应变速率对管线钢近中性pH值环境敏感开裂的影响

以X-70管线钢近中性pH值溶液(NS4和实际土壤溶液)为研究对象,研究了恒载荷、慢应变速率拉伸(SSRT) 和循环蓑荷等不同条件下的环境开裂行为.结果表明,在该体系中局部应变速率是联系各种不同断裂过程的纽带,决定着断裂的模式.当该局部应变速率低于发生应力腐蚀开裂(SCC)敏感局部应变速率的上限(即5×10-5 s-1)时, SCC才能够发生;在循环载荷作用下,当该局部应变速率高于此上限时,将发生力学因素起主导作用的腐蚀疲劳(CF)开裂;该局部应变速率继续升高时,将发生机械断裂.对X-70管线钢在近中性pH值的环境开裂,不论开裂过程是溶解或(和)氨的作用占主导,均受局部应变速率控制.在通常遇到的现场服役条件下, X-70管线钢在近中性pH值溶液中的开裂模式是SCC,不是CF,应称之为“近中性pH值应变促进腐蚀开裂”,实质上这是一种由局部应变速率决定的环境开裂行为.     

X70钢焊接接头在近中性溶液中应力腐蚀行为研究

采用慢应变速率试验(SSRT)、扫描电镜以及电化学测量技术研究了X70管线钢焊接接头在近中性模拟土壤溶液中的应力腐蚀开裂(SCC)行为。结果表明,断口和柱面SCC裂纹均发生在热影响区(HAZ)。在试验溶液中,随着外加极化电位降低管线钢SCC敏感性增强,电位负移到一定电位值后,SCC敏感性减弱;随着溶液pH值降低,腐蚀速率增大,敏感电位区间负移。施加阴极电位时,在试样断口观察到明显的准解理脆断特征,断口和柱面有穿晶SCC裂纹。分析了焊接接头试样HAZ的SCC机理,在试验介质中,管线钢应力腐蚀开裂主要受阳极溶解和氢致开裂两种机理的联合作用,适当的电位可以使阳极溶解和氢致开裂的联合作用达到最大,从而造成较严重的应力腐蚀开裂。     

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.     

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.     

X70管线钢在不同温度近中性pH溶液中的应力腐蚀破裂行为

采用慢应变速率实验(SSRT)研究了不同温度和电位下X70管线钢在近中性pH溶液中的应力腐蚀破裂(SCC)行为．结果表明，不同温度下，X70管线钢在近中性pH溶液中的开裂方式都是穿晶型的，具有准解理特征，并且随着外加阴极电位的降低，SCC敏感性增加，氢致开裂占主导．随温度的下降，溶液pH值略有降低，SCC敏感性增加。     

外加电位对316L不锈钢在硼酸溶液中应力腐蚀行为的影响

采用慢应变速率拉伸 (SSRT) 实验,结合不同扫描速率下的动电位极化曲线,对316L不锈钢在动电位极化曲线不同区下的应力腐蚀开裂 (SCC) 敏感性以及腐蚀机理进行了研究。通过断口的SEM形貌进一步分析了316L不锈钢在硼酸溶液中的应力腐蚀开裂机理。结果表明,在近中性硼酸溶液环境下,外加电位对应力腐蚀开裂敏感性具有一定影响;当外加电位处于钝化区和过钝化区时,其SCC机制是由阳极溶解控制,且随着电位的升高其SCC敏感性增大;外加电位为-600 mV时,开裂机制为氢致开裂,此时316L不锈钢有最大SCC敏感性。     

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.     

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.     

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.     

Oxide properties and stress corrosion cracking behaviour for Alloy 600 in leaded caustic solutions at high temperature

The stress corrosion cracking behaviour of Alloy 600 in caustic solutions with and without PbO at 315 °C was investigated by means of slow strain rate tension tests. The characterisation of the oxide that formed on Alloy 600 was derived from transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. Lead was incorporated into the oxide in a metallic lead state and a lead oxide state, which degraded the passivity and induced PbSCC susceptibility. NiB was used as an inhibitor. It reduced the lead incorporation level in the oxide layer and decreased PbSCC susceptibility.     

