API X70 Pipeline Steel Research Articles

Improving the Corrosion Resistance of API X56 and API X70 Pipeline Steels by Hot Dip Aluminization

The external surface of the pipeline steels can be protected from corrosion by cathodic protection and a suitable coating system. But to protect the internal surface of steel pipelines from corrosion is always a challenge. In the current study, a protective aluminum coating was applied on the internal surface of steel pipeline grades API X56 and API X70 by hot dip aluminization process to minimize the cost of inhibitors. The steel substrates were dipped in the hot aluminum bath, whose temperature was maintained at 720 °C, for three different dipping times (5, 7.5 and 10 minutes) and then permitted to cool to room temperature. The coated specimens were characterized in terms of their structure and composition by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM micrographs analysis reveal that the hot dip aluminized specimens gave an unwavering and well adherent interface which is essential for corrosion resistance of the substrates. The corrosion behavior was evaluated by weight loss method in case of sweet corrosion (in 1.0 % NaCl + CO2 environment) and, by electrochemical potentiodynamic polarization tests for marine corrosion (in 3.5% NaCl environment). The sweet corrosion rates showed severe material loss (14.17 mpy for API X56 and 6.6 mpy in case of API X70) of uncoated samples as compared to the aluminized samples which showed no corrosion at all even after one week due to the passive layer of Al2O3. Whereas the marine corrosion of aluminized specimens was also reduced to 0.11 mpy (coated specimens) from 0.57 mpy (uncoated specimens).

High cycle fatigue behaviors of API X65 pipeline steel welded joints in air and H2S solution environment

To investigate the mutual effect of hydrogen, microstructures and stress concentration on the fatigue failure, fatigue behaviors of X65 steel welded joints in both air and saturated H2S solution were investigated at high cycle regime. The experimental result demonstrates that due to lower dislocation density observed by electron backscattered diffraction (EBSD), the fine grain heat affected zone (FGHAZ) is prone to induce cyclic strain localization and further lead to fatigue crack propagating along the FGHAZ in air. Furthermore, the quasi-cleavage with brittle-like fatigue striations and secondary crack on the fracture surface in saturated H2S solution is attributed to hydrogen embrittlement. Moreover, compared with base metal (BM) and FGHAZ, the weld metal (WM) and coarse grain heat affected zone (CGHAZ) are composed of bainite and martensite/austenite (M/A) phase, and more sensitive to hydrogen. Therefore, the fatigue crack is prone to grow along the interface between WM and CGHAZ under the normal applied stress.

Effect of Rare Earth and Cooling Process on Microstructure and Mechanical Properties of an Ultra-Cleaned X80 Pipeline Steel

In order to explore the effect of a small amount of rare earth addition in ultra-cleaned pipeline steel and the influence of the cooling process on the tensile and impact properties, three API X80 pipeline steels were fabricated by varying RE addition and the cooling process at the same time. Three microstructures with different features for a low C high Nb microalloyed high-strength pipeline steel and the corresponding mechanical properties were investigated. The results showed that even in the ultra-cleaned steel with O and S contents less than 10 ppm, the addition of RE would still cause an increase in the volume fraction of inclusions consisting of complicated RE oxysulfide and RE sulfide. More inclusions formed in the 112 ppm RE steel were harmful to the low temperature toughness, while few inclusions formed in the 47 ppm RE steel had almost no influence on the low temperature toughness. The two RE additions had no effect on strength of the steels. As the finishing cooling temperature was increased and the cooling rate was decreased within a certain range, the volume fractions of polygonal ferrite and quasi-polygonal ferrite as well as the number density and size of martensite–austenite islands were increased. Under such combined effect, the strength of the steels had almost no change. As the finishing cooling temperature was increased from 481 to 584 °C and the cooling rate was reduced from 20 to 13 °C/s, for the steel with 112 ppm addition of RE, there was an obvious decrease in the low temperature toughness. The reduced value (about 33 J) of the USE of steel consisted of two parts including the influence (about 18 J) of more inclusions formed due to 112 ppm addition of RE and the effect (about 15 J) of the lower high-angle grain boundaries.

