Near-neutral pH Stress Corrosion Cracking Research Articles

Pipeline Circumferential Cracking in Near-Neutral pH Environment Under the Influence of Residual Stress: Dormancy and Crack Initiation

The purpose of this study is to identify the integrity challenges encountered by buried pipeline steels, specifically to address Circumferential Near-Neutral pH Corrosion Fatigue (C-NNpH-CF). Damage to the pipeline’s protective coating and corrosion conditions increase the risk of service failures caused by C-NNpH-CF. (Note that this mechanism has previously been termed near-neutral pH stress corrosion cracking.) Unlike axial cracking, circumferential cracking is primarily influenced by residual stress from pipeline bending, geohazards, and girth welds. External corrosion pits often lead to dormant cracks, with growth ceasing around 1 mm depth due to reduced dissolution rates. Investigating the impact of bending residual stress (an appropriate source of axial residual stress) and cyclic loading (simulated pipeline pressure fluctuation), the study employs the digital image correlation (DIC) method for stress distribution analysis. Factors like applied loading, initial notch depth, and bending conditions influence crack initiation and recovery from the dormancy stage by affecting stress distribution, stress cells, and stress concentration. Cross-sectional and fractographic images reveal time/stress-dependent mechanisms governing crack initiation, including dissolution rate and hydrogen-enhanced corrosion fatigue. The study emphasizes the role of various residual stress types and their interactions with axial cyclic loading in determining the threshold conditions for crack initiation.

The role of hydrogen embrittlement in the near-neutral pH corrosion fatigue of pipeline steels

This study confirms that "Near-neutral pH Stress Corrosion Cracking" in steel pipelines propagates by hydrogen-enhanced corrosion fatigue. A critical cyclic loading frequency where maximum growth rate per cycle exists. The critical frequency at different temperatures is verified to correspond to hydrogen diffusion predictions. Growth is limited by crack-tip blunting below this frequency and hydrogen diffusion above this frequency. The critical frequency occurs when crack-tip deformation mechanisms that blunt the crack during the stress cycle overwhelm additional enhancement from diffusion. Growth occurs along cleavage planes and ductile slip planes, indicating hydrogen embrittlement, hydrogen-induced plasticity, and fatigue contribute to crack growth.

Overview of Stage 1b Stress Corrosion Crack Initiation and Growth of Pipeline Steels

Stress corrosion cracking (SCC) can cause catastrophic failure of buried pipelines for oil and gas transmission. The life cycle of pipeline steels experiencing SCC consists of five stages: incubation stage, stage 1a (crack initiation), stage 1b (early-stage crack growth), stage 2 (sustainable crack growth caused by mechanical driving force), and stage 3 (rapid crack propagation to rupture). Stage 1b encompasses a large portion of the pipeline’s lifespan, which is of great significance to pipeline integrity management aimed at service life extension. However, this stage is less studied so far. This invited paper provides a brief review of the recent progress on stage 1b stress corrosion crack initiation and growth for buried pipeline steels. Emphasis is placed on the effects of loading conditions and their interactions on stage 1b growth of high pH SCC, while some progress of near-neutral pH SCC is included for the purpose of comparison. It first introduces SCC in pipeline steels which is followed by a definition of stage 1b and its significance in terms of the service life of pipeline steels. Then the most recent advancements in understanding early-stage crack growth in stage 1b are reviewed and discussed. In summary, stage 1b growth can be self-induced by existing cracks (the so-called mother-daughter analogy), involving crack initiation in the plastic zone ahead of the surface tip of a surface crack, which is quite different from the stochastic process of coalescence of randomly formed individual cracks. Stage 1b growth extends crack length and increases the stress intensity factor at the depth tip without a physical increase in crack depth, serving as a bridge to stage 2. Further, the need for future research on stage 1b SCC initiation and growth of pipeline steels is discussed.

Crack Propagation in Pipelines Under Extreme Conditions of Near-Neutral PH SCC

Stress Corrosion Cracking (SCC) process through which cracks occur in a variety of susceptible materials is a result of a combination of residual or applied stresses and corrosion. In oil and gas field, buried pipeline steels are made of low-alloy steels with a ferritic-pearlitic structure, such as X70. In dilute solutions, these materials are prone to SCC failure. The Near-neutral simulated soil solution (NS4) solution is established to imitate SCC conditions and subsequently became the industry requirement for crack growth experiments in the majority of laboratories. The strain-assisted active crack pathways are considered while modelling SCC growth as an oxide film rupture and anodic dissolution process. It’s been hypothesized that increasing the strain concentration can help with dissolution at the film-free crack tip. This research focuses on estimating the SCC crack growth rate under various environmental conditions in oil and gas pipelines using finite element modelling. The simulation is carried out using the J-integral theory in the COMSOL Multiphysics program. Simulations are performed to model the crack growth rate (CGR) using slip anodic dissolution (film rupture) mechanism. The plastic strain gradient is required to compute the SCC CGR (da/dt). Because the plastic strain located at crack tip increases proportionally to the crack length as it propagates, the CGR increases as the stress intensity factor (SIF) increases. The crack growth rates increase when constant loads are applied and as the temperature rises, and elevating the cathodic potential has a minimal influence on the propagation rate of cracks but raises the material yield strength and imparts brittle behavior to it.

