Comparative assessment of electrochemical hydrogen absorption by pipeline steels with different strength

The assessment of ability to absorb hydrogen of three API grade pipeline steels: X52, X70 and X100 have been evaluated. The factors of cathodic hydrogen charging, time of exposure, and applied stress were taken into account. It has been shown that all steels demonstrate the sensitivity to hydrogenating in deoxygenated, near-neutral pH NS4 solution under relatively “soft” cathodic polarisation, although the efficiency of hydrogen permeation in metal is quite low and depends on time of exposure. Applied tensile stress, which equivalent to gross hoop stress in pipe wall under operating conditions, can accelerate the hydrogen absorption in several times. For studied steels the resistance to hydrogen absorption decreases with decreasing of steel strength.     

Corrosion and Stress Corrosion Cracking Behavior of X70 Pipeline Steel in a CO2-Containing Solution

The electrochemical corrosion and stress corrosion cracking (SCC) behaviors of X70 pipeline steel in CO₂-containing solution were studied by electrochemical measurements, slow strain rate tensile tests, and surface characterization. The results found that the electrochemical corrosion of X70 steel in aerated, alkaline solution is an activation-controlled process, and a stable passivity cannot develop on steel. Corrosion rate of the steel increases with the CO₂ partial pressure. The enhanced anodic dissolution due to the additional cathodic reaction in the presence of CO₂, rather than the film-formation reaction, dominates the corrosion process. The mass-transfer step through FeCO₃ deposit is the rate-controlling step in corrosion of the steel. The susceptibility of steel to SCC and the fracture brittleness increase with the CO₂ partial pressure. The enhanced fracture brittleness is attributed to the evolution and penetration of hydrogen atoms into the steel, contributing to crack propagation. The formed deposit layer is not effective in reducing hydrogen permeation due to the loose, porous structure.     

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.     

Electrochemical corrosion behavior of X80 pipeline steel in a near‐neutral pH solution

In this work, the electrochemical corrosion behavior of X80 pipeline steel was investigated in a near‐neutral pH solution using electrochemical impedance spectroscopy (EIC) and photo‐electrochemical (PEC) measurements as well as X‐ray photo‐electron spectroscopy (XPS) technique. The effects of hydrogen‐charging and stress were considered. The results show that the steel is in an active dissolution state, and a layer of corrosion product is formed and deposited on the electrode surface, which is subjected to further oxidation to form ferric oxide and hydroxide. Photo‐illumination enhances anodic dissolution of the steel when it is under anodic polarization due to destroying of the corrosion product film. When the steel is under cathodic polarization, the cathodic current density decreases upon laser illumination due to the photo‐oxidation of hydrogen atoms generated during cathodic reactions, which behaves as an anodic reaction to offset the cathodic current density. Hydrogen‐charging and stress decrease the corrosion resistance of the steel and enhance the dissolution rate of the steel.     

X80管线钢在近中性pH溶液中腐蚀行为研究

目的研究X80管线钢在近中性p H溶液中的腐蚀与应力腐蚀裂纹萌生行为。方法采用电化学实验和浸泡实验研究X80管线钢在近中性p H溶液中的腐蚀行为,采用慢应变速率拉伸实验研究X80管线钢在近中性p H溶液中,在自腐蚀电位和外加电位下的应力腐蚀裂纹萌生行为。结果 X80管线钢在近中性p H溶液中的极化曲线只有活化区,没有钝化区,其自腐蚀电位约为-750 m V,浸泡195天后,试样表面没有氧化膜出现,但是观察到点蚀坑。在自腐蚀电位下,X80管线钢试验表面有大量的应力腐蚀裂纹;在-500 m V阳极外加电位下,X80管线钢试验表面几乎没有观察到应力腐蚀裂纹;在-850 m V阴极外加电位下,X80管线钢试验表面的应力腐蚀裂纹很少,但是随着外加阴极电位负移到-1300 m V时,X80管线钢试验表面的应力腐蚀裂纹增多。结论 X80管线钢在近中性p H溶液中发生均匀腐蚀,但是夹杂物剥落能在X80管线钢表面形成点蚀坑。在近中性p H溶液中,在自腐蚀电位下,X80管线钢应力腐蚀裂纹萌生敏感性最强;外加阴极电位抑制应力腐蚀裂纹萌生,但是随着外加阴极电位的负移,应力腐蚀裂纹萌生敏感性增强;外加阳极电位下,由于均匀腐蚀的作用,应力腐蚀裂纹萌生敏感性较弱。     

Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution

The local corrosion behavior of welded X70 pipeline steel in near-neutral pH solution was studied by micro-electrochemical measurements, including scanning vibrating electrode and local electrochemical impedance spectroscopy. The microstructure of the welded steel was observed by optical microscopy and scanning electron microscopy. It is demonstrated that the microstructure of weld metal consists of acicular ferrite and grain boundary ferrite, while that of heat-affected zone is a mixture of acicular ferrite, bainitic ferrite and a few martensite/austenite microconstituents. The microstructure of base steel is typically ferrite and pearlite. Electrochemical corrosion mechanism of welded X70 steel does not experience change upon hydrogen-charging, or stressing, or both. Hydrogen-charging is capable of enhancing the local anodic dissolution of the steel. The resistance of corrosion product layer decreases with hydrogen-charging, and heat-affected zone has the largest dissolution current upon hydrogen-charging. The increase of applied stress enhanced the anodic dissolution of welded X70 steel, especially the heat-affected zone, in near-neutral pH solution. Maximum current is observed in heat-affected zone, and increases with the increase of applied stresses. The total synergistic effect of hydrogen-charging (10 mA/cm²) and applied stress (550 MPa) on anodic dissolution of welded X70 steel in near-neutral pH solution is determined to be within the range of 5.7 and 6.5, with a maximum value encountering in heat-affected zone.     

Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel

Stress corrosion cracking behaviour of API-5L-X52 steel under cathodic protection in near-neutral and high pH conditions was studied using slow strain rate test method and electrochemical measurements. The slow strain rate test showed ductile and brittle fracture feature at low and high applied potentials, respectively. In order to identify the mechanism contributes in stress corrosion cracking; the electrochemical potentiodynamic polarisation test was done at fast and slow sweep rate. The results revealed that at near-neutral pH condition the anodic dissolution at crack tip was the dominant mechanism. While at high pH medium, the hydrogen based mechanism was dominant.     

Synergistic Effects of Hydrogen and Stress on Corrosion of X100 Pipeline Steel in a Near-Neutral pH Solution

In this work, scanning vibrating electrode technique and local electrochemical impedance spectroscopy measurements were used to investigate the effects of stress and hydrogen on electrochemical corrosion behavior of a X100 pipeline steel in a near-neutral pH solution. The stress distribution on the test specimen was calculated using the finite element method. Results demonstrated that the hydrogen-charging enhances the local anodic dissolution of the steel, contributing to the formation of a layer of corrosion product. However, there is little difference of the charge-transfer resistance between the regions with and without hydrogen-charging due to rapid diffusion of hydrogen atoms throughout the specimen with time. When the local stress concentration is not significant enough to approach the yielding strength of the steel, the steel is still in a relatively stable state, and there is a uniform distribution of dissolution rate over the whole surface of the steel specimen. Although the stress-enhanced activation is not sufficient to result in an apparent difference of current density of the steel, the activation of the steel would activate dislocations, which serve as effective traps to the charged hydrogen atoms. With the increase of hydrogen concentration, the hydrogen-enhanced anodic dissolution becomes dominant.     

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was found that an application of elastic stress would not affect the electrochemical stable state of the steel specimen at a macroscopic scale. Under a weak cathodic polarization, the interfacial charge-transfer process occurring on steel contains both cathodic and anodic reactions. Since the anodic reaction process is still significant, localized dissolution could occur even at such a cathodic potential, resulting in generation of corrosion pits. These pits could be the start sites to initiate stress corrosion cracks. Strain rate affects the corrosion reaction, which is associated with the generation of dislocation emergence points and slip steps on the specimen surface, resulting in a negative local additional potential to enhance the cathodic reaction locally.     