Comprehensive study on hydrogen induced cracking of electrical resistance welded API X52 pipeline steel

Different heat treatment cycles were designed in order to investigate the effect of microstructural changes on hydrogen induced cracking resistance (HIC) and mechanical properties of the electric resistance welded steel. The heat treating of the as-welded specimen improved the ductility and impact toughness. After heat treatment, the uniform hardness profile was obtained for the welded specimens. The removal of local hard zones reduced the risk of HIC. The chemical composition and clustering of inclusions have a deleterious effect on cracking resistance in the H2S environment. Aluminosilicate compounds and MnS inclusions were favorite sites for HIC. The most promising post weld heat treatment for improving mechanical properties and the resistance to HIC was the application of two-cycle quenching followed by tempering.

Effect of filler metal on solidification, microstructure and mechanical properties of dissimilar super duplex/pipeline steel GTA weld

In the present experimental work, an attempt has been made to investigate the effect of filler metal on solidification, microstructure, and mechanical properties of the dissimilar weld between super duplex stainless 2507 and high strength low alloy API X70 pipeline steel. This joint is widely used in offshore hydrocarbon drilling risers and oil-gas transportation pipelines. Welds have been fabricated by the gas tungsten arc welding process using super duplex 2594 and austenitic 309L grade filler. A comparative assessment of both the fillers has been done by investigating multiple aspects of weld's structural integrity. 309L fillers weld has skeletal ferrite morphology, whereas 2594 filler solidifies with precipitation of multiple reformed austenites in the ferrite matrix. Lower width unmixed zone is formed on the API X-70 side in 2594 filler as compared to 309L. The tensile and impact strength of 309L filler weld is superior to 2594 filler. Precipitation of reformed austenite in the weld zone leads to lower hardness in 2594 as compared to skeletal ferrite dominated 309L weld zone. High chromium concentration in the weld zone results in superior pitting corrosion resistance of 2594 filler. • Filler 309L solidifies with skeletal ferrite morphology in weld zone. • Filler 2594 weld zone has multiple reformed austenite formation. • Tensile, impact strength and hardness of 309L filler weld is superior to 2594 weld. • 2594 filler weld has better pitting corrosion resistance than 309L filler weld. • Unmixed zone on X70 side is narrower in 2594 filler weld.

The effect of natural inhibitor concentration of Fumaria officinalis and temperature on corrosion protection mechanism in API X80 pipeline steel in 1 M H2SO4 solution

In this investigation, the effect of Fumaria officinalis concentration, as a natural inhibitor, and the medium temperature on the corrosion prevention mechanism of an API X80 pipeline steel alloy in the solution of 1 M H2SO4 is studied. For this purpose, corrosion behavior of the pipeline steel in the solution of 1 M H2SO4 at the presence of 0, 2, 4 and 6 g/L Fumaria officinalis inhibitor at medium temperatures of 20 °C, 40 °C and 60 °C was assessed. To assess the corrosion protection mechanism of the API X80 pipeline steel under these conditions, Open Circuit Potential (OCP), Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy (EIS) tests were conducted. As well, Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) detector was used to analyze the corrosion products formed on the surface of the pipeline specimens. Results of OCP and potentiodynamic polarization tests showed that Fumaria officinalis reduces the speed of both anodic and cathodic reactions. Also, ΔGads0calculation showed that Fumaria officinalis led to the reduction of anodic and cathodic reactions through its physical-chemical adsorption on the surface and blocking of corrosion active locations. The results of EIS tests indicated that with increasing the Fumaria officinalis inhibitor, the corrosion resistance of the API X80 pipeline steel increased. As well, corrosion products analysis showed that Iron sulfide compounds were formed on the surface of the specimens. In terms of the medium temperature, results showed that increasing the temperature of the solution from 20 °C to 60 °C has led to an increased corrosion rate and efficiency reduction of Fumaria officinalis. Moreover, it was defined that the adsorption process of Fumaria officinalis on the surface of the API X80 pipeline steel at the temperature of 40°Cin the 1 M H2SO4 solution followed adsorption isotherm of Langmuir.

