Measuring and understanding the critical duration and amplitude of anodic transients

Anodic transients are known to affect buried pipeline cathodic protection (CP) systems; however, their actual effects on pipeline corrosion have not been sufficiently quantified due to the lack of direct experimental evidences. In this work, a novel methodology developed based on an electrochemically integrated multi-electrode array has been used to visualise the dynamic effects of anodic transients on localised corrosion processes occurring on buried steel surfaces. It is shown that anodic transients do not necessarily cause steel corrosion, as long as their amplitude and duration are below critical values. The electrode array is able to determine these critical values accurately due to its ability to detect localised corrosion initiation, while the conventional coupon electrode is only able to estimate these values. The critical anodic transient duration has been explained as the incubation period for the breakdown of passivity formed on the steel surface exposed to CP-generated high pH conditions.     

Novel corrosion experiments using the wire beam electrode: (II) Monitoring the effects of ions transportation on electrochemical corrosion processes

An electrochemically integrated multi-electrode system namely the wire beam electrode (WBE) has been applied for the first time to study the effects of the transportation of electrochemically active species on the process, rate and pattern of electrochemical corrosion. The objective of this work is to demonstrate the applicability of the WBE method for investigating ion transportation related corrosion processes. A series of experiments have been carried out using WBEs made from mild steel and stainless steel wires. The WBE working surfaces were exposed to simulated diffusion-controlled corrosion environments where there were diffusion induced ions concentration gradients (termed diffusion-corrosion environment). Corrosion potential and current distribution maps (CPCD maps) were measured from WBE surfaces in continuous bases. Typical patterns have been identified from CPCD maps and the characteristics of these patterns have been found to depend heavily upon the type of electrode material and the type of corrosive ion. For mild steel WBE surface exposed to a diffusion-corrosion environment containing NiSO₄ or FeCl₃, the characteristic pattern in CPCD maps was found to emulate NiSO₄ or FeCl₃ concentration gradients, suggesting an ion-concentration controlled corrosion behaviour. However, when the mild steel WBE surface was exposed to a diffusion-corrosion environment containing NaCl, the characteristic pattern was found to show higher cathodic currents along the WBE edges with the magnitude decreasing in a contour-like manner towards the centre of the WBE surface, suggesting an oxygen concentration-controlled corrosion behaviour. When a stainless steel (SS316L) WBE surface was exposed to a diffusion-corrosion environment containing NiSO₄ or NaCl, the corrosion pattern appeared to be mainly determined by the random distribution of weak sites in passive film. When the SS316L WBE was exposed to a diffusion-corrosion environment containing FeCl₃, the CPCD map revealed a characteristic pattern that shows localised damage to passive film. This work demonstrates that the recognition and analysis of characteristic maps from WBE measurements can be used as a means of studying diffusion, migration and other forms of mass transportation related electrochemical corrosion processes.     

Novel corrosion experiments using the wire beam electrode: (III) Measuring electrochemical corrosion parameters from both the metallic and electrolytic phases

The wire beam electrode (WBE) and the scanning reference electrode technique (SRET) have been applied in a novel combination to measure, for the first time, electrochemical parameters simultaneously from both the metallic and electrolytic phases of a corroding metal surface. The objective of this work is to demonstrate the application of this combined WBE-SRET method in obtaining unique information on localised corrosion mechanism, by investigating typical corrosion processes occurring over a mild steel WBE surface exposed to the classic Evans solution. The WBE method was used to map current and potential distributions in the metallic phase, and the SRET was used to map current or potential distribution in the electrolytic phase. It has been found that the combined WBE-SRET method is able to gain useful information on macro-cell electrochemical corrosion processes that involve macro-scale separation of anodes and cathodes. In such macro-cell corrosion systems, maps measured using WBE and SRET were found to correlate with each other and both methods were able to detect the locations of anodic sites. However the movement of the scanning probe during SRET measurements was found to affect the SRET detection of cathodic sites. In micro-cell corrosion systems where the separation of anodic and cathodic sites were less distinct, SRET measurement was found to be insensitive in detecting anodic and cathodic sites, while the WBE method was still able to produce results that correlated well with observed corrosion behaviour. Results obtained from this work suggest that the WBE-SRET method is applicable for understanding the initiation, propagation and electrochemical behaviour of localised corrosion anodes and cathodes, and also their dependence on externally controllable variables, such as solution pH changes and the existence of surface coatings.     