Effects of dissolved hydrogen and elastic and plastic deformation on active dissolution of pipeline steel in anaerobic groundwater of near-neutral pH

The mechano-electrochemical effects of deformation and hydrogen on active dissolution of pipeline steel in near-neutral pH groundwater are described. Thermodynamic analysis shows that the free energy increment due to elastic deformation is insufficient to alter active dissolution rate remarkably. The synergistic effect of stress field and dissolved hydrogen on active dissolution (corrosion) is negligible. The effect of plastic deformation on corrosion relies heavily on the heterogeneity of dislocation structures formed in the deformation. The plastic deformation cannot significantly change the corrosion rate at the open circuit potential. There is good agreement between these theoretical predictions and experimental observations, indicating that the stress corrosion cracking of pipeline steel in the anaerobic groundwater of near-neutral pH is unlikely to be related to the hydrogen-facilitated anodic dissolution mechanism.     

Inhibiting effects of some mercapto-triazole derivatives on the corrosion of mild steel in 1.0 M HCl medium

The effect of some mercapto-triazole derivatives synthesized in the laboratory containing different hetero atoms and substituents in the organic structures on the corrosion and hydrogen permeation of mild steel in 1.0 M HCl was investigated by weight loss and various electrochemical techniques. Results obtained reveal that all the mercapto-triazole derivatives perform excellently as corrosion inhibitors for mild steel in 1.0 M HCl. Potentiodynamic polarization studies have shown that all these compounds suppress both the anodic and cathodic process and they behave as mixed-type inhibitors. Double layer capacitance and charge transfer resistance values were derived from Nyquist plots obtained from AC impedance studies. Changes in impedance parameters are indicative of the adsorption of these compounds on the metal surface. The inhibition efficiency mainly depends on the nature of the investigated compounds. The values of the inhibition efficiency calculated from these techniques are in reasonably good agreement. The extent of reduction of hydrogen permeation current through mild steel surface was studied by the hydrogen electropermeation technique. The adsorption of these compounds on mild steel surface is found to obey Langmuir adsorption isotherm. The free energy of adsorption for inhibiting process was determined on the basis of Langmuir adsorption isotherm.     

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% CO₂/N₂ 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 CO₂, 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 CO₂ absorption (pH decrease) might shift E–pH points into a susceptibility region of near-neutral pH SCC.     

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 ?920 mV_SCE, 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.     

Corrosion Performance of Carbon Steel in CO_2 Aqueous Environment Containing Silty Sand with Different Sizes

Corrosion performance of carbon steel in CO_2 aqueous environment containing silty sand with different sizes was investigated by immersion tests and electrochemical measurements.Silty sand could form an adsorption layer on steel surface in initial period,and the sand adsorption layer was turned into a mixture film of silty sand with corrosion product in last period.The adsorption layer in 325 mesh condition(large size)had the fewest pores for H_2CO_3 transport,exhibiting the highest cathodic current inhibition.In spite of little corrosion product,the sand adsorption film formed in 325 mesh condition induced the lowest corrosion rate.For 1000 and 5000 mesh silty sand,the sand adsorption layer had some pores for H_2CO_3 transport,leading to low cathodic current inhibition and much matrix dissolution.But the adsorption layer for5000 mesh silty sand(small size)had the largest special surface area to accelerate heterogeneous precipitation of corrosion product FeCO_3.Therefore,the mixture film in 5000 mesh condition was more compact,exhibiting stronger anodic inhibition and lower corrosion rate than those in 1000 mesh condition.     

Stress corrosion cracking study of microalloyed pipeline steels in dilute NaHCO3 solutions

Studies were carried out to evaluate the stress corrosion cracking (SCC) behavior of a X-70 microalloyed pipeline steel, with different microstructures by using the slow strain rate testing (SSRT) technique at 50 °C, in NaHCO₃ solutions. Both anodic and cathodic potentials were applied. Additionally, experiments using the SSRT technique but with pre-charged hydrogen samples and potentiodynamic polarization curves at different sweep rates were also carried out to elucidate hydrogen effects. The results showed that the different microstructures in conjunction with the anodic applied potentials shift the cracking susceptibility of the steel. In diluted NaHCO₃ solutions cathodic potentials close to their rest potential values decreased the SCC susceptibility regardless the microstructure, whereas higher cathodic potentials promote SCC in all steel conditions. Certain microstructures are more susceptible to present anodic dissolution corrosion mechanism. Meanwhile concentrated solution did not promotes brittle fracture.     

