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Purpose The purpose of this study is to elucidate the effects of electric-arc-induced ablation on the corrosion behavior of pipeline steel in neutral and high pH environments. Design/methodology/approach Electrochemical testing, an atmospheric-pressure immersion experiment and various techniques (e.g. scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy) were used to examine the effects of electric-arc-induced ablation on the corrosion behavior of pipeline steel in neutral and high pH environment. Findings Electric-arc-induced ablation occurred preferentially in areas of inclusion. The corrosion resistance of an ablation pit was lower than that of non-ablation areas. In the neutral soil solution, general corrosion was the dominant corrosion that affected pipeline steel; the effect of ablation was small but pitting corrosion could still be induced. In a high pH environment, the samples without ablation were passivated, whereas the samples with ablation pits could not be passivated; the ablation pits were likely to develop pitting corrosion. Originality/value Electric-arc-induced ablation can reduce the corrosion resistance of pipeline steel under high-voltage direct current interference.

The characteristics of calcareous deposits under strong cathodic current and their effect on hydrogen uptake of X80 steel in simulated soil environments were studied by scanning electron microscopy (SEM), hydrogen permeation technique, and hydrogen content tests. When the impressed cathodic current density was in the range of 10–75 mA/cm2, granular or squamous CaCO3 deposits were generated on the surface, forming discontinuous surface layer with pores and gaps of various sizes. When the current density was increased to 100 mA/cm2 and 125 mA/cm2, a dense Mg (OH)2 film was formed on the surface. With the increase of current density, the sub-surface hydrogen concentration and average hydrogen content first increased and then decreased, reaching the turning point at 75 mA/cm2. Combined with the structure and component characteristics of calcareous deposits, the mechanism of their effect on hydrogen uptake was analyzed.

In this paper, the statistical analysis of fluctuation characteristics of pipeline potential under dynamic DC stray current interference was carried out based on field test data. Then a self-design...

In view of the important influence of mechanical damage on the bearing capacity of buried pipelines, the residual strength of dented pipelines has become a hot issue in the pipeline industry. In this paper, X52 steel grade, commonly used in buried pipelines, is taken as the research object. The material properties are obtained by tensile test. The difference of curvature radius at the tip of the dent is accurately considered. Based on finite element method, the limit depth and the failure pressure of the dented pipelines are studied. Results have shown that the ultimate displacement load increases exponentially with the increase in the radius of curvature. When dent depth is small, the dent will not reduce the failure pressure, no matter it is smooth or sharp. As the dent depth increases, the influence of the radius of curvature at the tip of the dent becomes significant. The critical radius of curvature from sharp dent to smooth dent is analyzed. And the corresponding relationship between it and dent depth is obtained. It is found that when the radius of curvature exceeds 50 mm, the dent depth does not affect the failure pressure of the pipeline. An expression about the two parameters is put forward, providing supplementary suggestions for the depth-based evaluation criterion. This paper affords a strong support for better assessment of dented pipelines.

The corrosion behaviour of 316L and Alloy 625 was investigated using cyclic polarization, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy and induced coupled plasma-optical emission spectrometer. The results indicated that Alloy 625 showed better corrosion resistance than 316L and the prolonging immersion time could enhance corrosion resistance of the two alloys. The passive film formed on the surface of 316L exhibited an electronic structure of p–p heterojunction, with Fe3O4 and Cr2O3 enriched in the outer and inner layers, respectively. However, Alloy 625 presented the electronic structure of n–p heterojunction dominated by the outer Fe2O3/NiFe2O4 and inner Cr2O3. This resulted in the opposite semiconductive properties of the passive films formed on the two materials. In the acid solutions, Fe and Mo suffered from selective dissolution while Cr and Ni were relatively stable. The corrosion rates were mainly dominated by the dissolution of iron. Alloy 625 presented better corrosion resistance than 316L due to the obviously lower content of Fe and the higher content of Cr and Ni in the passive film. The continuously selective dissolution of iron resulted in the increase in Cr/Fe ratio in the passive film, which was responsible for the enhancement in corrosion resistance.