A review of crack growth models for near-neutral pH stress corrosion cracking on oil and gas pipelines

This paper presents a review of four existing growth models for near-neutral pH stress corrosion cracking (NNpHSCC) defects on buried oil and gas pipelines: Chen et al.’s model, two models developed at the Southwest Research Institute (SwRI) and Xing et al.’s model. All four models consider corrosion fatigue enhanced by hydrogen embrittlement as the main growth mechanism for NNpHSCC. The predictive accuracy of these growth models is investigated based on 39 crack growth rates obtained from full-scale tests conducted at the CanmetMATERIALS of Natural Resources Canada of pipe specimens that are in contact with NNpH soils and subjected to cyclic internal pressures. The comparison of the observed and predicted crack growth rates indicates that the hydrogen-enhanced decohesion (HEDE) component of Xing et al.’s model leads to on average reasonably accurate predictions with the corresponding mean and coefficient of variation (COV) of the observed-to-predicted ratios being 1.06 and 61.2%, respectively. The predictive accuracy of the other three models are markedly poorer. The analysis results suggest that further research is needed to improve existing growth models or develop new growth models to facilitate the pipeline integrity management practice with respect to NNpHSCC.

Perspective on “Transgranular Stress Corrosion Cracking of High-Pressure Pipelines in Contact with Solutions of Near Neutral pH,” R.N. Parkins, W.K. Blanchard Jr., B.S. Delanty,Corrosion50, 5 (1994): p. 394-408

Abstract:This paper provides a perspective on the stress corrosion cracking (SCC) of buried pipelines based on a paper by R.N. Parkins, et al., and associated works. The paper by Parkins, et al., included an identification of factors controlling near-neutral pH SCC (NNPHSCC) and a probabilistic approach to evaluating multiple cracks. Considerable research since the publication of the paper has shed light on the mechanisms of NNPHSCC and the various contributing factors. However, a probabilistic framework advanced by this paper is still a work in progress. This perspective describes the progress that has been made and the gaps still remaining in this area.

Occurrence of Near-Neutral pH Stress Corrosion Cracking in X80 and X100 Pipe Steels Under Various Cathodic Protection Conditions

In this work, proof ring tests with a duration of 220 d were performed to investigate the effect of cathodic protection (CP) on the occurrence of stress corrosion cracking (SCC) of X80 and X100 pipe steels in near-neutral pH environment. Various CP conditions were used in the study to mimic the possible CP circumstances occurring in the field, i.e., under-protection, standard-protection, and over-protection. Also, the delayed CP conditions, i.e., 110-d without CP followed by 110-d with CP, were applied to simulate the interruptions of CP in the real world. The results showed that a sufficient continuous CP could prevent the occurrence of SCC cracks. Under a given condition X100 steel tended to form deeper cracks than X80 steel. The cracks formed in both X80 and X100 steels exhibited normal/quasi-normal depth distributions. Only a small fraction of cracks would propagate deep into the steel. For X80 steel, with CP no higher than −776 mV, CP exhibited an inhibiting effect on both SCC crack initiation and propagation, and the inhibiting effect increased with CP level. For X100 steel, it was indicated that hydrogen effect played a more significant role in crack initiation compared to that in X80 steel. The application of CP would retard the crack growth in X100 steel, but promote more cracks to form. Under the delayed CP conditions, the growth of cracks developed in X80 steel could be effectively controlled by the delayed CP of −776 mV; for the deeper cracks formed in X100 steel, a CP level higher than −776 mV was needed to achieve an effective control on the growth. For both materials, the delayed CP of −1,126 mV would enhance crack initiation.

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...

Comparative assessment of near‐neutral pH stress corrosion cracking resistance of pipeline steels with different strength

In order to comparatively assess near‐neutral pH stress corrosion cracking (NNpH‐SCC) resistance of pipeline steels with different strength, constant load tests in NS4 solution purged with 5%CO2–N2 gas mixture were conducted on American Petroleum Institute (API) grade X70, X80 and X100 pipeline steels. Experimental results show that pit cracks appeared on the surface of X70 steel and non‐pit cracks appeared on X100 steel. However, for X80 steel both pit cracks and non‐pit cracks were observed, but pit cracks dominated. The near‐neutral pH SCC resistance decreased in the order of X70, X100 and X80 at the same applied stress level.

