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。     

Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution

Stress corrosion cracking (SCC) of X70 pipeline steel in simulated solution of the acidic soil in Yingtan in China was investigated using slow strain rate test (SSRT), SEM and potentiodynamic polarization technique. Experiment results indicate that X70 steel is highly susceptible to SCC as applied potential reduces, which is manifested in loss of toughness and brittle fracture. Constant polarization current can detect the occurrence of SCC. The lower the polarization current is the sooner stress corrosion cracking occurs. The SCC mechanisms are different at varying potentials. When potential is higher than open circuit potential, anodic process controls SCC, whereas when potential is far lower than open circuit potential, cathodic process controls SCC, and between these two potential regions, a combined electrochemical process controls the SCC. Stress or strain has a synergistic effect with electrochemical reactions to accelerate the cathodic hydrogen evolution process, which makes the X70 pipeline steel to be more susceptible to SCC.     

X100管线钢在酸性土壤模拟溶液中的应力腐蚀行为

采用慢拉伸(SSRT)、动电位极化和SEM观察等方法,研究了在不同的阴极保护电位条件下X100钢在酸性土壤模拟溶液中的应力腐蚀行为.结果表明,X100钢发生穿晶裂纹的应力腐蚀,裂纹的萌生和发展与阴极保护电位有关.完全阳极过程控制时,X100钢无裂纹出现,但出现晶间腐蚀;在混合过程控制时,应力腐蚀敏感性较低,裂纹发展缓慢;在完全阴极过程控制时,氢脆机制起主要作用,裂纹扩展迅速.     

Stress corrosion cracking behavior of X70 pipe steel in an acidic soil environment

Stress corrosion cracking (SCC) behavior of X70 pipe steel was investigated in an extracted acidic soil solution by slow strain rate test (SSRT), potentiodynamic polarization curve measurements and surface analysis technique. The SCC process and mechanism of X70 steel in the acidic soil solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. With the different applied potentials, the dominance of SCC process changes. At a relatively less negative potential, the steel SCC is based primarily on the anodic dissolution mechanism. When the applied potential is shifted negatively, hydrogen is involved in the cracking process, resulting in a transgranular cracking mode. With the further negative shift of applied potential, the SCC of the steel follows completely a hydrogen-based mechanism, with a river-bed shaped brittle feature of the fracture surface. Heat treatment alters the microstructure of the steel, resulting in a change of SCC susceptibility. In particular, the quenched steel with a bainite microstructure has a high susceptibility to SCC in the acidic soil, while the as-received steel with a ferrite matrix have a low SCC susceptibility.     

电化学充氢对X80管线钢在鹰潭土壤模拟溶液中应力腐蚀行为的影响

采用电化学技术、慢应变速率拉伸实验和扫描电镜(SEM)对电化学充氢后的X80管线钢在鹰潭土壤模拟溶液中的应力腐蚀行为进行了研究。结果表明：X80管线钢静态充氢后在鹰潭土壤模拟溶液中具有较高的应力腐蚀(SCC)敏感性,其断口模式为穿晶断裂;随着电化学充氢时间的延长,氢致塑性损失不断增加,拉伸断口由韧窝状韧性断口向脆性解理断口发展,SCC敏感性增大;电化学充氢促进了点蚀坑的萌生,点蚀坑和第二相夹杂是SCC裂纹萌生的重要原因。     

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.     

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.     

Mechanismus des Rißwachstums bei der Spannungsrißkorrosion des austenitischen Stahls X6 CrNiTi 18 10 in wäßriger Chloridlösung bei erhöhten Temperaturen

SCC crack growth mechanism of austenitic stainless steel X6 CrNiTi 18 10 in aqueous chloride solution at elevated temperatures The SCC crack growth mechanism of steam generator heat transfer tubes from stainless steel X6 CrNiTi 18 10 under internal stress conditions at elevated temperatures is discussed. Based on crack tip characterization by means of Scanning and Transmission Electron Microscopy and the evidence of hydrogen originated throughout the corrosion process a crack propagation model is presented. The results refer to a microcrack induced gradual crack growth caused by local hydrogen embrittlement. Microcrack growth has been observed due to slip-band decohesion. The crack growth rate is mainly influenced by the stress state near the crack tip and the hydrogen evolution throughout the corrosion process.     

Effect of electrolyte composition on crack growth rate in pipeline steel

Electrolyte composition can strongly affect the rate of crack growth in pipe steel X70. A weakly acidic citrate buffer (pH 5.5) and a mixture of NS4 solution with a borate buffer (pH 7) were used as background solutions under static and cyclic strain, respectively. Various compounds that affect steel dissolution and hydrogen absorption rates were added to the solutions. Crack growth is accelerated in the presence of metal dissolution activators but hindered upon addition of a corrosion inhibitor. Hydrogen absorption promoters do not accelerate the crack growth at the corrosion potential.     