Corrosion of X65 steel in CO2-saturated oilfield formation water in the absence and presence of acetic acid

Electrochemical corrosion behavior of X65 steel in CO₂-saturated formation water in the absence and presence of acetic acid was studied by electrochemical measurements, scanning vibrating micro-electrode (SVME), localized electrochemical impedance spectroscope (LEIS) and surface analysis techniques. It is found that, when steel is immersed in formation water, the dissolution of Fe dominates the anodic process and the steel is in active dissolution state. Adsorption of intermediate product on the electrode surface results in generation of an inductive loop in the low frequency range of EIS plot. As corrosion proceeds, the concentration of Fe²⁺ in the solution increases. When the product of [Fe²⁺] × [] exceeds solubility product of FeCO₃, FeCO₃ will deposit on the electrode surface, and protects the steel substrate from further corrosion. The steel is in a “passive” state. When the electrode surface is completely covered with FeCO₃ film, the inductive loop in the low frequency range disappears. In the presence of acetic acid in formation water, the cathodic reaction will be enhanced due to the direct reduction of undissociated acetic acid. Addition of acetic acid degrades the protectiveness of corrosion scale, and thus, enhances corrosion of steel by decreasing the FeCO₃ supersaturation in solution.     

Effects of Plastic Deformation and Carbon Dioxide on Corrosion of Pipeline Steel in Near-Neutral pH Groundwater

This paper investigates the effect of plastic deformation on the anodic dissolution behavior of pipeline steel in deaerated groundwater with near-neutral pH. The plastic deformation is introduced via two different ways: cold-rolling and in situ tension. It is observed that the cold-rolling prior to the exposure to corroding environment reduces the corrosion rate but the in situ tension increases corrosion rate slightly. In accord with thermodynamic analysis, the impacts of residual stresses and plastic deformation on active dissolution are very small except a highly non-uniform dislocation structure is formed. A preliminary analysis suggests that the reduced corrosion rate of cold-rolled steel is related to competitive adsorption of CO₂ and H⁺ on the active sites over the surface.     

氢对X80钢焊接接头在近中性pH值溶液中腐蚀行为的影响

采用动电位极化和电化学阻抗技术研究了电化学充氢后X80钢母材和焊缝在近中性pH值溶液中的电化学行为,并结合光学显微镜对试样充氢前后的形貌进行了观察,测量了母材和焊缝的放氢量。结果表明,电化学充氢并没有改变钢的组织结构;相同电化学充氢条件下,焊缝比母材吸收更多的氢。在近中性pH值溶液中,未充氢时母材比焊缝有更好的耐蚀性能。充氢促进母材和焊缝在近中性pH值溶液中的阳极溶解。随充氢时间增加,母材和焊缝的自腐蚀电位负移,腐蚀电流增大,腐蚀产物膜电阻和极化电阻减小,母材与焊缝电化学腐蚀行为均加剧。充氢后焊缝中位错缺陷浓度高,对氢起钉扎作用的,使焊缝的耐蚀性大于母材。     

Effects of Microstructure on Corrosion of X70 Pipe Steel in an Alkaline Soil

Corrosion of X70 steel with different heat treatments (quenching, air cooling, and furnace cooling) in an alkaline soil was investigated by weight-loss, surface characterization and electrochemical measurements. The cathodic/anodic reactions of X70 steel in alkaline soil are dominated by the oxygen reduction and formation of iron oxides that deposit on the steel surface. The protection of the oxide deposit is through a physical block effect. The deposit layer formed on as-received steel has a compact, complete structure and thus, provides an effective protection over the underneath steel. However, the deposit layers on the heat-treated steels are generally loose, porous and defective, and provide minor protectiveness. Corrosion of steel is affected by its microstructure. Generally, steels with heat treatments have a higher corrosion rate than the as-received steel. The presence of more pearlite enhances the corrosion rate of ferrite by a galvanic effect. When the steel contains bainite and martenite phases, the activity of the steel is further increased.     

A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4

Phosphorous as a minor element (0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate.