A Fracture Probability Competition Mechanism of Stress Corrosion Cracking

The stress corrosion cracking (SCC) of austenitic stainless steel was studied via polarization, slow strain rate and scanning electron microscope (SEM) techniques. Many SCC mechanisms have been proposed in which hydrogen embrittlement and passive film rupture-repassivation theories are generally accepted, but they can hardly explain the SCC mechanism of austenitic stainless steel in acidic chloride solution adequately, because the steel is in active dissolution state and cathodic polarization can prevent it from occurring. Our experiment shows that the anodic current increases the creep rate and decreases the plastic strength of the material on single smooth specimen as well as at the SCC crack tip. The fractured surface was characterized as brittle cleavage, while the surface crack of smooth specimen was almost vertical to the tensile strength, which can confirm that the cracks were caused by tensile stresses. A fracture probability competition mechanism of SCC was proposed on the basis of the experimental results combined with the viewpoint of ductile-brittle fracture competition. When the anodic dissolution current is increased to a certain degree, the probability of fracture by tensile stress will exceed that by shear stress, and the brittle fracture will occur. The proposed SCC mechanism can riot only explain the-propagation of SCC cracks but can explain the crack initiation as well. The strain on the surface distributes unevenly when a smooth specimen is deformed, so does the anodic current distribution. The crack will initiate at a point where the anodic current density is large enough to cause the material at a specific point to fracture in brittle manner.     

埋地长输管道交流腐蚀现场状态及其风险评估

超高压输电线路对埋地管道的交流干扰问题日益突出,国内对其交流腐蚀缺乏风险评估.以裸露X65钢模拟埋地长输管道防腐蚀涂层缺陷,采用交、直流电流及电位的在线监测和腐蚀失重等方法,研究了高压输电线路交流干扰特征及其对管道阴极保护系统的影响规律.结果表明,当土壤介质条件基本不变时,电位与电流的变化存在相似性;适当调节阴极保护电...     

数值模拟计算在地下管线及储罐的阴极保护设计中的应用

介绍了用于阴极保护系统设计的几种数值模拟计算方法。描述了数值模拟计算在地下管线及储罐的阴极保护系统优化设计方面的应用。通过工程应用实例,重点介绍采用基于边界元的Beasy数值模拟软件系统得到地下管线及储罐罐底的阴极保护电位和电流密度分布及其干扰因素影响的计算结果。     

电极极化对铝合金应力腐蚀断裂敏感性的影响

为了探讨极化对铝合金应力腐蚀断裂(SCC)敏感性的影响,采用慢应变速率拉伸技术研究了不同恒电位极化条件下7075T6,7075T7351,7075RRA铝合金的应力腐蚀行为.结果表明,在-1 200～-735 mV(vs SCE)电位范围内,无论是阳极极化还是阴极极化,甚至弱极化,都会增加7075铝合金在3.5% NaCl溶液中的应力腐蚀断裂敏感性,无论哪一种热处理状态的7075铝合金在极化电位下的ISSRT均明显高于腐蚀电位下的ISSRT.同时,有阴极极化时ISSRT值随电极极化增强而减小,阳极极化时ISSRT 值有随电极极化增强而增大的趋势.但不同热处理状态的7075铝合金受极化电位的影响程度不同.由此认为,阳极溶解和氢效应都是导致7075铝合金应力腐蚀开裂的重要因素,电化学保护方法并不适用于7075铝合金应力腐蚀断裂的防护.     

Cathodic protection revisited: Impact on structural morphology sheds new light on its efficiency

Cathodic protection (CP) has been found as one of the most viable techniques for inhibiting chloride induced corrosion of steel in concrete structures. This contribution specifically pursues exploring the morphological alterations of corrosion products in reinforced mortars under cathodic protection. For this purpose, scanning electron microscopy (SEM) techniques are combined with energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD) for microstructural analysis and quantification of the corrosion products. The combined characterisation provides important insight into the structural alterations induced by cathodic protection, and therefore, help to explain the efficiency of CP techniques. Furthermore, electrochemical measurements are employed to monitor the electrochemical process at the steel–paste interfaces. The beneficial microstructural alterations result in better corrosion resistance of the protected specimens, and turn out to be the mechanisms underlying the efficiency of CP techniques.     