Monte Carlo simulation of stress corrosion cracking on a smooth surface of sensitized stainless steel type 304

Stress corrosion cracking (SCC) on a smooth surface of structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack propagation and multiple large crack formation or final failure under combination of material, stress and corrosive environment. In this paper, a Monte Carlo simulation of the SCC process is proposed based on stochastic properties for micro crack initiation and concepts in fracture mechanics for crack coalescence and propagation. The procedure is as follows: The possible number of grain-sized micro cracks which can be initiated is set for a given space and initiation times for all cracks are assigned by random numbers based on exponential distribution. Sites and sizes of cracks are assigned by uniform random numbers and normal random numbers, respectively. Coalescence and propagation of cracks are determined based on fracture mechanics. The emphasis in the model is put on the influence of semi-elliptical surface cracks. Numerical simulations are carried out based on the results of creviced-bent-beam tests for sensitized stainless steel type 304 under high-temperature and high-purity water containing dissolved oxygen and the influence of micro crack initiation rate and coalescence condition on the simulation results is discussed.     

Effect of AC on stress corrosion cracking behavior and mechanism of X80 pipeline steel in carbonate/bicarbonate solution

The influence of various AC current densities on stress corrosion cracking behavior and mechanism of X80 pipeline steel was investigated in carbonate/bicarbonate solution by polarization curves and slow strain rate tensile tests. With the increasing AC current density, the SCC susceptibility of the steel increases, especially at high AC current density. A significant difference in the SCC behavior and mechanism is found for the steels with or without AC application. In the absence of AC, the fracture mode is intergranular and the mechanism is attributed to anodic dissolution. Under AC application, the cracks propagation is transgranular, and the mechanism is mixed controlled by both anodic dissolution and hydrogen embrittlement.     

Evaluation of stress corrosion cracking phenomenon in an AISI type 316LN stainless steel using acoustic emission technique

This paper deals with the analysis of the acoustic emission (AE) signals to determine the micro-process during stress corrosion cracking (SCC) of AISI type 316LN stainless steel that cause the AE, and thus the mechanism of the SCC process. AE with amplitudes ranging from 27.6 to 46.5 dB with different counts, energy and rise times occurred during SCC of type 316LN stainless steel in 45% MgCl₂ at 413 K. The analysis of the AE signals in conjunction with fractography indicated that a surge in the AE counts and energy indicated initiation of SCC. AE was found to be continuous prior to the initiation. The time gap between AE events increased during initiation. AE events occurred in bursts during crack growth. Plastic deformation ahead of the crack tip was determined to be the major source of AE during propagation of SCC in type 316LN stainless steel. The cracking was found to initiate and propagate in the transgranular mode.     

Crack initiation model for sensitized 304 stainless steel in high temperature water

To investigate the initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel in high temperature water, a constant load SCC test method combined with in situ crack observation technique was employed. The in situ crack observation system allowed us to detect small cracks of at least 100 μm. As a result, a fracture time decreased with an increase in an applied stress. The first cracks were observed at most 3 h before the specimen was fractured under all the stress conditions. After that, many cracks were initiated in a short time to fracture. The fracture was caused by coalescence of multiple cracks rather than by growth of some primary cracks. The simulation model for surface crack initiation was newly developed using a Monte Carlo method, which was based on damage mechanics and stress analysis around the existing cracks. The simulation could represent the empirical results of changes in the crack distribution and the cumulative number of cracks during the SCC tests. It was concluded, therefore, that the crack initiation process should be considered in simulating the life prediction of the material in this SCC system.     

阴极极化下X80钢在鹰潭土壤模拟溶液中应力腐蚀行为研究

采用电化学阻抗和慢应变速率方法,结合扫描电子显微镜,研究了不同阴极极化电位下X80钢在鹰潭土壤模拟溶液中的应力腐蚀行为。结果表明：鹰潭土壤模拟溶液中,X80钢/溶液界面处电荷转移电阻随阴极极化程度增加先升后降。在自腐蚀电位条件下开裂机理为阳极溶解,当外加电位为-1000 mV (vs SCE),应力腐蚀敏感性最低,此电位为最佳保护电位;继续增大阴极极化程度,应力腐蚀敏感性增加,此时开裂机制为氢和应力协同作用下的氢致开裂。     

CORROSION BEHAVIOR OF X70 PIPELINE STEEL IN SIMULATED KUERLE SOIL SOLUTION WITH CO2

采用动电位扫描、交流阻抗电化学方法和慢应变速率拉伸实验(SSRT)研究了CO2对X70管线钢在库尔勒土壤模拟溶液中应力腐蚀开裂(SCC)行为的影响, 并利用扫描电镜分析了不同CO2分压下的断面形貌. 结果表明: CO2能够与腐蚀膜FeCO3反应, 生成可溶性Fe的络合物 Fe(CO3)2 2-, 加速X70管线钢腐蚀; CO2与H2O形成H2CO3和HCO3-, 为阴极反应提供H+. X70管线钢在含 CO2溶液中的SCC机理为氢脆--阳极溶解协同机理, 且随CO2 分压的增加, 氢脆作用增大.     