The cathodic reaction mechanisms in CO2 corrosion of low-Cr steels were investigated by potentiodynamic polarization and galvanostatic measurements. Distinct but different dominant cathodic reactions were observed at different pH levels. At the higher pH level (pH > ~5), H2CO3 reduction was the dominant cathodic reaction. The reaction was under activation control. At the lower pH level (pH < ~3.5), H+ reduction became the dominant one and the reaction was under diffusion control. In the intermediate area, there was a transition region leading from one cathodic reaction to another. The measured electrochemical impedance spectrum corresponded to the proposed cathodic reaction mechanisms.

Herein, the effect of high chloride ion (Cl−) concentration on the corrosion behavior and passive film breakdown of 13Cr martensitic stainless steel under CO2 environment was demonstrated. The Cl− concentration was varied from 30 to 150 g/L and cyclic potentiodynamic polarization was conducted to investigate the influence of the Cl− concentration on the corrosion potential (Ecorr), passive breakdown potential (Epit), and repassivation potential (Erep). The results of the polarization curves revealed that 13Cr stainless steel is susceptible to pitting under high Cl− concentration. The passive breakdown potential and repassivation potential decreased with the increase of Cl− concentration. The semiconducting behavior of the passive film was investigated by Mott-Schottky analysis and the point defect model (PDM). It was observed that the iron cation vacancies and oxygen vacancies were continuously generated by autocatalytic reactions and the higher Cl− concentration resulted in higher vacancies in the passive film. Once the excess vacancies condensed at the metal/film interface, the passive film became locally detached from the metal, which led to the breakdown of the passive film.

Pipeline steel has considerable risk of corrosion in the high voltage direct current interference cases. Thus, under high potential/current density conditions, the anodic polarization behaviour of X80 steel in Na2SO4 solution and the influence of Cl− ions were investigated using reversed potentiodynamic polarization, the current interrupt method, galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. In the Na2SO4 solution free of Cl− ions, steel was passivated above 0.120 A cm−2 and the potential increased from −0.32 V to 1.43 V. The passive film was composed of Fe3O4, γ-Fe2O3, and FeOOH. The addition of Cl− ions observably influenced the passivation by attacking the passivate film. Low concentration of Cl− ions ( 5 mg L−1 NaCl) completely prevented passivation, showing strong corrosiveness. Thus, the X80 steel was corroded even under high-current-density conditions. The corrosion products were mainly composed of Fe3O4, α-Fe2O3, and FeOOH.

Herein, a schematic model was established to assess the combined effect of deposited CaCO3 and thermophilic sulfate-reducing bacteria (SRB), at high CO2 and low O2 dissolution levels on the corrosion behavior and growth of corrosion scale on X65 pipeline carbon steel. Field emission scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, confocal laser scanning microscopy, open circuit potential, and electrochemical impedance spectroscopy were used to characterize the corrosion pattern of water injection pipe. The results showed that a synergistic effect exists between CaCO3 and thermophilic SRB, both uniform and localized corrosion of CaCO3-coated X65 steel were promoted when inoculated with SRB. The corrosion process was comprised of three stages depending on the metabolism of SRB.

The corrosion behavior of N80 carbon steel in a simulated formation water system saturated with CO2 under high pressure at 100 °C was investigated. The effect of the CO2 partial pressure on the electrochemical behavior and surface morphologies of the N80 carbon steel was analyzed by in situ electrochemical methods and surface analysis, combined with a series of thermodynamic calculations of the potential of anodic/cathodic reactions. While an increase in the CO2 partial pressure did not alter the corrosion mechanism of the N80 steel, it resulted in higher concentrations of H+ and HCO3− ions, thereby significantly enhancing the rate of the cathodic reactions. The precipitation rate of FeCO3 increased with the CO2 partial pressure, with small and fine grains nucleating and growing on the steel surface with poor protectiveness.

