Electrochemical evaluation of under-deposit corrosion and its inhibition using the wire beam electrode method

A new experimental method has been applied to evaluate under-deposit corrosion and its inhibition by means of an electrochemically integrated multi-electrode array, namely the wire beam electrode (WBE). Maps showing galvanic current and corrosion potential distributions were measured from a WBE surface that was partially covered by sand. Under-deposit corrosion did not occur during the exposure of the WBE to carbon dioxide saturated brine under ambient temperature. The introduction of corrosion inhibitor imidazoline and oxygen into the brine was found to significantly affect the patterns and rates of corrosion, leading to the initiation of under-deposit corrosion over the WBE.     

Understanding the effects of electrode inhomogeneity and electrochemical heterogeneity on pitting corrosion initiation on bare electrode surfaces

The effects of electrode inhomogeneity (EI) and electrochemical heterogeneity (EH) on pitting corrosion initiation have been analysed by revisiting research findings reported in the literature and experimental evidences obtained in our laboratories using the wire beam electrode (WBE) method. Two mechanisms of pitting corrosion initiation have been identified on bare metal surfaces exposed directly to electrolytes. For WBE surface under free corrosion or low anodic polarisation conditions the initiation of pitting corrosion was found to be due to the disappearance of minor anodes, leading to accelerated dissolution of a few remaining major anodes. The nucleation stage of pitting corrosion appeared to be controlled by EI, while the propagation stage appeared to be determined by EH. For WBE surface under large anodic polarisation the initiation of pitting corrosion was found to be due to the formation of active new anodic sites, which is in agreement with the conventional mechanism of pitting nucleation.     

利用丝束电极技术研究模拟微生物膜/金属界面的电化学不均匀性

利用丝束电极技术研究模拟微生物膜/金属界面局部腐蚀电位、腐蚀电流分布规律与膜内过氧化氢分布的相关性.海藻酸钙凝胶用来模拟微生物膜.结果表明,基于LabVlEW软件及模块化仪器构建的丝束电极测量装置,能快速、准确获得局部电极电位及电流信息.     

Understanding electrodeposition of polyaniline coatings for corrosion prevention applications using the wire beam electrode method

Polyaniline (PANI) films have been successfully electrodeposited on aluminium AA1100 electrode surfaces in acidic electrolytes as anti-corrosion coatings. The wire beam electrode (WBE) has been applied for the first time as a novel tool to monitor the electrodeposition processes; and also to understand the anti-corrosion performance and mechanism of the PANI coatings. During PANI electrodeposition, the WBE was polarised anodically, and anodic polarisation currents were measured from various locations over the WBE surface to produce anodic polarisation current maps (APC maps). Preliminary experiments have revealed that if an AA1100 electrode was not pre-treated, APC maps would show a localised anodic current distribution, resulting in a nonuniform PANI deposit. If the AA1100 electrode was pre-treated by a cathodic polarisation process, APC maps would show a random anodic current distribution, leading to a PANI coating covering the whole electrode surface. When exposed to a corrosive environment, PANI coatings were found to prevent localised corrosion of AA1100, primarily by enhancing its passive film rather than by a barrier mechanism. However, a nonuniform PANI coating has been found to accelerate general corrosion of AA1100. These results suggest that the WBE is a practical tool for monitoring, characterising and optimising PANI electrodeposition processes and for evaluating the anti-corrosion performance of PANI coatings.     

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.     

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.     

The heterogeneous electrochemical characteristics of mild steel in the presence of local glucose oxidase—A study by the wire beam electrode method

The influence of glucose oxidase (GOD) activity on the heterogeneous electrochemistry at artificial biofilm/mild steel interface was first characterized by the wire beam electrode (WBE) method. Potential/current distribution maps show that a cathodic zone can be formed at the GOD capsule site. The cathodic zone is gradually weakened due to the gluconic acid production in seawater. When GOD capsule is confined on rusted WBE surfaces, the formerly formed anodic zone is gradually changed into cathodic zone, in the presence of glucose. The novel device developed in our laboratory demonstrates powerful applications in heterogeneous electrochemistry measurements at the biofilm/mild steel interfaces.     