A new method of studying buried steel corrosion and its inhibition using the wire beam electrode

An electrochemically integrated multi-electrode system namely the wire beam electrode (WBE) has been applied for the first time to study corrosion of mild steel buried in sand, with and without the presence of corrosion inhibitor potassium dichromate (K₂Cr₂O₇). Measurements of galvanic current distribution maps have been carried out during the exposure of the WBE to dry, damp and chlorinated sand environments. Characteristic changes in galvanic current distribution maps have been observed during the initiation and propagation of localised corrosion. Specifically, during corrosion initiation in damped sand, new anodes were found to initiate and corrosion appeared to be in general form. When the WBE was later exposed to chlorinated sand, massive disappearance of anodic sites was found to occur, resulting in accelerated anodic dissolution of a small number of remaining anodic sites. Addition of corrosion inhibitor K₂Cr₂O₇ to the sand environment was found to significantly reduce galvanic current only after an initial increase in galvanic current. This result suggests that K₂Cr₂O₇ behaved as an anodic inhibitor.     

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。     

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.     

Novel corrosion experiments using the wire beam electrode. (I) Studying electrochemical noise signatures from localised corrosion processes

This series of papers presents four novel experiments that were designed to study localised corrosion phenomena using an electrochemically integrated multi-electrode array namely the wire beam electrode (WBE). This present paper reports a WBE based experimental method that has been employed, for the first time, to study electrochemical noise patterns (called noise signatures) from localised corrosion processes. The objective of this work is to demonstrate the applicability of the WBE for investigating the origin of spontaneous electrode potential/current fluctuations and their effects on electrochemical processes. The key strategy of this work is to apply the WBE in a novel experimental set-up to simultaneously measure electrode potential noise and WBE current distribution maps--an approach that allows the direct comparison and correlation of electrochemical noise and corrosion events. Preliminary experiments have been carried out using a classic pitting corrosion system: stainless steel in a solution containing FeCl₃. A large number of anodic sites were found to exist on WBE surface at the very beginning of its exposure to the corrosion environment. Correlation between characteristic patterns in electrode potential noise and corrosion behaviour has been observed. More specifically, the characteristic sharp peaks in potential noise data (called noise signature I) were found to correlate with the sudden disappearance of single unstable anode in WBE current distribution maps. The characteristic noise pattern of quick potential changes followed by partial or no recovery (called noise signature II) was found to correspond with the massive disappearance of minor anodes in WBE current distribution maps. This result suggests that, in the corrosion system under study, electrode noise activities were associated with the disappearance of minor anodic sites, which lead to the eventual disappearance of most anodic sites. Localised corrosion was the result of the accelerated anodic dissolution of a small number of remaining anodic sites. The characteristics features in electrochemical noise and in WBE maps were reproducible.     

Effects of alternating current interference on cathodic protection potential and its effectiveness for corrosion protection of pipelines

In this work, the effect of alternating current (AC) interference on cathodic protection (CP) potential on a X65 steel in a near-neutral pH bicarbonate solution was investigated, and the CP performance under AC was evaluated by weight-loss measurements. The CP potential applied on the steel cannot be maintained in the presence of AC interference. The shift of the CP potential depends on the applied CP level and AC current density. No matter if the direct current potential of the steel is shifted negatively or positively upon application of AC, the steel suffers from increased corrosion. The AC decreases the effectiveness of CP for corrosion protection. The CP standard at ?0.850?V (copper sulphate electrode) that does not consider the AC interference is not appropriate for AC corrosion protection.     

