A numerical study of damage caused by combined pitting corrosion and axial stress in steel pipes

Localized pipe wall damage accounts for many failures. Numerical modelling of pipes under increasing axial load and constant internal pressure when there is corrosion pits on the exterior surface of the pipe is reported herein. It is shown that for the assumed ideal elastic–plastic material the shape and volume of the plastic field depend on pit depth and its geometry. Pipe wall fracture around a pit can be associated with a critical plastic section. The results reported herein should be relevant for estimating of the risk of perforation and of loss of contents for steel pipes under different loading.     

Empirical model for dissolved oxygen depletion during corrosion of drinking water copper pipes

Predictive models characterizing the evolution and interaction of key parameters of water chemistry are needed to better understand corrosion events in drinking water pipes. We performed experiments with new copper pipes under combinations of pH, dissolved oxygen (DO), temperature, chlorine, and dissolved inorganic carbon. We found that DO consumption during 24 h of stagnation was not limited by diffusion, thus the DO consumption rate in the bulk water could be used to probe the processes occurring at the pipe surface. We propose a quantitative dependency of the DO consumption rate on a rather limited set of physicochemical parameters.     

-100℃低温无缝钢管的开发

通过对化学成分设计,炼钢、轧制、热处理等工艺控制,保证了-100℃低温无缝钢管的低温冲击韧性以及抗腐蚀性能。重点指出使用无缝钢管加工成的弯头产品要进行匹配的热处理,以保证弯头的质量不低于母材。开发的低温无缝钢管具有尺寸范围广、优良的低温韧性和抗腐蚀性能等优点,保证了其在低温环境下使用的安全性。     

长庆油田某采油厂集输管道内腐蚀原因及腐蚀机理

采用光学显微镜、扫描电镜(SEM)、能谱仪(EDS)、X射线衍射仪(XRD)等对长庆油田6个典型区块管道腐蚀失效的原因进行了分析,并对腐蚀机理进行了探讨。结果表明:6个典型区失效管道的组织为铁素体和少量的珠光体,元素分布均匀,化学成分符合20#钢的标准规定。1号、3号、4号、5号、6号典型区块腐蚀穿孔管节腐蚀产物的主要成分为铁的氧化物(Fe2O3、Fe3O4)。2号典型区块腐蚀穿孔管节腐蚀产物的主要成分为铁的氧化物(Fe2O3、Fe3O4)和铁的硫化物(FeS)。1号、4号、5号、6号典型区块管道主要发生了二氧化碳腐蚀。2号典型区块管道发生了二氧化碳和硫化氢腐蚀。3号典型区块管道发生了二氧化碳和氧腐蚀。     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.     

Influence of Ce surface treatments on corrosion behaviour of A3xx.x/SiCp composites in 3.5 wt.% NaCl

The influence of silicon carbide particles (SiCp) proportion and matrix composition on aluminium metal matrix composites A3xx.x/SiCp modified by cerium-based conversion or electrolysis coating was evaluated in 3.5 wt.% NaCl at 22 °C using potentiodynamic polarization and gravimetric measurements. Ce-treated surfaces presented better corrosion behaviour in chloride media than original composite surfaces without treatment. Both treatments preferentially covered the intermetallic compounds and SiCp. The electrolysis afforded a higher degree of protection than conversion treatment because the coating was more extensive. Coating microstructure and nature of corrosion products were analysed by scanning electron and atomic force microscopy (SEM, AFM) and low angle X-ray diffraction (XRD).     

Investigation of the corrosion behaviour of AA 2024-T3 in low concentrated chloride media

Aluminium alloy (AA) 2024-T3 is an important engineering material due to its widespread use in the aerospace industry. However, it is very prone to localized corrosion attack in chloride containing media, which has been mainly associated to the presence of coarse intermetallics (IMs) in its microstructure. In this work the corrosion behaviour of AA 2024-T3 in low concentrated chloride media was investigated using microscopy and electrochemical methods. TEM/EDS observations on non-corroded samples evidenced the heterogeneous composition within the IMs. In addition, SEM observations showed that intermetallics with the same nominal composition present different reactivity, and that both types of coarse IMs normally found in the alloy microstructure are prone to corrosion. Moreover, EDS analyses showed important compositional changes in corroded IMs, evidencing a selective dissolution of their more active constituents, and the onset of an intense oxygen peak, irrespective to the IM nature, indicating the formation of corrosion products. On the other hand, the results of the electrochemical investigations, in accordance with the SEM/EDS observations, evidenced that IMs corrosion dominates the electrochemical response of the alloy during the first hours of immersion in the test electrolyte.     