Effect of Strength and Microstructure on Stress Corrosion Cracking Behavior and Mechanism of X80 Pipeline Steel in High pH Carbonate/Bicarbonate Solution

The stress corrosion cracking (SCC) behaviors and mechanisms of X80 pipeline steels with different strength and microstructure in high pH carbonate/bicarbonate solution were investigated by slow strain rate testing and electrochemical test. The results showed that the cracking mode of low strength X80 steel composed of bulky polygonal ferrite and granular bainite in high pH solution was intergranular (IGSCC), and the SCC mechanism was anodic dissolution (AD). While the mixed cracking mode of high strength X80 steel consisted of fine acicular ferrite and granular bainite was intergranular (IGSCC) in the early stage, and transgranular (TGSCC) in the later stage. The decrease of pH value of crack tip was probably the key reason for the occurrence of TGSCC. The SCC mechanism may be a mixed mode of AD and hydrogen embrittlement (HE), and the HE mechanism may play a significant role in the deep crack propagation at the later stage. The cracking modes and SCC mechanisms of the two X80 steels were associated with its microstructure and strength.     

Stress corrosion cracking of gas pipelines - Effect of surface roughness, orientations and flattening

The primary corrosion mitigation of the external surface of high pressure steel gas pipelines is protective coatings with secondary protection usually by cathodic protection. Adhesion and resistance to cathodic disbondment of the coating is critical for its integrity and grit blasting is an important process in achieving this adhesion. The effect of surface roughness, from grit blasting, on the intergranular stress corrosion cracking resistance of X70 gas pipelines was investigated using slow strain rate testing in carbonate/bicarbonate solution at 75 °C. The effect of orientation of test pieces with respect to the axial direction of pipes was also investigated.Time to failure ratios decreased with increasing surface roughness indicating reduced stress corrosion cracking resistance. The reduced resistance to cracking with increasing roughness would be predominantly associated with stress concentration effects related to the surface roughness resulting from the grit blasting. Crack concentration decreased with increasing roughness, which is likely to be associated with the concentration of surface damage from the grit blasting using varying sized grit. As formed pipe surfaces, with no grit blasting, resulted in some of the lowest time to failure ratios and hence some of the lowest resistances to stress corrosion cracking. These also showed some of the deepest cracks. The influence of roughness and residual stresses on threshold stress is currently being investigated.Time to failure ratios indicated a greater resistance to stress corrosion cracks for circumferentially orientated test pieces compared to those longitudinally orientated. Whilst further testing would be required for confirmation, the current results suggest that flattening the test pieces had only a minor, if any, effect on stress corrosion cracking susceptibility as measured by slow strain rate testing to fracture.     

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.     

Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

In situ characterization by localized electrochemical impedance spectroscopy of the electrochemical activity of microscopic inclusions in an X100 steel

A localized electrochemical impedance spectroscopy (LEIS) technique was used to characterize in situ the micro-electrochemical activity of inclusions contained in an American Petroleum Institute (API) X100 steel in a near-neutral pH solution. It is found that there exists an electrochemical heterogeneity between inclusions and the adjacent steel matrix. Consequently, a galvanic couple is formed to result in the locally preferential dissolution. The local electrochemical activity of the inclusion depends on its composition. A Si-enriched inclusion is associated with a high electrochemical activity, and the preferential dissolution of the inclusion generates a local microvoid, whose further dissolution initiates a corrosion pit. An aluminum oxide-enriched inclusion is more stable than the adjacent steel matrix. The preferential dissolution would occur on the steel, causing the “drop-off” of the inclusion and generating a corrosion pit.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrite solutions

The stress corrosion cracking (SCC) susceptibility of pure copper and four α-brasses of different zinc alloy concentration in NaNO₂ 1 M solution, at various pH values, was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. The results confirmed that passivity rupture was a necessary condition for SCC of α-brass and copper in nitrite solutions. Susceptibility to SCC was observed only when the potential was equal to or higher than a certain critical value at which passivity breakdown was triggered by the slow dynamic straining of the metal. Cracks were found only in those metal areas where passivity rupture had taken place.     