Stress corrosion cracking and hydrogen embrittlement cracking of welded weathering steel and carbon steel in a simulated acid rain environment

Abstract

The stress corrosion cracking (SCC) and hydrogen embrittlement cracking (HEC) characteristics of welded weathering steel and carbon steel were investigated in aerated acid chloride solution. The electrochemical properties of welded steels were investigated by polarisation and galvanic corrosion tests. Neither weathering steel nor carbon steel showed passive behaviour in this acid chloride solution. The results indicated that weathering steel had better corrosion resistance than carbon steel. Galvanic corrosion between the weldment and the base metal was not observed in the case of weathering steel because the base metal was anodic to the weldment. However, the carbon steel was susceptible to galvanic corrosion because the weldment acts as an anode. Slow strain rate tests (SSRT) were conducted at a constant strain rate of 7.87 × 10⁷ s^-1 at corrosion potential, and at potentiostatically controlled anodic and cathodic potentials, to investigate the SCC and HEC properties in acid chloride solution. The welded weathering steel and carbon steel were susceptible to both anodic dissolution SCC and hydrogen evolution HEC. However, weathering steel showed less susceptibility of SCC and HEC than carbon steel at anodic potential because of Cr and Cu compounds in the rust layer, which retarded anodic dissolution, and at cathodic potential due to the presence of Cr, Cu, and Ni in alloy elements, which inhibit the reduction of hydrogen ions. SEM fractographs of both steels revealed a quasicleavage fracture in the embrittled region at applied anodic and cathodic potentials.     

SCC Behaviour of X-70 Pipeline Steel in Near-Neutral pH Solutions

The stress corrosion cracking (SCC) behaviour of X-70 pipeline steel in near-neutral pH solutions was studied via slow strain rate testing (SSRT). The results showed that the cracking mode of X-70 pipeline steel in near-neutral pH solutions was transgranular at different temperatures and applied potentials with the feature of quasi-cleavage. The pH value of the solution decreased with increasing the addition of CO2, which increased the susceptibility to SCC. SCC susceptibility increased as the applied potential moved towards the cathodic direction, suggesting that hydrogen induced cracking (HIC) dominated the cracking process at cathodic potentials. The slight decrease of pH values with decreasing temperature of the solution increased the susceptibility to SCC, which attributed to the change of solubility of CO2 in the solution at different temperatures. The propagating directions of SCC cracks were different at different potentials. At rather negative cathodic potentials, the cracks were almost perpendi     

Stress Corrosion Cracking of X80 Steel in the Presence of Sulfate-reducing Bacteria

The systematic laboratory studies on the roles of sulfate-reducing bacteria(SRB) in the stress corrosion cracking(SCC) susceptibility of X80 steel subjected to cathodic potential have been conducted in a nearneutral pH soil solution by slow strain rate tests.The cathodic potential and SRB increase individually the SCC susceptibility of the steel in the soil solution.The positive role of the SRB activities in SCC susceptibility depends on the prolongation of pre-incubation time,and the SCC susceptibility of the steel increases under more negative potentials.What’s more,the applied potentials and the presence of SRB work together in promoting the SCC susceptibility of the steel.But,the combined action becomes limited with decreasing cathodic potentials.The relationships between the plasticity loss and the permeable hydrogen concentration were established for the steel in the soil solution,regardless of under open circuit potential or cathodic potentials,in both the sterile and SRB inoculated conditions.The relationships are practically significant for the selection of safe cathodic protection(CP) potentials in the presence of SRB in soil environment.     

