCC) flaws by means of in-tube flaw detection and the subsequent elimination of these flaws). One important feature of the manifestation of the problem of SCC in main gas pipelines is the selectivity of cracking (SCC is identified in a small fraction of pipes), the causes of which are unknown. It is still not clear whether this feature is due to the time delay of SCC or this effect has another origin and is associated with the resistance of the greater part of pipes to stress corrosion damageability. This issue is directly related to forecasting of the further development of stress corrosion damageability both for gas pipelines in which SCC has already been detected and for segments in which SCC problems have not yet arisen. An especially pressing task is assessing the possibility of the development of SCC in process pipelines of compressor stations situated in regions where the presence of the SCC process on linear segments has been identified, because equipment operation and the presence of maintenance staff on the station territory involve increased risk.

Mechanistic investigation of hydrogen-enhanced anodic dissolution of X-70 pipe steel and its implication on near-neutral pH SCC of pipelines

The effects of hydrogen-charging on anodic dissolution of pipe steel under near-neutral pH condition were studied by electrochemical techniques. Hydrogen-charging enhances the anodic dissolution rate of the steel. The hydrogen-enhanced dissolution increases with increasing charging current density. The hydrogen effect is attributed to the alteration of chemical potential and exchange current density of steel. Hydrogen-charging affects the corrosion process of the steel. In particular, at a high charging current density, a layer of corrosion product forms on the electrode surface to change corrosion potential and interfacial double-charge layer capacitance as well as charge-transfer resistance. The hydrogen effect factor for enhanced anodic dissolution of steel at an anodic potential of −0.4 V (SCE) is 1.53 only. Hydrogen-enhanced anodic dissolution of steel by itself may not be the major factor contributing to the high rate of crack growth in pipe steel in near-neutral pH electrolyte. A further investigation of the synergistic effect of hydrogen and stress on dissolution at the crack-tip is essential to determine the mechanism of near-neutral pH stress corrosion cracking of pipelines.

Corrosion behavior of X-70 pipe steel in near-neutral pH solution

The mechanism of near-neutral pH stress corrosion cracking (SCC) of natural gas pipelines has not been well-established since the first accident was found in the 1980s. In particular, the role of hydrogen in near-neutral pH SCC has remained unknown. In this work, cyclic voltammetry was used to comprehensively investigate the fundamentals of the electrochemical corrosion reactions occurring at the steel/solution interface in diluted, 5% CO 2/N 2-purged, near-neutral pH bicarbonate solutions. It is shown that there is no stable oxide film formed on the steel surface in near-neutral pH solution. The dissolution-based cracking mechanism does not apply for near-neutral pH SCC of pipelines. The formation of a metastable Fe(OH) 2 deposit layer shows a catalytic activity on hydrogen evolution reaction, indicating that a significant amount of hydrogen could be generated under near-neutral pH condition. The presence of corrosive anions in the soil electrolyte enhances both the anodic polarization of the steel and the cathodic hydrogen evolution reaction, resulting in an increased hydrogen evolution rate. The introduction of oxygen could form a stable oxide film on the surface of steel, resulting in the loss of the surface catalytic effect on hydrogen evolution reaction. Thus, a hydrogen-based mechanism does not apply for SCC in the presence of oxygen.

Thermodynamically modeling the interactions of hydrogen, stress and anodic dissolution at crack-tip during near-neutral pH SCC in pipelines

A thermodynamic model was developed to clarify the interactions of hydrogen, stress and anodic dissolution at crack-tip during near-neutral pH stress corrosion cracking in pipelines by comprehensively considering the electrochemical reactions occurring in the pipeline steel in deoxygenated, near-neutral pH solution. By analyzing the change of the free-energy of steel due to the presence of hydrogen and stress, it is demonstrated that a synergism of hydrogen and stress promotes the cracking of steel. The enhanced hydrogen concentration in the stressed steel significantly accelerates the crack growth. An exact expression of the hydrogen concentration factor, i.e., the dependence of anodic dissolution rate of steel at crack-tip on the hydrogen concentration, is essential at the quantitative prediction of the crack growth rate.