Effects of corrosion product deposit on the subsequent cathodic and anodic reactions of X-70 steel in near-neutral pH solution

The effects of corrosion product deposit on the subsequent anodic and cathodic reactions of X-70 steel in a near-neutral pH solution were investigated by localized electrochemical impedance spectroscopy (LEIS), scanning vibrating micro-electrode (SVME) and macroscopic EIS measurements as well as surface analysis technique. It is found that the deposit layer formed on the steel surface is porous, non-compact in nature. The presence of a corrosion product layer would enhance adsorption, but significantly inhibit absorption and permeation of hydrogen atoms into steel. It is due to the porous structure of the deposit that generates a spatial separation of cathodic and anodic reaction sites, resulting in an increased effective surface area for hydrogen adsorption and, simultaneously, a “blocking” effect on hydrogen absorption and permeation. The deposit enhances greatly anodic dissolution of the steel, which is attributed to the adsorption of the intermediate species and the resultant “self-catalytic” mechanism for corrosion of the steel in near-neutral pH solution. In the presence of corrosion product deposit on the pipeline steel surface, pipeline corrosion, especially pitting corrosion, is expected to be enhanced. Stress corrosion cracks could initiate from the corrosion pits that form under deposit. However, deposit does not contribute to hydrogen permeation, although the hydrogen evolution is enhanced.     

Local additional potential model for effect of strain rate on SCC of pipeline steel in an acidic soil solution

Stress corrosion cracking (SCC) behavior of X70 pipeline steel in an acidic soil solution was investigated by slow strain rate test, surface characterization, potentiodynamic polarization curve measurement and electrochemical hydrogen permeation technique. A local additional potential model (LAPM) was developed to illustrate the critical role of strain rate in SCC of the steel. According to LAPM, both density and mobility of local active spots on the steel surface, i.e., dislocation emergence point, increase linearly with strain rate. Generation of such active spots introduces an additional negative potential locally, affecting the electrochemical reaction and, consequently, the susceptibility of the steel to SCC. It is found that a maximum of the SCC susceptibility occurs at strain rate of 10⁻⁶ s⁻¹, which is associated with an enhanced hydrogen evolution due to the local additional potential (LAP) effect. When strain rate is sufficiently high to exceed 10⁻⁶ s⁻¹, the mobility of the dislocation emergence points is so fast that the reactive species in solution cannot combine with them for cathodic reaction, resulting in a decrease of the SCC susceptibility. Similarly, a maximum of hydrogen permeation current observed at the strain rate of 10⁻⁶ s⁻¹ is also attributed to the effect of strain rate on the density and mobility of dislocations in the steel. Diffusion of hydrogen atoms in a strained steel is through both body diffusion and dislocation diffusion, with the latter enhanced by an increasing strain rate. When strain rate is so high that the dislocation mobility is sufficiently fast, hydrogen atoms become incapable of catching up with the dislocations. As a result, the hydrogen diffusion is dominated by the body diffusion mode.     

Effect of inclusions on initiation of stress corrosion cracks in X70 pipeline steel in an acidic soil environment

The effect of inclusions on the initiation of stress corrosion cracking (SCC) X70 pipeline steel was investigated in an acidic soil solution using slow strain rate test, scanning electron microscopy and energy-dispersive X-ray techniques. The results demonstrated that stress corrosion cracks are not initiated in X70 steel when it is under anodic polarization. At cathodic polarization, hydrogen evolution is enhanced, and hydrogen is actively involved in SCC processes. Two types of inclusions exist in the steel and play different role in crack initiation. The inclusions enriching in Al are brittle and incoherent to the metal matrix. Microcracks and interstices are quite easily to be resulted in at the boundary between inclusions and metal. There is no crack initiating at inclusions containing mainly Si.     

Mechanical behaviour of SSRT specimens optimized for IGSCC concerns

Simulating testing conditions leading to evaluate the intergranular stress corrosion cracking sensitivity of structural alloys is crucial to estimate the lifetime of in-service components. Former studies have pointed out that a simple modification of the design of slow strain rate tensile specimens was particularly convenient for evaluating the susceptibility to intergranular stress corrosion cracking of nickel-base alloys. The aim of the present work is to characterize and model the mechanical behaviour of such specimens. Validation of proposed modelling relies mainly on tensile tests carried on specimens equipped with strain gages. One of the striking results is that, for a given displacement rate of the heads of the specimen, a much slower strain rate can be obtained locally in comparison with the strain rate of an equivalent smooth specimen.