Effects of SRB on cathodic protection of Q235 steel in soils

The effects of sulfate reducing bacteria (SRB) on cathodic protection (CP) of the Q235 steel in the soils have been studied by bacterial analyses, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and energy‐dispersive X‐ray analysis (EDX). The results showed that the pH value of the soil around the steel gradually increased, the number of SRB and the corrosion rate of the steel decreased, and the CP efficiency increased with the increasing of applied cathodic potential. At the cathodic polarization potential of ?1050 mV, SRB still survived in the soils. At the same potential, the CP efficiency in the soil without SRB was higher than that with SRB, and the corrosion rate of the steel in the soil with SRB was much higher than that without SRB. The cathodic current density applied for the steel in the soil with SRB was bigger than that without SRB at the same cathodic potential.     

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.     

Assessment of cathodic protection effect on long-distance gas transportation pipelines based on buried steel specimens

Different severe corrosions are found at the damaged point of anticorrosive coating on natural gas pipelines, even though the cathodic protection (CP) potential is between −0.85 and −1.15 V. Therefore, there are totally 168 buried steel specimens (BSSs) and 28 soil samples at 28 buried points that are taken out in situ and then analyzed in the lab to assess the CP effectiveness. First, the CP status is simulated by BSSs at the damaged points, and the corrosion products on BSSs are analyzed microscopically. Second, the corrosion rates of the testing BSSs are calculated by the weight-loss method to perform the quantitative assessment. Third, the specific reasons for external corrosion are explored via analyzing the CP potential attenuation tendency and the corrosiveness of soil. The corrosion growth trend is also investigated by comparing the test results among three test cycles within 8 years. Finally, the control measures for external corrosion of pipelines are put forward. The study is beneficial to evaluate the CP system and determine the causes of external corrosion of pipelines, thereby formulating corresponding protective measures.     

土壤环境中管线钢硫酸盐还原菌腐蚀

腐蚀是世界各国共同面临的问题之一,每年因腐蚀造成的经济损失占全国GDP的3%~5%,其中土壤腐蚀约占总腐蚀的20%。金属的土壤腐蚀是一种自发的冶金逆过程,它不仅会导致埋地金属构筑物腐蚀破坏,还会引发管线泄漏、燃烧和爆炸等事故,给社会带来巨大的经济损失和社会危害。微生物腐蚀是埋地管线钢腐蚀的重要腐蚀类型之一,其中以硫酸盐还原菌引起的腐蚀最为严重。从环境因素、材料因素和微生物因素三个方面,对土壤环境中管线钢硫酸盐还原菌腐蚀进行了简要概述。埋地管线钢微生物腐蚀研究最多的是环境因素的影响,包括土壤类型、土壤含水量、土壤阴离子、化肥、农药、土壤宏电池和剥离涂层。材料因素的研究多集中在阴极保护、外加应力和杂散电流等因素的影响。相比前两种影响因素,微生物因素最为复杂,也是研究最少的一个方面。微生物因素的研究是一个全新的研究领域,包括膜内生物酶的影响以及膜内电子传递等。今后一段时间,埋地管线钢微生物腐蚀仍以环境和材料因素等多因素的耦合作用为主要研究方向。     

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.     

Early stage beneficial effects of cathodic protection in concrete structures

Over the last 25 years, cathodic protection (CP) of reinforced concrete structures suffering from chloride induced reinforcement corrosion has shown to be successful and durable. CP current causes steel polarisation, electrochemical reactions and ion transport in the concrete. CP systems are designed based on experience, which results in conservative designs and their performance is a matter of wait‐and‐see. CP systems can be designed for critical aspects and made more economical using numerical models for current and polarisation distribution. Previously, principles of numerical calculations for design of CP systems were reported. The results were satisfactory, except in terms of current density for active corroding systems. This was suggested to be due to neglecting beneficial effects of CP current flow. One of the beneficial effects is pH increase at the steel surface due to oxygen reduction. As the pH increases, the corrosion rate decreases and the current demand decreases. A simple model was set up for this transient process, suggesting that such effects should occur on the time scale of hours to days. This model was validated from start up data of a CP field trial system on part of a bridge. Field results confirmed the modelling proposed here.     