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.     

A novel device for the wire beam electrode method and its application in the ennoblement study

A novel device used in the wire beam electrode (WBE) method was developed in our lab. The system consists of standard modules, and is controlled by a LabVIEW software (National Instruments Co., NI). Since all the components in this WBE test device are commercially available, and the detailed information of the modules can be readily accessed by researchers, the system is easy to be built and modified, depending on experimental designs. The device can perform desired measurements, which is controlled by programs written in LabVIEW, for which the researchers can easily design and modify to create user-defined tests and control applications. This device has a faster measurement rate than that in published literature. In our study, we have used this device for the first time to investigate the relationship between local potential/current patterns and H₂O₂ distribution in an artificial biofilm/stainless steel interface. The results showed that the electrochemical distribution of the biofilm-related system is closely correlated with the biofilm heterogeneity. The device improves the WBE measurement techniques and is expected to be widely used in academic and industrial fields.     

Organic molecules showing the characteristics of localised corrosion aggravation and inhibition

The behaviour of imidazoline and an acid functionalised resorcinarene as steel corrosion inhibitors in carbon dioxide (CO₂)-saturated brine solutions has been studied using an electrochemically integrated multi-electrode array namely the wire beam electrode (WBE). Both imidazoline and resorcinarene acid provided excellent inhibition to general CO₂ corrosion; however imidazoline was found to aggravate localised corrosion by creating a small number of major anodes that focused on a small area of the WBE surface, leading to highly concentrated anodic dissolution. The resorcinarene acid showed distinctively different behaviour by generating a large number of minor anodes randomly distributing over the WBE surface, leading to insignificant general anodic dissolution. These results indicate that resorcinarene acid provided effective localised corrosion inhibition by promoting a random distribution of insignificant anodic currents.     

An experimental comparison of three wire beam electrode based methods for determining corrosion rates and patterns

Laboratory experiments have been carried out to examine the advantages and limitations of three wire beam electrode (WBE) based techniques, including the noise resistance R_n-WBE method, the overpotential-galvanic current method, and the galvanic current method, in determining corrosion rates and patterns. These techniques have been applied simultaneously to several selected corrosion systems of different characteristics. It has been found that the R_n-WBE method has advantages over other WBE based methods when applying to WBE surfaces under uniform corrosion. However, the R_n-WBE method has been found to be unsuitable for low noise level corrosion systems. It has also been found that both R_n-WBE and overpotential-galvanic current methods are similarly applicable to WBE surfaces under nonuniform corrosion. However, the galvanic current method has been found to be suitable only for WBE surfaces under highly localised corrosion. Some related issues regarding R_n calculation such as trend removal and its effects on corrosion mapping have also been discussed.     

苯并三唑对碳钢/铜合金电偶腐蚀行为的影响

目的 研究NaCl溶液中苯并三唑(BTA)对碳钢/铜合金电偶腐蚀行为的影响.方法 使用丝束电极(WBE)技术和电化学阻抗谱(EIS)技术研究在未添加和添加BTA的NaCl溶液中,丝束电极表面的电位分布、电流密度分布和电化学阻抗谱演化,同时对比分析碳钢区域与铜合金区域的阻抗谱特征.结果 在未添加BTA的条件下浸泡72 h...     

Monitoring pitting‐crevice corrosion using the WBE‐noise signatures method

An electrochemically integrated multi‐electrode array namely the wire beam electrode (WBE) and noise signatures analysis have been applied in novel combinations to study crevice corrosion behaviour in the presence of pits. Characteristic electrochemical noise signatures were found to correlate with characteristic changes in WBE current distribution maps, which indicate corrosion rates distributions, corrosion patterns and the degree of pitting and crevice corrosion. Specifically, two characteristic noise patterns were observed: (i) the characteristic noise pattern of quick potential changes towards more negative direction with no recovery (termed noise signature I) was found to correspond with the initiation and stabilization of the anode inside crevice; and (ii) the characteristic noise pattern of the cyclic potential oscillation at a constant frequency (termed noise signature II) was found to correspond with the stable anodic dissolution in the occluded cavity site in WBE current distribution maps. A new parameter namely the localization parameter (LP) has been proposed to describe the degree of localization. The LP for crevice corrosion was found to be low compared to that for pitting corrosion.     

