CPB和TU对X70钢在含SO_2酸性溶液中的缓蚀作用

采用交流阻抗法和极化曲线法研究了溴代十六烷基吡啶(CPB)和硫脲(TU)在复配前后对X70钢在H2SO3溶液中的缓蚀作用.研究发现:H2SO3的浓度为10 mmol/L、pH=3.7的HAc-NaAc缓冲体系中,溴代十六烷基吡啶(CPB)与硫脲(TU)的缓蚀率随着浓度的增加而增加,溴代十六烷基吡啶(CPB)的浓度达到5×10-6kg/L时的缓蚀性能最佳,而硫脲(TU)的浓度达到50×10-6kg/L时的缓蚀性能最佳.复配实验结果表明:当缓蚀剂的总浓度为25×10-6kg/L,溴代十六烷基吡啶(CPB)和硫脲(TU)复配质量浓度比为1∶1时缓蚀效果最好.

X70管线钢CO_2湿气顶部的腐蚀行为

采用高温高压冷凝反应釜模拟CO2湿气顶部腐蚀环境,研究了X70钢在不同腐蚀时间和温差条件下的腐蚀行为。采用SEM和EDS等分析方法究了腐蚀产物膜的宏观、微观形貌和结构特征,明确了湿气条件下腐蚀产物膜与局部腐蚀之间的关系,探讨了CO2湿气顶部腐蚀机理。结果表明,在不同的腐蚀时间条件下,X70钢的局部腐蚀与腐蚀产物膜脱落位置呈对应关系;在不同温差条件下,当腐蚀产物膜达到一定厚度后,腐蚀产物膜上也可能存在温度梯度,从而导致腐蚀膜更容易发生局部脱落。

直流电作用下X70钢在砂土中腐蚀的电化学行为

采用极化曲线和电化学阻抗谱方法,研究了在砂土中直流电通电时间对X70钢腐蚀行为的影响,分析了直流电条件下X70钢在砂土中的腐蚀机理。结果表明,直流电加速了X70钢在砂土中的腐蚀,通电时间越长,腐蚀速率总体越大,腐蚀程度不断深入;整个通电过程中,自腐蚀电位经历了先正移后负移再正移的过程,砂土中孔隙水、含氧物质及腐蚀性离子都参与了电极反应;随着腐蚀的进行,阻抗谱的时间常数由两个变为三个再退化为两个,经历了感抗弧出现和消失的过程。

中性环境中锈斑对X70/X80钢早期腐蚀的作用

采用NS4典型中性土壤模拟溶液模拟中性土壤环境,将X70和X80管线钢电化学试件在模拟溶液中进行短期浸泡,发现溃疡状锈斑均匀分布在试件的工作表面,锈迹疏松易脱落,微观呈现颗粒状形貌,面积较大的锈斑处出现浅腐蚀坑。对带锈件和不带锈件进行了极化曲线、交流阻抗等电化学实验并进行了对比。对比结果发现,金属界面早期锈斑的产生加重金属的腐蚀,加快腐蚀速率。这是因为早期锈斑的不完整性与稀疏性降低金属基体与腐蚀环境电化学反应的极化电阻,其粗糙的表面增加试件的工作面积,增大腐蚀电流。

CPB和TU对X70钢在硫酸溶液中的缓蚀行为的电化学研究

采用交流阻抗法和稳态极化曲线法研究溴代十六烷基吡啶(CPB)与硫脲(TU)复配对X70钢在0.5mol/L硫酸溶液中的缓蚀作用.研究发现:随着CPB浓度的增加,缓蚀率增加,在浓度为10mg/L时,出现极值现象;CPB与TU复配后缓蚀率提高.