The effect of pH on the structure and protectiveness of passive films formed in H2S-containing environment was investigated using potentiodynamic polarization, X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). The corrosion resistance of 316L stainless steel decreased with pH under H2S-containing condition. The passive films formed at higher pH contained lower content of Cr and higher content of S. This was responsible for the passive film degradation. Ni was enriched at the film/metal interface, while Cr was enriched in the passive film. The content of NiO in the passive film increased with pH, which resulted in the weakened Ni enrichment at the film/metal interface at higher pH. The results also indicated that H2S and pH had a combined effect on the passive film structure. The oxide films presented a three-layer structure, where the outermost layer was mainly composed of hydroxides, and the intermediate and inner layers were dominated by sulfides and oxides. The presence of H2S could inhibit the formation of iron oxides. Therefore, iron oxides and iron sulfides were enriched in the inner and intermediate layer, respectively.

The effect of pH on the electrochemical behaviour and passive film composition of 316L stainless steel in alkaline solutions was studied using electrochemical measurements and a surface analysis method. The critical pH of 12.5 was found for the conversion from pitting corrosion to the oxygen evolution reaction (OER). OER was kinetically faster than pitting corrosion when both reactions could occur, and OER could postpone pitting corrosion. This resulted in pitting being initiated during the reversing scan in the cyclic polarization at the critical pH. According to the X-ray photoelectron spectroscopy analysis, the content of Cr and Mo decreased with pH, while Fe content increased. This induced the degradation of the passive film, which resulted in the higher passive current densities under more alkaline conditions. The selective dissolution of Mo at high pH was found, which demonstrated that the addition of Mo in austenitic stainless steels might not be beneficial to the corrosion resistance of 316L in strong alkaline solutions.

The effect of Ca2+ on the corrosion behavior of 3Cr steel was investigated using immersion tests and electrochemical measurements. The results demonstrated that with an increase in Ca2+ concentration in the solution, the corrosion rate of 3Cr steel decreased and the Ca2+ content in the corrosion scale increased. Ca is involved in the formation of both the inner and outer corrosion scales of 3Cr steel. Polarization curves revealed that Ca2+ could enhance the pseudopassivation of 3Cr steel. EIS results demonstrated that Ca2+ can decrease the porosity of corrosion films. Additionally, Ca2+ could accelerate the precipitation of FeCO3 (CaxFe1−xCO3) by reducing its solubility and saturation degree.

The effect of calcareous deposits in simulated soil environment on the hydrogen permeation of X80 steel was investigated using electrochemical hydrogen permeation technique in association with hydrogen content tests and cross-section analysis by scanning electron microscopy (SEM). A raising trend of hydrogen permeability with the formation of calcium hydroxide deposits on X80 steel surface was obtained. This can be attributed to many factors, such as the cathodic current density significantly contributed to increase in hydrogen permeation, while the excessive cathodic current density was likely to break the integrity of the calcareous layer. Furthermore, the hydrogen content and thickness of calcareous deposits increased with respect to the charging current density and hydrogen charging duration, which were in accordance with the change trend of hydrogen permeation in presence of calcium hydroxide deposits.

The susceptibility of super 13Cr steel to stress corrosion cracking (SCC) was assessed through slow strain rate testing in simulated formation water saturated with CO2 under a high-temperature and high-pressure (HTHP) environment. The evolution, morphology, and chemistry of fracture and corrosion products on the steel surface were evaluated using in situ electrochemical methods and surface analysis. Results indicate that the occurrence of pitting corrosion increases SCC susceptibility. At 150 °C, the degradation of a surface film induces pitting corrosion because of an increase in anodic processes. The presence of Cl− causes film porosity, and CO2 reduces the Cr(OH)3/FeCO3 ratio in the inner film, which further promotes Cl−-induced porosity.

During the application of the API X80 grade pipeline steel in long distance gas transportation, the risk of hydrogen induced degradation for this kind of high strength material needs to be clarified from a safety point of view. Commercial X80 steel samples were hydrogen charged electrochemically at different current densities and immersion time. Hydrogen damage in the form of blisters and cracks was observed by optical microscope and scanning electron microscopy. Average hydrogen content in the sample measured by thermal desorption method and the sub-surface concentration of diffusible hydrogen detected by hydrogen permeation cell were both employed to indicate the correlation between hydrogen and surface damage. The influence of hydrogen on the mechanical properties of X80 steel was also characterized by slow strain rate tests. The results showed that the hydrogen damage can be detected on the steel surface if the hydrogen content could reach a certain threshold value during the process of rapid hydrogen uptake. It is suggested that there exists a critical hydrogen level before blisters initiate on X80 steel and that the level is 2–3 ppm, under the test conditions used in this work.