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。     

Correlation between electrochemical impedance and current distribution of carbon steel under organic coating

The current distribution of carbon steel beneath intact organic coating exposed to 3.5% NaCl solution was investigated by the wire beam electrode (WBE) method. The electrochemical impedance spectroscopy (EIS) of individual electrodes with high current was characterized to analyze the behavior of the coating degradation and metal corrosion at coating/metal interface. The polarity switch of the electrode under the coating was observed. The mechanism of coating degradation and carbon steel substrate corrosion was discussed from a point of electrochemical distribution.     

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.     

WBE联合LP技术研究海水/海泥界面碳钢的腐蚀行为

利用阵列电极技术捕获了在不同浸泡时间下每个微电极与其它99个微电极整体的电偶电流,分析获得了海水/海泥界面附近阴、阳极分布区域的变化规律;利用线性极化和电化学阻抗技术对垂直海水/海泥界面的一行微电极进行测试,获得了距海水/海泥界面不同距离的微电极的腐蚀电流、腐蚀电位,并观察了腐蚀形貌,探讨了其腐蚀机理。结果表明：平行于海水/海泥界面的海泥区域为电偶腐蚀阳极区、对应的海水区域为电偶腐蚀阴极区域;海水/海泥界面区金属的腐蚀是由电位差、溶解氧浓度、海泥阻抗等多因素控制;腐蚀主要区域是近海水/海泥界面的海泥区域及远海水/海泥界面的海水区域。海水中较高浓度的氧促进了腐蚀产物的致密化,而海泥中缺氧环境及微生物抑制了腐蚀产物的致密化。     

Heterogeneous corrosion behaviour of carbon steel in water contaminated biodiesel

Biodiesel has been widely used as an additive to traditional fuel supplies, but the corrosion of metals used in biodiesel infrastructure is becoming an increasing concern. In this study, the influence of water contamination and corrosion behaviour of carbon steel in biodiesel, were characterized using the wire beam electrode (WBE) technique. In situ local current distributions among the electrodes showed a distinct corrosion pattern, with the anodes formed in the area that was exposed to water, and the cathodes formed along the water–biodiesel interface. The anodic current distribution showed a positive correlation with the biodiesel concentration gradient in water.     

Monitoring localised corrosion in inhibited solutions using wire beam electrode–noise signatures method

Abstract

The objective of this work is to determine the nature of localised corrosion in inhibited solutions by establishing possible relationships between characteristic features in electrochemical noise and corrosion processes using an electrochemically integrated multielectrode system, namely the wire beam electrode (WBE) in combination with noise signatures method. Experiments have been carried out to simultaneously measure electrode potential noise and WBE current distribution maps from stainless steel (SS316L) WBE exposed to inhibited solutions containing 6%FeCl₃ solution with inorganic inhibitors including 2 wt-% sodium chromate (Na₂CrO₄), cerium chloride (CeCl₃) and lanthanum chloride (LaCl₃). Characteristic electrochemical noise signatures were found to correlate with characteristic changes in WBE current distribution maps that show corrosion rates distributions and the degree of localised corrosion. A new concept namely localisation parameter (LP) has been proposed to identify the degree of inhibition. Na₂CrO₄ effectively inhibited the pitting corrosion process and the LP successively decreased through out the corrosion process. With the presence of CeCl₃ and LaCl₃, pits on stainless steel in 6%FeCl₃ solution were not repassivated and an increase in the LP was observed for both inhibitors. The results suggest that the WBE method could be used in combination with the noise signatures as a sensitive technique for monitoring inhibiting effect on localised corrosion.     

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.