NaCl液滴下304不锈钢表面电化学性质研究

利用丝束电极技术研究NaCl液滴下304不锈钢表面电化学参数分布及其随腐蚀时间的变化规律。结果表明,液滴下不锈钢丝束电极表面的腐蚀电位分布和电偶电流分布均呈现空间、时间上的不均匀性;丝束电极表面随机形成局部阴极区和阳极区,而且随着腐蚀时间的延长,局部区域的极性发生反转。液滴下电极表面的腐蚀程度和不均匀程度均随腐蚀时间的延长先增加后减小,在腐蚀12 h时达到最大。     

Study of localized corrosion of 304 stainless steel under chloride solution droplets using the wire beam electrode

Localized corrosion of 304 stainless steel under droplets of 1 M sodium chloride solution was investigated by the wire beam electrode (WBE) method. It was found that the current distributions were heterogeneous with isolated anodic current peaks mostly located near the edge of the droplet. During the corrosion process, the stainless steel WBE exhibited the stochastic characteristics with the disappearance of some anodic sites. In addition, stainless steel suffered more serious localized corrosion with the increase of the droplet size. The increase of the cathodic area and the three-phase boundary (TPB) length was believed to be the reason.     

Effect of AC interference on the corrosion behavior of cathodically protected mild steel in an artificial soil solution. Part I: Investigation on formed corrosion product layer

AC-induced corrosion is a big threat even for cathodically protected pipelines nowadays. While this phenomenon was intensively investigated in the last decades, the corrosion mechanisms due to AC interference remain unclear. In the present work, investigations on the surface processes on cathodically protected mild steel during AC polarization have been performed. They utilized high-speed potential measurements that have demonstrated the influence of the polarization parameters on the resulting alternating voltage. The corrosion product layer was characterized with scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy, which clearly show the effect of the parameters of the applied alternating current on the surface under different cathodic protection (CP) conditions. It was demonstrated that the properties of the formed corrosion product layer, meaning the layer thickness, amount of oxygen, and so on, is not only dependent on the AC polarization parameters but also on the CP potential itself.     

Effect of pulse cathodic protection on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on pipeline steel

Coating disbondments on pipeline steels are regions with high resistivity where conventional cathodic protection (CP) could not fully protect. Therefore, in an attempt to mitigate this challenge, this study investigates the effect of pulse CP on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on X-52 pipeline steel. In this regard, conventional and pulse CP of ?870 mV_SCE were applied to the open mouth of a simulated coating disbondment. For pulse CP, frequencies of 1, 5, and 10 kHz were used. Results showed while the conventional CP was not able to fully protect the 20 cm simulated coating disbondment, for the pulse CP with increase in frequency from 1 to 5 kHz, and from 5 to 10 kHz, improve in CP potential protection under the simulated coating disbondment was achieved. This was accompanied by considerably lower corrosion and a more uniform pH distribution under the simulated coating disbondment.     

Novel corrosion experiments using the wire beam electrode. (IV) Studying localised anodic dissolution of aluminium

An electrochemically integrated multi-electrode array namely the wire beam electrode (WBE) has been applied in novel experiments to study the anodic dissolution behaviour of aluminium (AA1100), which was exposed to corrosive media with and without the presence of corrosion inhibitor potassium dichromate. The objective of this work is to demonstrate the applicability of the WBE for investigating corrosion processes under anodic polarisation. Anodic current measurements and mapping have been made, for the first time, with the WBE surface being anodically polarised. Pitting potential as well as anodic dissolution profile has been successfully determined by mapping anodic dissolution currents over the anodically polarised WBE surface. The pitting potential determined using the WBE method was found to correlate well with that determined using the conventional pitting scan method; and the anodic dissolution profile determined using the WBE method was found to correlate with maps obtained using the scanning reference electrode technique (SRET). Potassium dichromate was found to significantly affect the pitting potential, anodic dissolution profile and pitting initiation characteristics of aluminium. Two mechanisms of localised corrosion initiation have been identified. For WBE surface under free corrosion or low anodic polarisation conditions, the initiation of localised corrosion was found to be due to the disappearance of minor anodes, which lead to accelerated dissolution of a few major anodes. For WBE surface under large anodic polarisation, the initiation of localised corrosion was found to be due to the formation of active new anodic sites. This work suggests that the WBE method is useful for understanding the electrochemical behaviour of localised anodic processes, and their dependence on externally controllable variables.     

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.