The protectiveness and characterization of passive films formed at various potentials in H2S-containing environments were studied using electrochemical measurements and surface analysis method. The corrosion resistance of 316L in H2S-containing environment decreases with the applied potential. The Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) results indicate that Ni participates in the film formation, which results in the corresponding enrichment in the passive film. The oxidization degree analysis indicates that metallic elements are present in the passive film. Sulfide ions are significantly favored in the passive film at higher potentials, which is responsible for the breakdown of passive film.

The effect of H2S on the corrosion behavior of 316L stainless steel was investigated using electrochemical methods by changing the gas condition from CO2 to H2S and then back to CO2. The presence of H2S showed an acceleration effect on the corrosion of 316L stainless steel in comparison with CO2. The acceleration effect remained even after the complete removal of H2S by CO2, indicating that the passive film was irreversibly damaged. X-ray photoelectron spectroscopy (XPS) analysis indicated that the passive film was composed of Cr2O3, Fe2O3, and FeS2 after being immersed in H2S-containing solutions. The semiconducting property of the passive film was then investigated by using the Mott–Schottky approach. The presence of sulfides resulted in higher acceptor and donor densities and thus was responsible for the deterioration of passive films.

To establish an adequate thermodynamic model for the mechanism of formation of hydrogen sulfide (H2S) corrosion products, theoretical and experimental studies were combined in this work. The corrosion products of API X60 pipeline steel formed under different H2S corrosion conditions were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. A thermodynamic model was developed to clarify the environmental boundaries for the formation and transformation of different products. Presumably, a dividing line with a negative slope existed between mackinawite and pyrrhotite. Using experimental data presented in this study combined with previously published results, we validated the model to predict the formation of mackinawite and pyrrhotite on the basis of the laws of thermodynamics. The established relationship is expected to support the investigation of the H2S corrosion mechanism in the oil and gas industry.

The effect of the cathodic current density (i) on the permeation of hydrogen through API X80 pipeline steels under cathodic protection was investigated using an electrochemical permeation technique. A proper equation was presented for determining the sub-surface concentration of hydrogen (c0). With the aim to enhance the predictive capabilities of the proposed equation, actual environmental parameters such as temperature were considered. The experimental results reported herein clarified the relationship between c0 and i. It was clearly shown that the ratio of c0 to the specimen thickness (L) was proportional to √ i when i was lower than its critical value (iH). On the other hand, for i values above iH, c0 was equal to the saturation hydrogen content because of reaction equilibrium. Furthermore, iH was found to be dependent on temperature (i.e., iH increased with the test temperature).

The corrosion inhibition effect and adsorption behaviour of 1-phenyl-3-(phenylamino)propan-1-one (PPAPO) on N80 steel in hydrochloric acid solution have been investigated by Fourier transform infrared (FTIR), electrochemical method and scanning electron microscopy. The corrosion inhibition mechanism of PPAPO mixed with Na 2 WO 4 was interpreted from the thermodynamic point of view. The results indicated that PPAPO mixed with Na 2 WO 4 acted as a mixed-type inhibitor. The inhibition film formed on N80 steel surface can increase the charge transfer resistance and prevent the occurrence of corrosion reaction, thereby reducing the corrosion rate of metal surface. The inhibition efficiency was up to 96.65%; the inhibitor PPAPO with Na 2 WO 4 showed good synergistic effect on N80 corrosion behaviour in HCl solution. The adsorption behaviour of inhibitors on N80 steel surface was in accordance with the Langmuir adsorption model and mainly belonged to chemisorption. The adsorption process of PPAPO on N80 surface was spontaneous and irreversible endothermic reaction.