X70管线钢在海水中的腐蚀特性与防腐涂层性能研究

研究常温和40℃的温度环境下,X70钢在模拟海水中的腐蚀性能。介质为模拟海水,分别对表面抛光和表面带有防腐涂层的X70钢试样进行浸泡腐蚀试验,对其产物观察取样并进行SEM和XRD扫描,得到其腐蚀垢层微观形貌图和成分分析图。对X70钢进行电化学腐蚀测试,结果表明,同温度下,腐蚀电流密度随着浓度的增加而增大,腐蚀速率随着浓度的增加而增大;同浓度下,腐蚀电流密度随着温度的升高而增加,腐蚀速率随着温度的升高而增大。对X70钢在海水条件下的腐蚀速率、腐蚀产物以及防腐涂层性能等进行了全面的分析,所得数据真实可靠,为海水所用X70钢的腐蚀性能分析及防腐方法提供了理论和试验依据。

剥离涂层下X70钢阴极极化效果试验研究

在总结现有国内外相关研究试验装置与试验研究方法的基础上,自行设计组装了新的试验装置,该试验装置能够较好地模拟工程实际中发生轴向涂层缺陷的埋地管道的腐蚀介质环境;同时通过开路电位测试,针对模拟破损、剥离涂层下的金属基体,就通电点通电、调压、断电、断电后再通电过程中表面的电位分布进行了试验研究。结果表明,阴极极化的增大在一定程度上增大缝隙深处极化效果的同时,也增大了缝隙内部的电位梯度;此外,阴极保护停运会造成缝隙内部金属短时间内腐蚀的发生,以及阴极保护再启动后阴极极化效果的降低。

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. Constaat 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.

STUDY OF THE X70 PIPELINE STEEL CORRODING IN 3.0wt% NaCl SOLUTION USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY TECHNIQUE

The corrosion process of the X70 pipeline steel in 3.0wt% NaCl solution were studied using polarization method, and the chronological characteristics during the entire polarization plot were investigated in detail using EIS technique. In the active region of X70 steel, such as 20mV potential bias applied on open circuit potential (OCP), the impedance spectra was comprised of three parts: a high-frequency capacitive loop, a middle-frequency capacitive loop and a low-frequency inductive component. When positive polarization potential increased, the capacitive loops at high and middle frequency range merged, and the inductive component at low frequency shrunk. At high positive polarization potential bias (500-800mV vs. OCP), the high-frequency capacitive loop and the low-frequency inductive loop exhibited as disheveled points due to the synergism of the inhomogeneity of the corroding material and the localized corrosion. The results were fitted utilizing the equivalent circuits to simulate the impedance spectra and to interpret the electrochemical features shown during the experiments.

Effect of deteriorated microstructures on stress corrosion cracking of X70 pipeline steel in acidic soil environment

In order to investigate stress corrosion cracking （SCC） of X70 pipeline steel and its weld joint area in acidic soil environ- ment in China, two simulating methods were used： one was to obtain bad microstructures in heat affected zone by annealing at 1300 ℃ for 10 min and then, quenching in water; the other was to get different simulating solutions of acidic soil in Yingtan in south- east China. The SCC susceptibilities of X70 pipeline steel before and after quenching in the simulating solutions were analyzed using slow stain rate test （SSRT） and potentiodynamic polarization technique to investigate the SCC electrochemical mechanism of different microstructures further. The results show that SCC appears in the original microstructure and the quenched microstructure as the polarization potential decreases. Hydrogen revolution accelerates SCC of the two tested materials within the range of-850 mV to -1200 mV vs. SCE. Microstructural hardening and grain coarsening also increase SCC. The SCC mechanisms are different, anodic dissolution is the key of causing SCC as the polarization potential is higher than the null current potential, and hydrogen embrittlement will play a more important role to SCC as the polarization potential lower than the null current potential.

