Effect of cathodic protection on corrosion of pipeline steel under disbonded coating

In this work, a test rig was developed to study the effect of cathodic protection (CP) on corrosion of X70 pipeline steel in the crevice area under disbonded coating through the measurements of local potential, solution pH and dissolved oxygen concentration. Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.     

Effect of alternating current on cathodic protection on pipelines

In this work, the effects of alternating current (AC) on the performance of cathodic protection (CP) and the CP potential readings were investigated on a 16Mn pipeline steel in a simulated soil solution. The presence of AC interference decreases the CP effectiveness to protect the steel from corrosion. Only when CP potential is sufficiently negative, the steel is under a complete protection even when the AC current density is 400 A/m². Moreover, the AC would shift CP potential from the designed value. The effect of AC on the CP performance depends on the cathodic potential applied on the steel.     

Conventional and pulse cathodic protection of reinforced concrete: Electrochemical behavior of the steel reinforcement after corrosion and protection

The applicability and efficiency of an alternative for impressed current cathodic protection (CP) for reinforced concrete, based on pulse technology, was investigated. The technique, denoted as pulse CP (pCP), was evaluated on the basis of a comparative analysis to reference (non-corroding), corroding, and conventional CP conditions, in terms of long-term monitoring of electrochemical parameters for the embedded steel with time of corrosion and protection. The hereby reported results are for the total duration of the experiment, i.e., 460 days of conditioning, also presented in comparison with earlier stages. Protection was applied after corrosion was initiated (using corrosion medium of 5% NaCl), at different time intervals (here reported are starting points 60 and 150 days of age). Both CP regimes used current density of 5 or 20 mA/m² steel surface. The pulse CP was applied as a pulse-shaped block current (square wave) with the current itself being the feedback control signal, using 12.5–50% duty cycle at 500 Hz to 1 kHz frequency. Under equal environmental conditions and for a comparatively long period of application, the pulse CP was found to perform as effectively as the conventional CP with regard to electrochemical behavior of the steel reinforcement. Furthermore, the pulse CP technique was found to achieve more rapidly the so called “open circuit potential (OCP) passivity” as a result of an enhanced ion transport (chloride withdrawal) and more favorable cement chemistry (increased alkalinity around the steel reinforcement).     

Microbiologically influenced corrosion of underground pipelines under the disbonded coatings

The microbiologically influenced corrosion (MIC) of underground pipeline was identified. Corrosion was typically found under the disbonded area of girth welds coating where cathodic protection (CP) current could not penetrate. Sulfate reducing bacteria (SRB) and fermentative acid producing bacteria (APB) were confirmed as the microbes involved in the corrosion process. This corrosion occurred despite the fact that the CP level was well below the criteria of -850 mV (Cu/CuS0₄). Detailed field surveys and laboratory analysis revealed the presence of high numbers of microbes at these sites, metallurgical and mineralogical fingerprints of MIC. The results indicate that the synergistic effects of disbonding, the ineffective input of CP current under the disbonded coatings, and environments favorable to bacterial growth were the corrosion mechanism.     

剥离涂层下管线钢腐蚀研究进展

综述了在有阴极保护的条件时剥离涂层下缝隙内电位电流的分布和化学环境的变化、阴极保护防止缝隙腐蚀的机理、微生物腐蚀的机理以及前人提出的关于剥离涂层下微生物腐蚀与阴极保护相互作用的规律,并展望了对于解决剥离涂层下微生物腐蚀难题需进一步的实验研究.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

Integral diagnostic in the failure causes of external corrosion of a natural gas transport pipeline

The objective of this study consisted in investigating the possible causes which give rise to the presence of low wall pipe thicknesses on a 16″ natural gas transport pipeline, even though during the last 12‐year period cathodic protection (CP) potentials were kept in the protection range at which external corrosion should not occur. Results from in‐line inspection from a 16″ natural gas transport pipeline showed 46 indications with more than 80% wall thickness lost due to external corrosion in the second segment of the pipeline. Direct inspection at the indication locations, review of the CP system performance, pipeline maintenance programs and studies, allowed to make an integral diagnostic where it was found out that the main cause of external corrosion was an inappropriate coating application since the pipeline construction, this situation has originated the increase of CP shielding effects through time.     

Characterization of corrosion of X65 pipeline steel under disbonded coating by scanning Kelvin probe

Corrosion of X65 pipeline steel under a disbonded coating was studied by scanning Kelvin probe measurements. Three types of specimen were designed and prepared to investigate the effects of immersion time, oxygen concentration and wet-dry cycle on Kelvin potential profile and thus corrosion behavior of the steel. Kelvin potential measured on “intact” area is shifted negatively with time, indicating an increasing water uptake under the “intact” coating. With the increase of the amount of solution, it is expected that the electrolyte concentration and electrochemical reaction rate change, resulting in a significant decrease of interfacial potential. Moreover, there is a more negative Kelvin potential on disbonded area than that on “intact” area. The negative shift of Kelvin potential is attributed to corrosion reaction of steel occurring under the disbonded coating. Due to the narrow geometry of coating disbondment, an oxygen concentration difference exists along the depth of the disbondment. The corrosion behavior under disbonded coating strongly depends on the oxygen partial pressure and local geometry. With continuous purging of nitrogen and removing of oxygen, Kelvin potential tends to be identical throughout the disbonded area. During wet-dry cycle, the thickness of solution layer trapped under disbonded coating decreases due to evaporation of water. With the decrease of solution layer thickness, the measured Kelvin potential decreases, indicating that the effect associated with the reduction of oxygen solubility in the concentrated solution during drying of electrolyte is favored over that related to the enhanced oxygen diffusion and reduction. There exists a critical thickness of solution layer, below which the oxygen solubility is sufficiently low to support the electrochemical corrosion reaction of steel.     

Effect of CO2 and R-ratio on near-neutral pH stress corrosion cracking initiation under a disbonded coating of pipeline steel

This study investigates the role of CO₂ and cyclic stress R-ratio (R = minimum stress/maximum stress) on near-neutral pH SCC initiation mechanism(s) under a disbonded coating of pipeline steel protected by cathodic protection (CP). It was found that depending on CO₂ concentration and level of CP, different localized environments with various pH could be formed under the disbonded coating. When cyclic loading was applied, different SCC initiation mechanisms were involved depending on the pH of the localized environments. Reducing the R-ratio had different effects on the initiation mechanisms.     

Characterization of electroless Ni-based alloys for use in bipolar plates of direct methanol fuel cells

In this study, an electroless Ni-P deposit is employed as an anodic coating layer to protect the metallic bipolar plate of a direct methanol fuel cell (DMFC). An inductively coupled plasma-mass spectrometer, grazing incidence X-ray diffractometer, and potentiodynamic polarization measurements were used to study changes in the composition, crystalline structures and corrosion resistance of the Ni-P deposits at various bath pHs and temperatures. The crystalline structure of the Ni-P deposits was observed to change from crystal to amorphous as the P content increased, thus enhancing the corrosion resistance of the plate. The optimal conditions for obtaining the highest P content were found to be at pH 4.3 and 70 °C. Furthermore, the potentiostatic test for the Ni-P deposits prepared under the optimal condition was performed in a simulated anode working environment (0.5 M H₂SO₄ + 10 vol.% methanol), with the test showing that a negative corrosion current was observed at all times, therefore indicating that cathodic protection was employed throughout. Even after 10 h of potentiostatic treatment, no metal ions were found in a test solution. In addition, the result of a DMFC performance test demonstrated that bipolar plates using an anticorrosion coating of Ni-P deposits obtained a lower bulk resistance and an enhanced cell performance when compared to commercially available plates. Hence, the low-cost electroless Ni-P deposit demonstrates high potential for use as a corrosion protection layer in a DMFC bipolar plate with a Cu interlayer.     

Corrosion behavior of API 5L X‐60 pipeline steel exposed to near‐neutral pH soil simulating solution

Steel gas pipelines are exposed externally to damage by surface corrosion and cracking phenomena. They are the main deterioration mechanism under coating failure and cathodic protection (CP) that can reduce the structural integrity of buried gas transmission pipelines where the soil aggressiveness and bacterial activity appear. Corrosion phenomenon is accentuated by the soil parameters influence such resistivity, pH, temperature, moisture content and chemical composition of electrolytes contained in the soil. Soil parameters influence on pipeline steel corrosion behaviour exposed in near‐neutral pH soil simulating solution has been investigated by potentiodynamic polarisation and EIS method. Results showed that the steel corrosion increases, corrosion current density increases with temperature in the range from 20 to 60 °C. The associated activation energy has been determined. Impedance curves showed that the charge transfer resistance (R_t) increases with increasing immersion duration. Parameters such as corrosion current density (I_corr), polarization resistance (R_p), and soil resistivity (ρ) can serve as the parameters for evaluation of soil corrosivity.     

Kinetic mechanisms for cut edge corrosion of coil coated architectural cladding

Abstract

Cyclic dip testing of coil coated architectural claddings was carried out in order to understand the mechanisms of cut edge corrosion. Thus, a number of nominally identical materials from the same manufacturer, but with varying steel thickness (and, hence, zinc to steel area ratios at the cut edge) were examined. The degree of disbondment of the organic coating from the cut edge was measured after 1000 h. For a constant thickness of zinc coating, the results demonstrate a linear relationship between the degree of coating disbondment and the thickness of the steel substrate. Further, the trend line showed a positive amount of disbondment when ex trapolated to zero thickness of steel. It is concluded that the cathodic disbondment mechanism plays an important role in the initiation of organic coating failure at the cut edge. However, the kinetics of cut edge disbondment is dominated by the anodic undermining mechanism.     

酸性土壤环境中剥离涂层下X80钢应力腐蚀行为及机理

目的研究酸性土壤环境中剥离涂层下X80管线钢应力腐蚀行为及机理。方法采用电化学极化曲线测试、慢应变速率拉伸试验和腐蚀形貌扫描电子显微镜观察,对服役于鹰潭土壤环境的X80管线钢在剥离涂层下滞留液中的应力腐蚀行为及机理进行了分析研究。结果 X80管线钢在剥离涂层下的滞留液中具有一定的SCC敏感性,应力腐蚀开裂类型属于TGSCC,敏感性较大位置为近漏点处、剥离区中下部及剥离区底部,且近漏点处滞留液体系中X80钢的SCC机理受阳极溶解(AD)机制控制,剥离区底部滞留液中SCC机理受阳极溶解+氢脆(AD+HE)的混合机制控制。结论服役于酸性土壤中的X80管线钢在外防腐涂层破损后,除开放破损处将发生腐蚀外,剥离涂层下的管线钢还会存在一定的应力腐蚀敏感性。     

油气开采环境下管道的协同腐蚀及防护研究进展*

服役于高温高压 CO₂ / H₂S 环境下的管道腐蚀是油气田中急需解决的重要问题。CO₂、H₂S 及 Cl^- 是油气田管道中常见的腐蚀介质，其与温度、压力、pH值、含水率、流速等外界因素间的协同腐蚀作用会导致管道严重腐蚀，研究这些腐蚀介质与外界因素的协同腐蚀机制以及减缓管道腐蚀的措施有着重要的科学意义和经济价值。针对油气开采过程中金属管道的腐蚀问题，综述了 CO₂、H₂S 及 Cl^- 在协同腐蚀过程中起到的作用，讨论了温度、压力、pH 值、含水率及流速等实际工况条件下外界因素对腐蚀过程的影响。论述了现有管道腐蚀防护技术与工艺的特点：合金元素的掺杂可以改善腐蚀形貌，提高腐蚀产物层的致密性，等离子体扩渗与镀膜技术能够制备一层致密的保护层来吸收部分腐蚀介质并减缓腐蚀速率，缓蚀剂的添加可以减缓管道的阴极或阳极反应或形成减缓腐蚀速率的吸附层。最后展望了未来油气田管道防护技术的发展方向：为了有效地对油气开采环境下的管道进行保护，需要进一步研究腐蚀介质和外界因素间的协同腐蚀作用，模拟实际工况下的腐蚀环境， 对等离子体扩渗与镀膜技术、缓蚀剂等现有的防护技术进行系统的试验测试。     

套管穿越处的阴极保护

套管作为管道穿越处的保护管，在穿越时能对管道的防蚀层超到很好的保护作用。但在管道的运行过程中，由于套管的屏蔽作用，套管内的管段处于线路阴极保护系统的屏蔽区，随极保护电流不能对其起保护作用，使其成为线路防蚀的薄弱环节。针对这一情况，介绍了一种在套管内管段上补加特殊开头的牺牲阳极的方法，解决了这一特殊环境中的腐蚀问题。     

高膜厚PU防腐系统在油气管道上的应用

高膜厚PU防腐系统可解决防腐工程上的死角，包括异形、补口、三通、弯头、法兰、阀门等几何形状复杂金属外表面的防腐，以及老龄在役管线的防腐结构的修复。这种防腐系统的使用寿命可达30a。对国内油气管道防腐涂层常用的国产环氧粉末防腐结构和高膜厚PU防腐结构各项技术性能指标进行了对比，又以环氧粉末和高膜厚PU防腐系统用于管道防腐和储罐防腐进行了详细的对比，论证了使用高膜厚PU防腐系统技术的可行性和经济性。     

杂散电流干扰和阴极保护作用下碳钢腐蚀规律研究

目的探讨杂散电流和阴极保护二者共同作用对碳钢腐蚀的影响。方法在碳钢管表面手工涂刷涂层并制造小块破损点,研究Q235碳钢在涂层破损后,受单纯直流杂散电流干扰、单纯阴极保护以及二者共同作用时随时间变化的电化学交流阻抗图谱(EIS),通过图谱信息以及图谱数据拟合进行分析。结果所有条件下,Bode图低频阻抗和Nyquist图容抗弧半径都随时间延长而逐渐增加。通过图谱和数据拟合发现,单纯杂散电流条件下,杂散电流越大,电化学阻抗越小,浸泡15天时,20 m A杂散电流条件下的极化电阻达到200 m A条件下的4倍。阴极保护对杂散电流腐蚀具有防护作用,无论是单独施加阴保,还是杂散+阴保共同作用,-1000 m V(vs.CSE)与-850 m V(vs.CSE)横向对比,总是-1000 m V条件下的极化电阻更高。一定程度上,阴保电位越负,极化电阻越大,保护效果越好。结论在一定范围内,不论是单独施加,还是共同作用,总是杂散电流越小,阴极保护电位越负,对碳钢的保护效果越好,腐蚀程度越轻。利用电化学交流阻抗技术监测管道腐蚀状况是可行的。     

Mechanistic aspect of near-neutral pH stress corrosion cracking of pipelines under cathodic polarization

This paper investigates mechanistically stress corrosion cracking (SCC) of an X70 pipeline steel that is under cathodic protection (CP) in a near-neutral pH solution. It was found that there is a critical potential range, i.e., ?730 and ?920 mV_SCE, where the steel is in a non-equilibrium electrochemical state, and anodic dissolution (AD) reaction may occur when the steel is polarized cathodically. When the applied potential is more positive than this range, SCC is AD-based; while the applied potential is more negative, SCC of pipelines is under hydrogen embrittlement (HE) mechanism. When the polarization potential is within the range, SCC of the steel is under the combined effect of AD and HE. Therefore, AD may still occur on pipeline steel that is under CP with the potential within this critical range, contributing to the cracking process.     

Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment

A novel test setup has been used in this study to simulate stress corrosion cracking initiation under a disbonded coating on an X-65 pipeline steel. In this setup, the synergistic effects of cyclic loading, cathodic protection and soil solution environment under disbonded coatings have been considered. When the X-65 pipeline steel was exposed to the test environment, there existed a wide range of corrosion products on the steel surface in the gradient of cathodic protection. Increasing the test time and the maximum stress increased the possibility of stress corrosion cracking initiation in regions with a high susceptibility to pitting corrosion.     

油气管线钢土壤环境硫酸盐还原菌腐蚀研究进展

结合国内外埋地管线钢微生物腐蚀的研究,综述了腐蚀性土壤微生物种类和特点、环境因素对硫酸盐还原菌腐蚀的影响、生物腐蚀研究方法和进展,以及微生物腐蚀防护与监检测技术.最后,对埋地管线钢微生物腐蚀研究进行了展望.埋地管线钢服役环境复杂,受到土壤类型、杂散电流、阴极保护、应力、剥离涂层和微生物等多种因素的影响,而各种因素之间又存在着相互的耦合作用.多因素耦合作用下埋地管线钢微生物腐蚀将成为土壤微生物腐蚀今后的主要研究方向.土壤微生物腐蚀研究涉及土壤学、材料学、腐蚀科学和微生物学等多学科,是一个多学科交叉的研究课题,而化学和电化学分析技术、微生物分析技术以及材料表征技术等的联用也将为土壤微生物腐蚀行为和机制的研究提供更多的研究方法,这也有助于更好地理解微生物/材料之间的相互作用机制.随着对微生物腐蚀研究的深入,人们对硫酸盐还原菌腐蚀机理的认识也更加全面,"生物阴极催化还原"理论从生物能量学和生物电化学角度解释了微生物腐蚀的过程和机理.抗菌涂层开发和耐微生物腐蚀管线钢研发为MIC防治提供了一个新的研究路径.