Pipeline steel has considerable risk of corrosion in the high voltage direct current interference cases. Thus, under high potential/current density conditions, the anodic polarization behaviour of X80 steel in Na

X80 steel has great risk of corrosion in high voltage direct current (HVDC) interference cases. In this study, the anodic polarization behavior of X80 steel under high potential and current density in Na2SO4 solution was investigated. The I &times, R drop was eliminated using current interrupt technique during the potentiodynamic measurement. Therefore, the real polarization curve was obtained. The corrosion behavior was investigated by galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results show a new form of passivation route. The steel dissolved actively below &minus, 0.388 VSCE, then became partly passivated from &minus, 0.388 to 1.448 VSCE, and fully passivated above 1.448 VSCE. The passive film was formed containing Fe2O3 and FeOOH, and resistant to SO42&minus, ions. It not only blocked the direct dissolution of steel, but also facilitated oxygen evolution. The corrosion rates of steel samples decreased after the passivation.

The corrosion behavior of N80 carbon steel in a simulated formation water system saturated with CO

The general and localised corrosion of super 13Cr SS under various CO2 partial pressures (pCO2) in the static conditions and rotated speeds of 188 rpm at 200 °C were evaluated. The results show that super 13Cr SS exhibits lower localised corrosion risk at 188 rpm compared to the static conditions. The small velocity retards the localised corrosion through the formation of uniform corrosion product scales, which mainly comprises nanocrystalline FeCr2O4 at 2.7 bar pCO2. A double-layered film consists of an inner layer of Cr(OH)3/undissolved austenite and an outer crystalline FeCO3 layer was observed at 28.5 bar pCO2.

Effect of small content of chromium (Cr) on wet–dry acid corrosion behavior of low alloy steels has been investigated. The results show that the corrosion resistance of the steels increased with increasing Cr content from 0.10 to 0.50 wt%. Higher content of Cr promotes initial corrosion and accelerates the formation of dense and protective rust in long-term corrosion. The enhanced protectiveness of the rust is closely related to its composition. High content of Cr increases the content of amorphous phases and decreases the content of γ-FeOOH in the rust, resulting in the high compactness of the rust and low electrochemical activity in acid condition. Cr dopes in rust and depresses the transformation from amorphous phases to α-FeOOH, as well as the growth process of FeOOH particles, which is responsible for the enhanced compactness of rust in long-term corrosion.

The corrosion behavior of a semi-automatic gas tungsten arc welded joint in 3Cr steel was investigated in a CO2-containing environment. Assessment of the galvanic corrosion interactions of the different regions in the joint was carried out through microelectrochemical measurements and immersion tests. The weld zone (WZ) was determined to be the most cathodic region while the base metal (BM) was the most anodic region. The heat affected zone (HAZ) underwent little galvanic interaction, as its free potential was close to the coupled potential. The galvanic coupling effects theoretically derived from the corrosion current densities before and after coupling were in good agreement with the results of the immersion tests.

Corrosion rate of 6.5%Cr steel was considerably lower than that of carbon steel and 3%Cr steel. Current density underwent a drop in certain potential range when potentiodynamic polarization curve was tested. Microscope with camera was used for in situ recording of the change in steel surface. The flaking off of the film, which occurred at a certain potential range, was recorded. The results of electrochemical measurements indicated that the film, which mainly consisted of Cr(OH)3, was a pseudopassive film. When 6.5%Cr steel was polarized or corroded, pseudopassive film formed. Pseudopassivation of 6.5%Cr steel was closely related to low corrosion rate.

The evolution of the corrosion scale formed on 3Cr steel was investigated by scanning electron microscopy (SEM) and X-Ray Diffraction (XRD). The inner layer was characterized by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Thermogravimetric analysis-differential scanning calorimetry (TG-DSC). The results show the inner layer is an amorphous-like structure containing some nano-quasicrystalline and nanocrystalline grains which is more uniform and less porous than the crystalline FeCO3 layer. At the 2nd stage, the transition of cathodic reactions was found, and the growth of the FeCO3 phase in the inner layer dramatically decreased the film porosity, resulting in the passivation.