铈离子对高强铝合金应力腐蚀开裂的缓蚀作用

基于慢应变速率拉伸实验(SSRT),采用恒电流极化、电化学噪声(ECN)与电化学阻抗(EIS)等方法,研究7A04铝合金在3.5%(质量分数)NaCl水溶液中的应力腐蚀开裂(SCC)行为以及Ce~(3+)对其SCC的缓蚀作用,探讨Ce~(3+)对裂纹孕育与发展过程的抑制机理。结果表明:无论是阳极还是阴极极化,均会促进7A04的SCC倾向,前者增加了裂尖的阳极溶解,后者则加速了裂尖的氢脆效应。Ce~(3+)的加入能延缓7A04的SCC断裂时间,但其有效性仅限于裂纹的萌生阶段。由于Ce~(3+)能够抑制铝合金表面的亚稳态点蚀发育和长大,因而使裂纹的孕育时间显著延长,降低了SCC的敏感性。不过一旦裂纹进入扩展阶段或者试样表面有预裂纹,则由于Ce~(3+)很难迁移到裂纹尖端或在裂尖区难以成膜,不能对裂纹的生长起到有效抑制作用,因而无法降低7A04的SCC发展速率。SEM分析表明7A04铝合金光滑试样SCC主要源于亚稳态或稳态点蚀的诱导作用。     

纳米镀锌层在NaCl溶液中的腐蚀行为研究

通过电镀的方法制备了不同晶粒尺寸的纳米镀锌层和微米镀锌层，应用极化曲线和交流阻抗的方法研究了它们在5％NaCl溶液中的电化学腐蚀行为，结果表明，纳米镀锌层较微米镀锌层腐蚀电流密度增加，纳米镀锌层的电化学腐蚀行为存在纳米尺寸纳应，镀锌层纳米化使参与反应的原子数增加，阳极交换电流密度提高，氧的阴极还原更为容易，阴极交换电流密度也提高。     

交流干扰下管道最佳保护准则探究

为了探究交流干扰下抑制管道发生腐蚀的最佳保护准则,通过试验确定交流干扰下达到最佳保护状态时的阴极保护电位,首先验证经典的-0.85 V[vs饱和硫酸铜参比电极(CSE)]阴极保护电位准则和负向偏移100mV阴极保护准则在交流干扰下是否还具有良好的保护效果;接着探究阴极保护电位对保护效果的影响,确定保护效果较好时保护电位的范围.在此基础上,再结合保护程度和保护效率2个评价指标全面分析,找出最佳保护状态下的阴极保护电位取值.结果表明:存在交流干扰时,经典的-0.85 V(vs CSE)阴极保护电位准则和负向偏移100 mV阴极保护准则已经失效;在-1.20 V(vs CSE)以内,阴极保护电位负向升高,对交流干扰下的腐蚀抑制作用会增强;综合腐蚀速率、保护程度和保护效率3个因素,得到交流电流密度在50 A/m2以内时,建议采用-1.00 V(vs CSE)的保护电位;交流电流密度大于50A/m2时,建议采用-1.10 V(vs CSE)的保护电位.     

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, i _crit, and passive current density, i _p, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, t _f, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, K _ISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.     

Propagation of steel corrosion in concrete: Experimental and numerical investigations

This paper focuses on experimental and numerical investigations of the propagation phase of reinforcement corrosion to determine anodic and cathodic Tafel constants and exchange current densities, from corrosion current density and corrosion potential measurements. The experimental program included studies on RC specimens with various binder compositions, concrete cover thicknesses, and concrete cover crack widths. Modelling and fitting of experimental data using an electrochemical model allowed for the determination of parameters, which are key parameters for electrochemical modelling tools. The numerical model was, furthermore, used to identify electrochemical parameters, which are independent of concrete cover thickness and crack width and at the same time allow for determination of the corrosion current density and corrosion potential of concrete structures within an acceptable error.Very good comparisons between the experimentally measured and numerically simulated corrosion current densities and corrosion potentials were found for the various RC specimens. Anodic and cathodic Tafel constant between 0.01 and 0.369 V/dec and 0.01 and 0.233 V/dec, respectively, were found in the present study through numerical simulations of the experimental data. Anodic and cathodic exchange current densities ranged from 1.0E–12 to 1.0E–09 A/mm² and 1.0E–12 to 1.1E–09 A/mm², respectively.     