Effects of Cl- and SO_4^(2-) Ions on Corrosion Behavior of X70 Steel

[ Manuscript received October 8, 2003, in revised form March 2, 2004]Corrosion behaviors of X70 steel were studied by means of electrochemical experiments and morphology observation. First, through potentiodynamic polarization in solution of various Cl- ions concentration, it was found that Epit began to appear in solution of Cl- concentration above 0.1 mol/L, and there was a critical point of Cl- concentration between 0.05 mol/L and 0.1 mol/L, below which the extent of pitting and general corrosion were trivial, while in solution of Cl- concentration above 0.1 mol/L, general and pitting corrosion became greater as the increasing of Cl- concentration. All of them were confirmed by the SEM observations after anodic polarization. Second, via the potentiodynamic polarization curves of X70 steel in 0.5 mol/L Cl- solution with 0, 0.05, 0.5 and 1 mol/L SO42-ions, it was found SO42- ions were able to inhibit corrosion aroused from Cl- ions, accordingly a model was set up to describe the process. In addition, to further explore the inhibited effect of SO42- ions, EIS was used in solutions of different Cl- and SO42- concentrations, the results revealed that the electrochemical resistance has a relation with the [SO42-]/[Cl-], that was, the bigger the value of [SO42-]/[CI-], the greater the electrochemical resistance.

SCC of X70 and Its Deteriorated Microstructure in Simulated Acid Soil Environment

In order to study the stress corrosion cracking （SCC） of X70 pipeline steel and its weld joint in acid soil environment of southeast of China, two simulating methods were used here. The one was to obtain the bad microstructures in heat affected zone by annealing at 1300℃ for 10 min and air cooling to room temperature, the other was to get a series of simulating solutions of the acid soil environment. SCC susceptibilities of X70 pipeline steels＇before and after being normalized in the simulated solutions were studied by slow strain rate test （SSRT） and microstructural observation of fracture areas. Potentiodynamic polarization curves were used to study the electrochemical behaviour of different microstructures. SCC does occur to both the as-received material and normalized microstructure after heat treatment as the polarization potential decreased. Hydrogen embrittlement （HE） is indicated occurring to all tested materials at -850 mV （vs SCE） and -1200 mV（vs SCE）. The SCC mechanisms are different within varying potential range. Anodic dissolution is the key cause as polarization potential higher than null current potential, and HE will play a more important role as polarization potential lower than the null current potential.

DECREASING PITTING SUSCEPTIBILITY OF PASSIVE FILMS ON X70 PIPELINE STEEL IN NaCl SOLUTIONS BY ILLUMINATION

The influence of UV illumination on passivity and pitting susceptibility on X70 pipoeline steel in a borate buffer （pH=8.4 ） solution containing NaCl is described. It is observed that illumination of the sample leads to a decrease in its pitting susceptibility as indicated by pitting potential and incubation time measurements in chloride containing electrolytes. This effect is strongly dependent on the applied potential during passivation. The electronic properties of the passive films on X70 steel were studied by Mott-Schottky analysis and photocurrent transient measurements. The results indicated that illumination during passivation led to modifications in the electronic properties of the passive films, mainly to a decrease of the bulk doping and an increase in the surface state density. The cause for the decrease in the pitting susceptibility is preliminary explained in terms of such modifications of the passive flm.

Study on the microstructure and hardness of in-service welded joint of X70 pipeline steel

Hydrogen induced cracking （HIC） is one of the main problems of in-service welding onto active pipeline. Microstructure and hardness of welded joint have a vital effect on hydrogen induced cracking. The microstructure and hardness of welded joint of XTO pipeline steel were studied using simulation in-service welding device. The results show that the main microstructures of in-service welded seam are grain boundary ferrite , intracrystalline acicular ferrite , as well as small amount of widmanztatten structure. The main microstructures of coarse grain heat-affected zone （CGHAZ） are coarse granular bainite, lath ferrite and martensite. Metastable phases such as martensite and lath ferrite are found in CGHAZ because of the too quick cooling velocity a＇nd the hardness of the CGHAZ is high.