The corrosion behavior of corrosion resistant steel (CRS) in a simulated wet–dry acid humid environment was investigated and compared with carbon steel (CS) using corrosion loss, polarization curves, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), N2 adsorption, and X-ray photoelectron spectroscopy (XPS). The results show that the corrosion kinetics of both steels were closely related to the composition and compactness of the rust, and the electrochemical properties of rusted steel. Small amounts of Cu, Cr, and Ni in CRS increased the amount of amorphous phases and decreased the content of γ-FeOOH in the rust, resulting in higher compactness and electrochemical stability of the CRS rust. The elements Cu, Cr, and Ni were uniformly distributed in the CRS rust and formed CuFeO2, Cu2O, CrOOH, NiFe2O4, and Ni2O3, which enhanced the corrosion resistance of CRS in the wet–dry acid humid environment.

Effects of W and Mo additions on wet-dry acid corrosion behavior of low-alloy steels under different O2 concentrations were investigated. The results showed that W and Mo-containing steels presented excellent wet-dry corrosion resistance under O2 concentrations of both 1 and 5%. The roles of W and Mo played in increasing corrosion resistance of the steels were different. W could improve the thermodynamic stability of the steel substrate, while Mo could promote the formation of protective rust. Interaction between W, Mo and O2 concentration greatly influenced wet-dry acid corrosion of low-alloy steels. W addition was more effective for depressing charge transfer process at the rust/substrate interface and presented higher enhanced effect on increasing corrosion resistance than Mo under O2 concentration of 1%. Mo was more effective on increasing rust compactness and showed higher enhanced effect on increasing corrosion resistance than W under O2 concentration of 5%.

Performance of magnesium sacrificial anode under alternating current was investigated in 4 g/L Na2SO4 simulated backfill solution by weight-loss measurements for 96 h, electrochemical measurements and surface examination. Alternating current increased the dissolution of magnesium, and at the same time, its DC potential shifted positively. The output cathodic protection current decreased at alternating current density of 50 A/m2. Polarity reversal occurred to magnesium sacrificial anode as soon as alternating current interference of 100 A/m2 (or larger) was applied. Besides the “instant effect”, the polarity reversal showed a “time effect”. It disappeared gradually at alternating current density of 100 A/m2. When alternating current density was more than 200 A/m2, the polarity reversal remained throughout the experiment (96 h). Based on the results of electrochemical measurements and surface examination, mechanism of polarity reversal was proposed. Keywords: Magnesium, Sacrificial anode cathodic protection, Alternating current, Polarity reversal, Instant effect, Time effect

In this work, the crystal structures and micromorphology of H 2 S corrosion products were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The corrosion behavior of X60 steel under different conditions was analyzed. The results show that mackinawite (FeS) is a tetragonal crystal and pyrrhotite (Fe 1− x S) is a hexagonal crystal. Mackinawite mainly forms at low temperatures and low H 2 S partial pressures while the formation of pyrrhotite takes place at the higher temperatures and higher H 2 S partial pressures because of the difference between their thermodynamic properties. Due to the protection of pyrrhotite, corrosion rate of X60 steel was reduced.

The effects of alternating current on the galvanic coupling zinc/pipeline steel were investigated in 4 g/L Na2SO4 simulated backfill solution by weight-loss measurements for 96 hr, electrochemical measurements, and surface examinations. Alternating current accelerates the dissolution of zinc significantly but reduces the corrosion rate of pipeline steel. Under an applied alternating current, zinc DC potential shifts negatively while the DC potential shift direction of pipeline steel is closely related to the value of alternating current density. When the alternating current density is low the DC potential shifts negatively. It, however, shifts positively under higher alternating current density. Besides, the coupling current increases with alternating current density.