Influence of intermittent loading on SCC behavior for an X70 pipeline steel in a near neutral pH solution

The influence of intermittent loading on stress corrosion cracking (SCC) behavior for an X70 pipeline steel was studied using slow strain rate tests (SSRT) in a near neutral pH soil solution at various applied potentials. The d.c. potential drop method was used to measure the crack initiation and propagation. The crack length at the cross section of the fractured sample was observed using scanning electron microscopy (SEM). The results showed that the crack initiation stress in solution was lower than the yield stress in air. SEM observation showed that the crack length was longer in intermittent loading than that in continuous loading. At anodic potential, the crack initiation stress was the lowest and the crack length was the longest. The crack kept growing during constant loading period.     

Influence of local stress on initiation behavior of stress corrosion cracking for sensitized 304 stainless steel

To investigate the influence of local stress on initiation behavior of stress corrosion cracking (SCC) for sensitized Type 304 stainless steel, cracking process during constant load SCC test was monitored and recorded with an in situ crack observation system. The changes in number of cracks, sum of crack length and cracked area on the specimen surface with test time were identified from the cracking images analyzed by image processing. In the SCC tests, many cracks were initiated and coalesced on the surface, and the coalescence of cracks played an important role to primary crack growth. The influence of applied stress on crack initiation was different from that on crack growth. In addition, there was a difference between influences of stress on incubation period to crack initiation and crack initiation rate. Due to these differences, a stress of 0.8S_y was thought to cause relatively many cracks compared with 0.5S_y and 1.3S_y (S_y = 200 MPa). Through quantitative estimation of distribution in local stress around a crack by finite element analysis method, it was deduced that the crack initiation is influenced not only by bulk stress applied at the end of the body, but also by local stress formed around pre-existing cracks. According to pre-existing cracks, stress enhancement accelerates the crack growth, while the stress relaxation causes the suppression of new crack initiation. Based on the experiment and analysis results, three types of growth process were suggested, which are caused by propagation itself, by new crack initiation at vicinity of the crack tip, and by coalescence of approaching cracks. Then, it was concluded that, in order to predict/simulate the cracking behavior of this SCC system, the influence of local stress on the crack initiation should be taken into account.     

外加电位对X70管线钢在库尔勒土壤模拟溶液中应力腐蚀开裂敏感性的影响

采用慢应变速率拉仲试验(SSRT)研究了不同外加电位下X70管线钢在库尔勒土壤模拟溶液中的应力腐蚀开裂(SCC)行为,并用扫描电镜分析了不同电位下的断面形貌.结果表明,X70管线钢在库尔勒土壤模拟溶液中具有SCC敏感性;在Ecorr附近施加弱极化时,应力腐蚀开裂敏感性增加;施加强阳极电位时,发生强烈阳极溶解,导致阳极溶解断裂;施加强阴极电位时,析氢过程加强,导致氢致应力腐蚀断裂.     

Study on H2S stress corrosion test of welded joint for X65pipeline steel and numerical analysis

The susceptibility of welded joint for the X65 pipeline steel to H2S stress corrosion cracking (SCC) is investigated.SCC tests on the steel are carded out in the environment based on NACE TM-O1-77 solution with saturated gaseous H2S. The threshold stress intensity factor and crack propagation velocity are calculated according to wedge-opening loading (WOL)specimens. The three-dimensional elastic-plastic finite element analysis of WOL specimens is pesformed by using the FEM programming package ANSYS. Stress field and concentration of hydrogen distribution property ahead of the crack tip are obtained. This paper surveyed the microstructure of welded joint and studied on the mechanical properties of X65 pipeline steel.It provides experimental basis for studying stress corrosion. The results of numerical analysis are consistent with conclusions of stress corrosion test.     

Cl–对冷变形316L奥氏体不锈钢在H2S环境下应力腐蚀的影响

目的研究H2S环境下不同Cl^-浓度对冷变形316L奥氏体不锈钢应力腐蚀行为的影响,探究Cl^-造成影响的原因,为不锈钢安全服役提供理论数据。方法采用力学方法研究了冷变形316L奥氏体不锈钢的力学行为,通过计算延伸率损失表征材料的应力腐蚀敏感性,通过电化学手段表征了点蚀电位。最后为了研究点蚀与基体中氢含量的关系,进行了扩散氢含量的测试,通过测量试样的扩散氢含量,进一步理解应力腐蚀行为。结果随着Cl^-浓度的增加,316L奥氏体不锈钢的延伸率损失逐渐增大,应力腐蚀敏感性增强。断口形貌从杯状的等轴韧窝转变为解理型脆性断裂。动电位极化测试表明,Cl^-浓度的增加,点蚀电位逐渐降低,直至–0.0228V,试样更容易发生点蚀。扩散氢含量的测量进一步显示了点蚀坑的存在促进了氢进入到金属内部。结论 Cl^-对316L奥氏体不锈钢在H2S环境中的应力腐蚀行为有重要影响,随着Cl^-浓度的增加,应力腐蚀敏感性增强,结合点蚀电位的测量结果,可能是由于Cl^-破坏金属表面的钝化膜,产生点蚀坑,裂纹形核并扩展,同时点蚀坑还促进了氢进入金属内部,应力腐蚀敏感性增强。