Effect of Microstructure on the Mechanical Properties and Fracture Toughness of API X65 Pipeline Steel in the Presence of Hydrogen

This study investigated the influence of microstructure on the mechanical properties and fracture toughness of API X65 pipeline steel in the presence of hydrogen. In this study, electrochemical method was used for hydrogen charging and indentation technique was applied to obtain the fracture toughness. The results showed that in the presence of hydrogen, elongation (EL%), reduction of area (RA), ductile fracture percentage, and fracture toughness of all microstructures decreased. The microstructure of martensite (M) + bainite (B) + ferrite (F), had the highest hydrogen trapping and uptake (Capp) as 8.58 × 10–6 mol cm−3 and the lowest apparent hydrogen diffusivity (Dapp) as 5.68 × 10−10 m2 s−1; thus, the maximum decrements of 33% in fracture toughness, 40% in ductile fracture percentage, 47% in RA, and 35% in EL% were observed. However, the microstructure of ferrite (F) + degenerated perlite (DP) + martensite-austenite micro constituent (M/A), where the lowest value of 5.85 × 10–6 mol cm−3 for Capp and the highest value of 8.5 × 10–10 m2 s−1 for Dapp had the minimum decrements as 2% in fracture toughness, 10% in ductile fracture percentage, 4% in RA, and 7% in El%. According to the obtained results, depending on the type of microstructures, hydrogen-induced work softening or hardening were observed by decreasing or increasing the yield stress respectively.

Hydrogen embrittlement susceptibility of X70 pipeline steel weld under a low partial hydrogen environment

The present study investigates the effects of a low partial pressure of hydrogen in a mixture with methane gas on the mechanical properties of welded API X70 pipeline steel using a series of smooth tensile and notched tensile, testing approaches. The microstructures and mechanical property performances of the weld metal are compared to the results obtained for the base metal. The weld metal was found to be more vulnerable to hydrogen-induced crack initiation than the base metal. A significant loss of ductility occurred in the weld metal, even when tested under 10 MPa of a gas mixture with 1% H2 using the smooth tensile specimen. The notched reduction in the area (RA) decreased markedly under 10 MPa gas mixtures with 1% H2. The microstructural inhomogeneity in the metal matrix was associated with a more severe hydrogen-assisted fracture in the weld metal. The hydrogen-induced crack initiation primarily occurred at the interface between the dendritic pro-eutectoid ferrite and bainitic ferrite and propagated with the dendritic ferrite in which the diffusion of hydrogen was promoted by the stress/strain incompatibility and localized deformations.

Role of texture and inclusions on the failure of an API X70 pipeline steel at different service environments

The demand for pipeline steels has been increased in several recent decades. These steels carry oil and natural gas over long distances. In this research, the microstructure and texture of an API X70 pipeline steel in the middle and top layers have been characterized by using SEM and EBSD analyses. Then, the effect of different sour and acidic environments on the mechanical properties of pipeline steels and hydrogen-induced cracking susceptibility is investigated. Moreover, the effect of texture, mesotexture and inclusions on HIC crack initiation and propagation is studied at different conditions in acidic and sour environments. The results show that two types of detrimental inclusions, including CaS and Al2O3, lead to the failure of pipeline steels in harsh environments. Moreover, it is concluded that an acidic environment does not alter the nature of fracture in the mentioned steel, while electrochemical hydrogen charging causes a brittle fracture. The 〈111〉ǁND oriented grains are not observed around the HIC cracks while the grains mostly tend to orient in 〈100〉ǁND direction around these cracks. The martensite phase accumulated around the HIC crack is recognized as the weakest microstructure that could resist against HIC propagation. Finally, the effect of mesotexture on HIC crack propagation is studied. According to results, Σ11, Σ29a and Σ13b CSL grain boundaries indicate a high resistance to the HIC propagation.