Influence of yield-to-tensile strength ratio(Y/T) on failure assessment of defect-free and corroded X70 steel pipeline

The effect of yield-to-tensile strength ratio(Y/T) on failure pressure of X70 pipeline without and with corrosion defects was investigated.The stress-strain response of materials was characterized by a power-law hardening curve.Two formulas to estimate the strain hardening exponent n for a special Y/T were obtained by least squared regression method and the influence of Y/T on n was analyzed.As an application of n-Y/T expression,the analytical solutions of burst pressure for X70 pipeline without and with corrosion defects were also obtained.The results indicate that the burst pressure of defect-free X70 pipe without corrosion defects is a function of the Y/T,pipe geometry t0/D0 and engineering tensile strength,and increases as Y/T or t0/D0 increases; whilst the burst pressure of corroded X70 pipe decreases with the increase of defect depths,d/t.Comparisons indicate that the present analytical solutions closely match available experimental and numerical data.

Study on M/A islands in pipeline steel X70

The fine structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microt winning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.

Oblique Y-groove cracking test of the welding cold cracking susceptibility of domestic X-70 pipeline steel

Gas Transmitting From West to East Project' is significant. It should ensure the welding quality and safety of pipeline. The task is very arduous to guarantee the quality of the project in the condition of long line, complex weather and geology features. In this paper, the welding cold cracking susceptibility of domestic X 70 pipeline steel adopted by the project, which is one of the most interesting questions of welding quality about petrol pipeline, was studied by means of oblique Y groove cracking test. The crack ratio of surface and section was tested under the conditions of different welding materials and preheat temperature .The thickness of plate steel was 14.7 mm and 10.3 mm . The results reveal that X 70 pipeline steel has good crack resistance. The research has important value for the construction of large scale pipeline engineering and the application of domestic X 70 pipeline steel.

Properties of Surface Film on X70 Pipeline Steel in CO_3^(2-)-/HCO_3^- Environment

The electrochemical behavior of X70 pipeline steel in (0.5mol·L-1 Na2CO3+1 mol·L-1 NaHCO3) solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyze the composition and microstructure of the surface film. The results showed that there were two anodic peaks at -600 mV and -350 mV. The surface film formed at -600 mV mainly consisted of ferrous carbonates and ferrous hydroxycarbonates. It had a small reaction resistance. It was metastable and possessed poor protective property. Numerous pits and microcracks existed on the film, which could be the active paths for the initiation of stress corrosion cracking. The surface film formed at -350 mV, mainly consisted of ferric oxides. It has high reaction resistance and offered good protection for the substrate.

Influence of cooling conditions on X70 pipeline steel in-service welding HAZ

The chamber device was designed and set up to simulate the in-service welding. The results show ： the t8/5 , t8/3 and inner wall peak temperature Tp decrease with the cooling rate increases. The welding energy is carried off by flowing medium, the cooling rate increases, and many unbalanced microstructures such as granular bainite, martensite and M-A generate ; it worsens the properties of HAZ. Under air-cooling, the cooling rate is slow, the austenite grain grows obviously, the lath ferrite crosses the whole austenite, and it causes the hardness value is also big. The change of HAZ width is not obvious with the increase of cooling rate; and burn-through is not susceptible to the cooling rate. The quench microstructures increase and the hydrogen does not outflow from the HAZ easily when increase the cooling rate, so the susceptibility of hydrogen cracking increases.

Study of microstructures and properties for girth weldingof domestic X70 pipeline steels

With the development of domestic pipeline steels, it is necessary to develop suitable welding technology which can improve the properties of the welded pipeline. In this paper, the microstructures and mechanical properties of domestic XTO pipeline steels and welded joints are discussed. The welding consumables of BOHLER E6010 and HOBART 81N1 are matched for girth welding. The following characteristics in heat-affected zone（ HAZ） are indicated that microstructures of intercritical HAZ（ ICHAZ） is finer and more uniform, the grain sizes of fine-grain HAZ （ FGHAZ） and subcritical HAZ （ SCHAZ） are smaller than that of coarse-grain HAZ（ CGHAZ）. The hardness, tensile strength and toughness of welded joints come up to the standard. The micrographs of impact specimens in welded joints are cleavage, quasi-cleavage and dimple which shows there is typical ductile rupture.