Corrosion properties of Nd:YAG laser-GMA hybrid welded AA6061 Al alloy and its microstructure

The paper presents the corrosion behaviour of the Nd:YAG laser-gas metal arc (GMA) welds of AA6061-T6 alloy. The laser-GMA hybrid welding enhances the corrosion susceptibility of AA6061 alloy. The surface morphology observation and composition analysis were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. An increasing of the precipitate phase is observed in the weld fusion zone (WFZ). The WFZ suffers more severe pitting and cracks are associated with pitting. It is proposed that the increased precipitate phase increases the galvanic corrosion couples and results in the aggravation of pitting and cracking in the WFZ.     

Effect of biofilm on cast iron pipe corrosion in drinking water distribution system: Corrosion scales characterization and microbial community structure investigation

Effect of biofilm on corrosion scales of cast iron pipe was studied with the biofilm community structure investigated by PCR-DGGE to give an explanation to MIC from the viewpoint of microbial phase. Corrosion scales were identified with XRD and XPS. It was demonstrated that biofilm can greatly affect element composition and crystalline phase of corrosion scales. Biofilm can accelerate corrosion in 7 d, but inhibit corrosion after 7 d, which was due to iron bacteria and iron reducing bacteria (IRB), respectively. DGGE fingerprinting gave a well explanation to this transition, which might be contributed to the change of biofilm microbial diversity.     

Corrosion of friction stir welded magnesium alloy AM50

The microstructure of a friction stir welded magnesium alloy AM50 was examined by means of optical light microscopy. The chemical composition, particularly the iron content, and morphology of the oxide film were analyzed and discerned via auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Corrosion behaviour of the welds and base materials were investigated by virtue of neutral salt spray tests and potentiodynamic polarization measurements in conventional cells and in a mini cell. The results demonstrate that minor increases in iron concentration as might be speculated to occur as a consequence of tool/work piece interaction during the welding process on the corrosion resistance of the weld can be ignored. The corrosion morphology was predominantly influenced by the distribution of the Mg₁₇Al₁₂ phase. Here, it was also found that the corrosion resistance of the friction stir weld varied in response to changes in the joint microstructure.     

Microstructure and corrosion of Pd-modified Ti alloys produced by powder metallurgy

A method for the fabrication of titanium alloy parts with enhanced corrosion resistance by a powder metallurgy route is presented in this paper. Commercial purity titanium powders modified with Pd have been hot isostatically pressed (HIPped) and the microstructure and distribution of the noble metal characterised by optical and scanning electron microscopy. The electrochemistry of the HIPped alloy has been assessed and the effect of powder size fraction evaluated. Results show that the phase composition and electrochemistry of the HIPped Pd-modified alloy is equivalent to that of wrought grade 7 Ti.     

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.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

Morphology and phase composition of corrosion products formed at the zinc-iron interface of a galvanized steel

The corrosion products formed on the inner wall of pipes made of galvanized low carbon steel, exposed for ∼2 years to water flowing in a large household heating system, were analysed using X-ray diffraction, Mössbauer and Raman spectroscopic techniques, as well as metallographic techniques. Products grew in the form of large-sized tubercles that gradually developed causing base metal losses up to perforation of the steel pipe. Considerable differences in the phase composition were found between the products formed in contact with the steel and those constituting the outer part of tubercles. The former were mainly made of FeCO₃ (siderite), with small amounts of Zn₅(CO₃)₂(OH)₆ (hydrozincite), ZnCO₃ (smithsonite), (Fe,Zn)CO₃ mixed carbonate and CaCO₃ (calcite), the latter mainly by Fe(III) oxyhydroxide goethite. Both parts of the tubercles also contained small amounts of other ferric oxyhydroxides, γ-FeOOH (lepidocrocite) and β-FeOOH (akaganeite), and very small amounts of hematite. The procedures used proved effective for an adequate identification of both the iron-containing and iron-free compounds in the corrosion products as well as for suggesting a corrosion mechanism.     