Electrochemical impedance spectroscopy analysis of corrosion product layer formation on pipeline steel

Pipeline steels exhibit intergranular corrosion (IGC) and stress corrosion cracking at active dissolution potentials in carbonate-bicarbonate solutions. The evolution of electrochemical behavior of API X70 pipeline steel during active dissolution in 1 M NaHCO3 was investigated by electrochemical impedance spectroscopy (EIS). Electrochemical modeling of EIS revealed that the metal dissolution rate is limited by slow inward diffusion of CO3−2 ions through a porous precipitated corrosion product layer to the steel surface. The porosity of the carbonate layer decreases over time as its thickness increases, both factors contributing to a strongly suppressed corrosion rate due to impeded CO3−2 diffusion. Decreasing steel corrosion rates with time in carbonate-bicarbonate solutions can be understood on this basis. Growth of the carbonate layer at the steel-carbonate interface intensifies tensile wedging stress in corroded grain boundaries, thereby facilitating intergranular crack initiation.

Application of modified normalization method for J-R curve determination using clamped SENT specimens with varying in-plane and out-of-plane constraints

In the present work, the normalization (NM) method was used to obtain J integral resistance (J-R) curves based on clamped single edge notched tension (SENT) specimens. The SENT fracture toughness tests using API X80 pipeline steel were carried out for a series of crack depth to specimen width ratios (a/W) and thickness to specimen width ratios (B/W), corresponding to varying in-plane and out-of-plane constraint conditions. To validate the normalization method, the unloading compliance (UC) method was adopted to derive the J-R curves for those clamped SENT specimens. In particular, two NM methods (modified NM method and unmodified NM method) are discussed and compared, and the experimental results indicated that the J-R curves calculated by the modified NM method have a better accuracy. Then, to quantify the modified NM method, the J-R curves, power law curve fitting parameters and the tearing modulus obtained from two methods (UC method and modified NM method) are compared. The comparison shows that good agreements are achieved among those methods. Moreover, three-dimensional finite element method (FEM) with GTN constitutive model is also used to determine J-R curves based on clamped SENT specimens. It is demonstrated that the modified NM method is an effective alternative tool for determining J-R curves of X80 steel based on clamped SENT specimens with different a/W and B/W ratios.

Derivation of the mechanistic relationship of pit initiation on pipelines resulting from cathodic protection potential fluctuations

In this work, a square wave polarization (SWP) technique was used to simulate the cathodic protection (CP) potential fluctuations on an API X100 pipeline steel to investigate the effect of potential fluctuations on pit initiation. The derived numerical relationship between the potential of the double-charge layer at the steel/solution interface and the CP potential fluctuation is consistent with the testing results. When the upper potential of the SWP potential is shifted positively (or negatively) while the lower potential is fixed, the potential of the double-charge layer linearly increases (or decreases), causing the increase (or decrease) of the pit density.

Numerical simulation of the erosion of pipe bends considering fluid-induced stress and surface scar evolution

Erosion–corrosion is the main cause of pipeline failures, especially in pipe bends, plugged tees, and pipe fittings with different cross sections. In this study, the erosion of a stressed pipe bend (arising from the internal high-pressure fluid) caused by particles entrained in the conveying fluid was investigated based on a numerical analysis. A numerical model based on the authors’ previous work was combined with a new erosion equation that considers the effect of the tensile load on erosion to analyze the erosion mechanism of a pipe bend under high pressure. The numerical model was verified by the erosion test of API X65 pipeline steel, and a systematic parametric study was also carried out. Moreover, a new numerical simulation method that considers the surface scar evolution of a pipe bend based on the erosion history, was proposed. This work provides an alternative means of understanding erosion behavior and predicting erosion damage of in-service, highly stressed oil and gas pipelines.

A Comparative Study of the Role of Hydrogen on Degradation of the Mechanical Properties of API X60, X60SS, and X70 Pipeline Steels

Three grades of high strength pipeline steel (API X60, X60SS, and X70) are subjected to cathodic hydrogen charging to determine the susceptibility of each steel grade to hydrogen‐induced cracking (HIC). The specimens are subjected to tensile stress until failure to generate stress‐strain, ductility, and toughness data. The fractured specimens are analyzed using a scanning electron microscope equipped with an EDS detector to determine crack initiation sites for the investigated grades of steel. The expectation that the X60SS steel grade will perform better than other grades is not met, instead it is found that under the cathodic hydrogen charging condition, the X60SS is more susceptible than the other two steel grades with respect to loss of toughness, ductility, and ultimate tensile strength. The presence of Ca‐Al‐based inclusions is the most common along the fracture surfaces. Therefore, these inclusions also act as fairly strong trapping sites that under plastically strained conditions, they can result in fisheye features which severely impact the mechanical properties of the steel specimens.