The formation and evolution of corrosion product scales on 13Cr stainless steel at 200 °C and various CO2 partial pressures were investigated. Scanning electron microscope was used to show the physical nature of the scales formed at 0.27 MPa CO2. The thin non-uniform layer was identified as FeCr2O4 by Raman Spectroscopy with a thick Cr(OH)3 layer being determined at 2.85 MPa CO2. These results provide a guide to understanding the mechanisms of CO2 corrosion for 13Cr stainless steel exposed to high temperature and pressure and demonstrate the influence of thermodynamics and kinetics on the formation and evolution of the corrosion products.

The extrusion and scraping action of rocks at the bottom of buried pipelines can lead to dent and scratch defects. Beside internal pressure, the pipelines should also bear other forms of loads during its operation, such as bending moment and axial force. The combination of internal pressure and these loads can make the pipe easy to damage. In order to study the influence of bending and axial loads on the limit internal pressure of dent-scratch pipelines, large numbers of finite element calculations are carried out in this paper. Through the comparison with experimental results, a failure criterion is proposed for finite element analysis (FEA). It is found in actual buried pipelines during pipe inspection that the existed maximum bending strain is equal to 0.4%. Take the bending moment which leads to the axial strain of 0.4% as the reference moment. A series of finite element calculations are conducted to analyze the influence of various bending loads on the limit internal pressure of pipelines with dent-scratch defects. In addition, the effect of axial compression loads on the limit internal pressure is also studied by FEA. According to the finite element results, it is concluded that both axial load and bending moment can decrease the limit internal pressure compared to one subjected to internal pressure only. Moreover, the tendency of the impact is approximately linear. Finally, an expression of limit internal pressure with respect to pipe material, pipe and defect dimensions and the magnitude of axial force is fitted, which provides a strong guidance for the safety assessment of the defective pipelines.

In this work, corrosion behavior of carbon steel under the application of alternating current (AC) was investigated in electrolyte solutions with five different pH levels, from near-neutral to alkaline condition, through weight loss measurements, potential monitoring techniques, potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) tests, and surface examination. It is found that the corrosion rate of carbon steel decreases with increasing pH value toward the alkaline end while increases with increasing AC current density. When the solution pH becomes highly alkaline, say, 12, passivation occurs on carbon steel surfaces, with an enlarging passive region as pH increases. However, it is worth noting that the passive film suffers from mechanical breakdown when the applied AC current density reaches a critical level. Based on these results, an AC corrosion mechanism of carbon steel in near-neutral and alkaline environment is proposed.

The influence of Cr on the initial corrosion behavior of low-alloy steels exposed to a CO2–O2–H2S–SO2 wet–dry corrosion environment was investigated using weight-loss measurements, scanning electron microscopy, N2 adsorption tests, X-ray diffraction analysis, and electrochemical impedance spectroscopy. The results show that the corrosion rate increases with increasing Cr content in samples subjected to corrosion for 21 d. However, the rust grain size decreases, its specific surface area increases, and it becomes more compact and denser with increasing Cr content, which indicates the enhanced protectivity of the rust. The results of charge transfer resistance (R ct) calculations indicate that higher Cr contents can accelerate the corrosion during the first 7 d and promote the formation of the enhanced protective inner rust after 14 d; the formed protective inner rust is responsible for the greater corrosion resistance during long-term exposure.

The welded joints of 3Cr pipeline steel were fabricated with commercial welding wire using the gas tungsten arc welding (GTAW) technique. Potentiodynamic polarization curves, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS) were used to investigate the corrosion resistance and the growth of a corrosion film on the weld zone (WZ). The changes in electrochemical characteristics of the film were obtained through fitting of the EIS data. The results showed that the average corrosion rate of the WZ in CO2 environments first increased, then fluctuated, and finally decreased gradually. The formation of the film on the WZ was divided into three stages: dynamic adsorption, incomplete-coverage layer formation, and integral layer formation.

The formation and the growth of the corrosion film that forms on 3Cr steel surface in CO2 environments were characterized and investigated by scanning electron microscopy, energy dispersive spectrometry, quadrant back scattering detector, x-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy (EIS). A model for the formation of the corrosion film is proposed in this paper. The data obtained using the proposed equivalent circuit, which is based on this model, exhibits close agreement with the experimental EIS results, indicating that the proposed film formation model is reliable.