基于改进BP神经网络优化的管道腐蚀速率预测模型研究

目的构造金属管道腐蚀速率预测模型,预测管道的使用寿命。方法分析了二氧化碳(CO2)和硫化氢(H2S)对金属管道的腐蚀过程,给出了管道腐蚀的化学反应方程式。引用了BP神经网络构造金属管道腐蚀速率的数学模型,采用了改进粒子群算法对预测模型进行优化。以45号金属管道为例,借助于Matlab软件对管道腐蚀速率进行仿真验证,并与实验测量数据进行对比和分析。结果金属管道腐蚀速率随着CO2或H2S压强的增大而逐渐增大,仿真结果显示CO2和H2S的最大腐蚀速率分别为7.20×10-5 mm/h和5.76×10-5mm/h,而实验测量结果显示CO2和H2S的最大腐蚀速率分别为7.14×10-5 mm/h和5.65×10-5 mm/h,采用改进BP神经网络预测模型所产生的相对误差在5%以内。结论金属管道在不同压强条件下,采用改进BP神经网络预测模型能够近似地预测其腐蚀速率,为金属管道的更换提供了参考依据。     

The effect of T4 heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy

The effect of heat treatment on the microstructure and corrosion behaviour of Zn27Al1.5Cu0.02Mg alloy was examined. The alloy was prepared by melting and casting route and then thermally processed (T4 regime). Corrosion behaviour of the as-cast and heat treated alloy was studied in 3.5 wt.% NaCl solution using immersion method and electrochemical polarization measurements. The applied heat treatment affected the alloy microstructure and resulted in increased ductility and higher corrosion resistance of the heat treated alloy. Electrochemical measurements of the corrosion rate at the free corrosion potential are in agreement with the results obtained using the weight loss method.     

Corrosion characteristics of plasma-sprayed Ni-coated WC coatings comparison with different post-treatment

The effects of vacuum annealing and laser remelting on the microstructure and corrosion behaviour of plasma-sprayed Ni-coated WC coatings on steel substrate have been investigated. The laser remelting was operated in a continuous way while the vacuum annealing was operated with clamping the coating on the graphite face in order to avoid decarburization of WC. When compared with the as-sprayed coating, the microstructure of the post-heating treatment coatings has been found to consist of different phases. Moreover, the denser microstructure can be obtained after heating treatment, especially the laser remelting coating. Electron probe micro analyzer (EPMA) shows that the chemical composition remained largely unchanged except the “bumps” at the interface for as-sprayed and vacuum annealing coatings. The more uniform composition was obtained for laser remelting coating. The Vickers microhardness measurement shows a very slightly enhancement for post-heating treatment coatings, which may be duo to the lamellar structure, lower contemt and bulky of carbide for coatings. However, salt spray corrosion (SSC) show the laser remelting coating has the best corrosion resistance, which is due to its low number defects and uniform distribution of the phase and composition.     

Study of the corrosion product films formed on the surface of Mg–xZn alloys in NaCl solution

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.     

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.     

Influence of anodising current on the corrosion resistance of anodised AZ91D magnesium alloy

The thickness, chemical composition and microstructure of anodised coatings formed on magnesium alloy AZ91D at various anodising current densities were measured. It was found that all these parameters could be affected by anodising current density, and hence the coatings formed at different anodising current densities had different corrosion resistances. This suggests that the corrosion performance of an anodised coating could be improved if a properly designed current waveform is used for anodising. In addition, based on the experimental results, some physical, chemical and electrochemical reactions involved in the anodising process were proposed to explain the anodising behaviour in this paper.