Electrochemical characteristic and stress corrosion behavior of API X70 high-strength pipeline steel under a simulated disbonded coating in an artificial seawater environment

The electrochemical and stress corrosion cracking (SCC) behavior of API X70 pipeline steel under a disbonded coating in artificial seawater was studied using in situ electrochemical measurements and ex situ slow strain rate tensile (SSRT) tests. Results indicated that the electrochemical behavior differs significantly with distance from defect opening. SCC mechanism of X70 steel under the disbonded coating was typically controlled by the synergistic action of anodic dissolution (AD) and the effect of hydrogen, and the susceptibility to SCC at the bottom of the crevice was much higher than that close to the defect opening. Furthermore, the effect of hydrogen on the SCC process is discussed in particular using a quantitative relationship between the cathodic current density and SCC susceptibility.

Effect of electrochemical hydrogen charging on an API X70 pipeline steel with focus on characterization of inclusions

Environmental conditions of service affect the resistance of pipeline steels to failure, as a result, the applied stress which would be otherwise considered safe can be responsible for fracture due to hydrogen induced cracking (HIC). The presence of hydrogen sulfide in natural gas is one of the main sources of hydrogen in steel. HIC testing showed that cracks initiated from inclusions and propagated through the steel matrix along the crack path that was occupied by inclusions. The number of inclusions and the distance between them as well as the grain size between them was considered to be important for predicting failure. Shorter distance between inclusions and smaller grains were found to facilitate HIC propagation. The hydrided specimens were subjected to tensile stress until failure to generate stress-strain curves, ductility and toughness data. It was revealed that even though most of the cracks initiate from rectangular and spinel inclusions, the globular inclusions have also contributed to reducing the tensile strength, ductility and toughness. Synchrotron radiation 3D imaging of steel structure was used to explain the features of the HIC cracks formation during tensile tests.

The Effects of Chloride Droplet Properties on the Underoil Corrosion of API X100 Pipeline Steel

The corrosive environment expected to form in diluted bitumen pipelines was explored by simulated exposure with a paraffin oil-covered chloride droplet on API X100 pipeline steel. The effects of dr...

Effects of Chloride and Carbonate on Corrosion of API-X100 Pipeline Steel

Effects of chloride, introduced with 0.03, 0.05, 0.10, 0.12, 0.22, 0.25 M, on the corrosion behavior of API-X100 pipeline steel in 0.15 M carbonate solutions were studied. The evaluations were carried out with monitoring the open-circuit potentials and electrochemical impedance spectroscopy (EIS). The open-circuit potentials gave an electrochemical evidence on the weakening opposite kinetics of the passive film formation to the kinetics of dissolution with higher chloride concentrations. The open-circuit potentials were less with higher chloride concentrations. The EIS tests indicated that the corrosion resistance decreased, and species diffusion increased with the chloride concentration, in multi-time constant interactions. In all tests, slippery transparent films formed with no presence of pitting on the sample surfaces.

Effect of Microstructure on the Strain Aging Properties of API X70 Pipeline Steels

Effect of Microstructure on the Strain Aging Properties of API X70 Pipeline Steels

Experimental investigation of hybrid laser arc welding of X80 pipeline steel

Hybrid laser/MIG welding was carried out for API X80 pipeline steel using different welding parameters. Influence of welding current on weld appearance, microstructure variation and hardness across the weld cross-section was investigated. Increasing welding current is benificial for larger weld penetration and high welding efficiency. However, increase of welding current can result in coarser microsturctures in CGHAZ near ArcZ. Maximum hardness value occurs in the vicinity of fusion zone of LaserZ, and a slight reduction is observed with larger welding current.