国产管线钢的环境开裂性能研究

采用电化学测试和慢应变速率实验(SSRT)研究了国产X70管线钢在近中性pH和高pH溶液中的应力腐蚀破裂(SCC)行为.结果表明,X70钢在高pH溶液和近中性pH溶液中的阳极极化曲线表现出明显的差异:在高pH溶液中有明显的活化-钝化转变而在近中性pH溶液中则无;在近中性pH溶液中,X70管线钢的开裂模式是穿晶型的,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,氢致破裂占主导;随温度的下降以及溶液中CO2含量的增加,溶液pH值降低,SCC敏感性增加.在高pH溶液中,在阴极极化时,X70钢表现出与在近中性pH溶液中类似的破裂行为和特征,即SCC敏感性随电位降低而增大,裂纹数目少而大,裂纹易扩展;但在阳极极化时,裂纹数目多而小,易萌生但难扩展.在2种溶液中阳极极化时,均存在SCC敏感电位区.     

Temperature influence on SCC behaviour of duplex stainless steel

Some studies have related damages in duplex stainless steel (DSS) structures due to stress corrosion cracking (SCC), however, different mechanisms are observed depending of the electrolyte, temperature, electrochemical potential and steel composition. Wherefore, the SCC mechanisms of the AISI 318 DSS in a 115?000?ppm of chloride solution at 25 and 70°C were investigated. The results showed that the SCC cracks propagated in both phases, ferrite and austenite. A reduction in elongation was observed at anodic potentials caused by electrochemical dissolution and at cathodic potentials below ?650?mVECS at 70°C and ?750?mVECS at 25°C related to SCC mechanism. Accordingly, the more susceptible and protective potentials were determined for DSS in both temperatures.     

A filiform corrosion and potentiodynamic polarisation study of some aluminium alloys

The aim of the present investigation is a combined study of filiform corrosion of aluminium alloys by accelerated exposure tests and potentiodynamic polarisation measurements. The accelerated exposure tests are performed on binary Al-Cu, Al-Mg, Al-Si and Al-Zn model alloys, a ternary Al-MgSi alloy and on the two commercial alloys, AA2024-T351 and AA7075-T651, with variations of composition and surface treatments. The surface treatments cover simple degreasing, chromate and cerium based treatments. A trend of a higher filiform corrosion susceptibility with increasing alloying elements concentrations was observed for all model systems. Furthermore, the filiform corrosion susceptibility varies with the solute atom, in particular Cu was found to have a detrimental effect on the filiform corrosion properties. Both chromating and cerating improve the filiform corrosion resistance of the alloys significantly. To explain the trends observed in the exposure tests, polarisation measurements were performed on the untreated Al-Cu and Al-Zn alloys in bulk anolyte and catholyte solutions which are characteristic for the local anodic and cathodic sites in the filaments on the aluminium substrates. From these measurements a filiform corrosion current, defined as the intercept of the anodic and cathodic curves, can be determined. The present set of experiments shows a correlation between the filiform corrosion properties during accelerated exposure tests and the potentiodynamic polarisation measurements for the Al-Cu alloys. When comparing the results for the Al-Cu and Al-Zn binary alloys it can be concluded that the correlation factor differs significantly with the solute atom and the filiform corrosion current proves to be a non-uniquely discriminating parameter for the filiform corrosion susceptibility of the model alloys. The difference in correlation factor for the Al-Cu and Al-Zn alloys is attributed to differences in the electrochemical behaviour of these alloys with local variations in substrate composition. For the Al-Cu and Al-Zn model alloys the filiform corrosion initiation characteristics are related to the passive range and thus implicitly to the ease of pitting of the alloy. A smaller passive range corresponds to a higher filiform site density for both the Al-Cu and Al-Zn alloys.     