埋設ラインパイプ鋼のニア・ニュートラルpH SCC (NNpHSCC) ー腐食き裂発生の環境条件ー

埋設ラインパイプ鋼のニア・ニュートラルpH SCC (NNpHSCC) ー腐食き裂発生の環境条件ー

Effects of Soil Type of Shanghai and Electrode Potential on Near-Neutral pH Stress Corrosion Cracking of Pipeline Steel X60

The effects of soil type of Shanghai and electrode potential from cathodic protection point on near-neutral pH stress corrosion cracking (SCC) behavior of pipeline steel X60 has been investigated for structural integrity assessment. The SCC behavior was evaluated through slow strain rate tests (SSRT) at various electrode potentials in three solutions containing different typical soils. Results showed that the SCC susceptibility generally increased with decreasing electrode potential in all the three environments, with quick increase happening in the range from-1500 to-700mV(SCE). It is suggested that hydrogen induced cracking should dominate the SCC at low potentials. In cathodic potential range, the rank of the SCC susceptibility for soil type was Near-city soil > Cyan-purple soil > Cyan-yellow soil. In anodic potential range, SCC susceptibility curves mixed each other in the three solutions.

APPLICATION OF INVERSE BROYDEN'S METHOD FOR COMPUTATIONAL ANALYSIS OF HYDROGEN CONTRIBUTION TO THE NEAR-NEUTRAL PH STRESS CORROSION CRACKING

An Inverse Broyden's Method was applied in the context of a molecular statics approach for the analysis of the contribution of hydrogen to the near-neutral pH stress corrosion cracking. A 3D crystal structure was tested numerically. It is shown that the Inverse Broyden's Method provides a computationally efficient technique to evaluate the effect of hydrogen on the material degradation. Simulation results demonstrated that atoms of hydrogen placed near the crack tip produced a strong effect on deformation and crack propagation in bcc iron leading to a 15–20% loss in a residual strength of numerically tested samples.

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.

Effect of CO 2 and R-ratio on near-neutral pH stress corrosion cracking initiation under a disbonded coating of pipeline steel

This study investigates the role of CO 2 and cyclic stress R-ratio ( R = minimum stress/maximum stress) on near-neutral pH SCC initiation mechanism(s) under a disbonded coating of pipeline steel protected by cathodic protection (CP). It was found that depending on CO 2 concentration and level of CP, different localized environments with various pH could be formed under the disbonded coating. When cyclic loading was applied, different SCC initiation mechanisms were involved depending on the pH of the localized environments. Reducing the R-ratio had different effects on the initiation mechanisms.

Near-Neutral pH Stress Corrosion Cracking Susceptibility of Plastically Prestrained X70 Steel Weldment

The application of strain-based design for pipelines requires comprehensive understanding of the postyield mechanical behavior of materials. In this article, the impact of plastic prestrain on near-neutral pH stress corrosion cracking (SCC) susceptibility of welded X70 steel was investigated with a slow strain rate tensile (SSRT) test. Generally, plastic prestrain reduces the SCC resistance in various welded zones. The SCC susceptibility of the test materials can be put in the following order: heat-affected zone (HAZ) > weld metal (WM) > base metal (BM). Fractographic analysis indicates that there are two cracking modes, mode I and mode II, during SSRT tests. Mode I cracks propagate along the direction perpendicular to the maximum tensile stress, and mode II cracks lie in planes roughly parallel to the plane where the maximum shear exists. The SCC of the BM is governed by mode I cracking and fracture of the HAZ, and the WM is dominated by mode II cracking. Damage analysis shows that the detrimental impact of plastic prestrain on the residual SCC resistance cannot be evaluated with the linear superposition model. A plastic prestrain sensitivity, a material constant independent of plastic prestrain, is proposed to characterize the susceptibility of SCC resistance to plastic prestrain, and it increases with the SCC susceptibility of the steels. The enhanced SCC susceptibility caused by plastic prestrain may be related to an increase in yield strength. The correlation of the ratio of the reduction in area in NS4 solution to that in air (RASCC/RAair) with the yield strength is microstructure dependent.

Effect of Dissolved Oxygen on Stress Corrosion Cracking of X70 Pipeline Steel in Near-Neutral pH Solution

Abstract To clarify the role of dissolved oxygen in near-neutral pH stress corrosion cracking (SCC), slow strain rate test (SSRT), dynamic polarization technique, and electrochemical impedance spec...

Local environment under simulated disbonded coating on steel pipelines in soil solution

Taking advantage of microelectrode technique, the local potential and pH in a crevice simulating disbonded coating on X70 pipeline steel were investigated as a function of cathodic protection (CP) in a near neutral pH soil bulk solution bubbled with 5% CO 2/N 2 gas. The experimental potential–pH (E–pH) diagrams were established for the steel in the crevice. Stress corrosion cracking (SCC) susceptibility of the steel in the local environment in the crevice was analyzed based on the experimental E–pH diagrams. The results showed that the local steel potential in the deep of the crevice was independent on CP potential applied at the opening. Due to the effect of the atmospheric CO 2, a near-neutral pH local environment promoting near-neutral pH SCC (also known as transgranular SCC, TGSCC) might be harbored in the crevice even with normal CP at the opening. During CP interruption, the steel potential decay and CO 2 absorption (pH decrease) might shift E–pH points into a susceptibility region of near-neutral pH SCC.

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.

Research and Cracking Implications from an Assessment of Two Variants of Near-Neutral pH Crack Colonies in Liquid Pipelines

Abstract One of the outstanding problems in understanding near-neutral pH stress corrosion cracking (NNpHSCC) in pipelines is the determination of the factors that cause a very small number of the ...