Stress corrosion cracking and hydrogen embrittlement of cold worked AISI type 304 austenitic stainless steel in mode I and mode III

Abstract

Comparative testing of cold worked AISI type 304 austenitic stainless steel in mode I and mode III under conditions of cathodic charging and chloride stress corrosion cracking (SCC) has been used to assess the role of hydrogen in SCC. The experimental results of these tests and those previously published have been used to deduce the mechanism and rate controlling step for SCC. The mechanism for chloride SCC in mode I is anodic dissolution of active slip planes containing hydrogen with the rate controlling step being the transport of hydrogen to these slip planes. The mechanism of SCC in mode III is tunnelling corrosion followed by overload again occurring on a plane of maximum hydrogen concentration.

MST/348     

Corrosion inhibition of AA6063 alloy by vanadates in alkaline media

Vanadium aqueous compounds are considered as promising environmentally friendly inhibitors for heterogeneous aluminium alloys. In this study, mechanism of corrosion inhibition of AA6063 in alkaline media by vanadate inhibitors is presented. A combination of weight loss and open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy was used to compare corrosion behavior of AA6063 alloy in aerated pH 10 0.05 M sodium chloride solutions in the absence and presence of 0.003 mol/l sodium orthovanadate. Tetrahedrally coordinated mono‐ and polyvanadates, that are main forms in such solutions, cause a decrease in corrosion current density and significantly increase amplitude of impedance spectra and impedance modulus |Z| values due to mixed cathodic‐anodic inhibition effect mainly associated with the formation of adherent surface film on cathodic and anodic intermetallic particles of alloy.     

Stress corrosion cracking of 2205 duplex stainless steel in H<Subscript>2</Subscript>S–CO<Subscript>2</Subscript> environment

Stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in H₂S–CO₂ environment was investigated by electrochemical measurements, slow strain rate test (SSRT), and scanning electron microscopy (SEM) characterization. Results demonstrated that the passive current density of steel increases with the decrease of solution pH and the presence of CO₂. When solutions pH was 2.7, the steel SCC in the absence and presence of CO₂ is expected to be a hydrogen-based process, i.e., hydrogen-induced cracking (HIC) dominates the SCC of the steel. The presence of CO₂ in solution does not affect the fracture mechanism. However, the SCC susceptibility is enhanced when the solution is saturated simultaneously with H₂S and CO₂. With elevation of solution pH to 4.5, the hydrogen evolution is inhibited, and dissolution is involved in cracking process. Even in the presence of CO₂, the additional cathodic reduction of H₂CO₃ would enhance the anodic reaction rate. Therefore, in addition to the hydrogen effect, anodic dissolution plays an important role in SCC of duplex stainless steel at solution pH of 4.5.     

吗啉多元胺对混凝土钢筋的阻锈作用

迁移性阻锈剂对钢筋混凝土中的金属材料有很好的保护作用,国内这方面的研究报道较少.采用动电位极化曲线和电化学阻抗谱研究了吗啉多元胺在模拟混凝土空隙液及混凝土中对钢筋的保护作用.在含有2 000mg/L NaCl的模拟液中,吗啉多元胺可以提高钢筋的抗点蚀能力.吗啉多元胺的加入导致混凝土钢筋的腐蚀电位正移,对钢筋的阴极和阳极电化学过程均有抑制作用,它是一种混合型缓蚀剂.吗啉多元胺能够在混凝土中扩散迁移,在钢筋表面形成吸附膜,阻挡侵蚀性离子对钢筋表面的侵入,抑制钢筋腐蚀电化学反应的进行.