Long term assessment of a hybrid electrochemical treatment

Monitoring the performance of systems installed to protect against/halt the corrosion of reinforcing steel in concrete is essential in ensuring that the reinforcing bar (rebar) is adequately protected and remains in a non‐corroding state. A chloride contaminated concrete test block containing a hybrid anode system has been periodically monitored over the past 4.5 years to assess the anode current output and thus, the protection offered to the steel. De‐polarisation and impedance analysis data have been acquired to calculate steel corrosion rates. Both tests indicated that the steel was passive in the concrete environment and that the anode was able to protect all of the embedded steel despite the difficult geometry of the sample. It was also found that after 2 years it was still possible to drive a high current from the installed anode.     

Protection of the opposite reinforcement layer of RC‐structures by CP – results of numerical simulations

If cathodic protection (CP) is applied to reinforced concrete structures the anode is usually attached at the concrete surface closest to the corroding reinforcement in order to minimise voltage and protection current that is required. In specific circumstances an anode installation at the concrete surface next to the corroding reinforcement is not practicable, i.e. due to difficult access. In such cases it would be helpful to know whether it is possible to protect the reinforcement by an anode being installed at the opposite surface of the structural element. The question is: does the second reinforcement layer obtain sufficient current to be effectively protected? Since a general answer to this question is not possible because the distribution of protection current depends on several parameters like the geometry and amount of the reinforcement, concrete resistivity, the polarisation behaviour and geometry of corroding and passive zones, a 3 D‐FEM model of the current and potential distribution was developed and its accuracy was verified by laboratory results. This paper presents the model itself as well as the results of parameter studies carried out to show the impact of the reinforcement distribution, the concrete resistivity and the polarisation behaviour of the reinforcement on the current distribution within the reinforcement.     

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.     

Use of the polarisation shift criterion to evaluate cathodic protection of on-grade steel storage tanks

The cathodic protection (CP) of on-grade coated steel storage tanks deployed on reinforced concrete slabs in soils of 20–30?kΩ?cm resistivity was evaluated for two different time periods 9 years apart. The required CP current was calculated and the ohmic drop was measured using the instant-off potential method. The resulting polarisation shift is a useful indicator of the level of protection afforded and whether the potential criterion or the polarisation shift criterion is fulfilled. It was also noted that the greater the ground resistivity, the less corrosive the environment for an on-grade steel structure.     

Electrochemical measurements of cathodic protection for reinforced concrete piles in a marine environment using embedded corrosion monitoring sensors

This study developed a sensor to monitor the corrosion of reinforced concrete structures. Concrete pile specimens with embedded sensors were used to obtain data on corrosion and cathodic protection for bridge columns in a real marine environment. Corrosion potential, cathodic protection current density, concrete resistivity, and the degree of depolarization potential were measured with the embedded sensors in concrete pile specimens. The cathodic protection (CP) state was accurately monitored by sensors installed in underwater, tidal, splash, and atmospheric zones. The protection potential measurements confirmed that the CP by Zn-mesh sacrificial anode was fairly effective in the marine pile environment. The protection current densities in the tidal, splash zones were 2–3 times higher than those in underwater and atmospheric zones. The concrete resistivity in the tidal and splash zones was decreased through the installation of both mortar-embedded Zn-mesh (sacrificial anode) and outside an FRP jacket (cover). Considering the CP, the cathodic prevention was more effective than cathodic protection.     

Cathodic protection of steel in concrete using magnesium alloy anode

Corrosion of steel embedded in concrete structures and bridges is prevented using cathodic protection. Majority of the structures protected employ impressed current system. Use of sacrificial system for the protection of steel in concrete is not as widely employed. The use of magnesium anodes for the above purpose is very limited. This study has been carried out with a view to analyse the use of magnesium alloy anode for the cathodic protection of steel embedded in concrete.Magnesium alloy anode, designed for three years life, was installed at the center of reinforced concrete slab, containing 3.5% sodium chloride with respect to weight of cement, for cathodic protection. Potential of the embedded steel and the current flowing between the anode and the steel were monitored, plotted and analyzed. Chloride concentration of concrete at different locations, for different timings, were also determined and analyzed.The magnesium anode was found to shift the potential of the steel to more negative potentials initially, at all distances and later towards less negative potentials. The chloride concentration was found to decrease at all the locations with increase in time. The mechanism of cathodic protection with the sacrificial anode could be correlated to the removal of corrosive ions such as chloride from the vicinity of steel.     

钢筋混凝土桥桩的阴极保护

本文概述了半浸于微咸水里工业桥桩及其上方横梁中阴极保护系统(CP)的设计和安装等施工技术，该CP系统由安装在支撑桩上的一展开式外加电流钛合金阳极罩和安装在梁上的热喷涂镀锌牺牲阳极系统组成，借助一分线整流设备来控制外加电流阴极保护系统并为其提供电力。再通过一程序控制电路且以数字方式实施分线整流设备的控制和监测操作。结果证明：桥桩上采用这种混合式CP系统，既经济有效，又方便实用。根据国外报道，铝／锌／铟合金(Al／Zn／In)是对钢筋混凝土结构达到有效阴极保护的最佳阳极材料。本文对于钢筋水泥结构建筑物实施阴极保护具有重要指导意义。     

Linear polarisation resistance measurements using a potentiostatically controlled guard ring

The use of a sensor controlled guard ring has been developed in recent years to enhance the accuracy of linear polarisation corrosion rate measurements on reinforced concrete structures. The established method of measurement uses a galvanostatically controlled guard ring device. In the method reported in this article the reinforcing steel is polarised potentiostatically by an inner auxiliary electrode and the real time plot of the current response is displayed on a laptop computer which controls the guard ring device. The area of steel polarised is confined by a current applied from an outer guard ring electrode which is controlled by two sensor electrodes positioned between the inner auxiliary and outer guard ring electrode. The potential between the two sensor electrodes is continually monitored, and the current output from the guard ring electrode varied to maintain a constant potential difference between the two sensor electrodes. The method has been validated on an electrical test circuit simulating active and passive reinforcement corrosion. Further testing has been conducted on a reinforced concrete specimen and the results compared with those of commercially available galvanostatically controlled equipment and those of standard unconfined linear polarisation resistance measurements.     

Investigations on the re-passivation of carbon steel in chloride containing concrete in consequence of cathodic polarisation

Although the complex changes at the steel–concrete interface due to cathodic polarisation are widely acknowledged to have a beneficial influence concerning the cathodic protection (CP) of steel in concrete, some questions concerning the repassivation of carbon steel in consequence of cathodic polarisation are still not satisfactorily clarified. In the recent literature, some indications are presented that repassivation occurs after a certain time of polarisation. Therefore, the investigations discussed in this paper aim to clarify, to what extent the re-passivation of carbon steel due to cathodic polarisation occurs, and if the ennoblement of OCP is a sufficient indication for repassivation. In a first step, the corrosion state of five nominal equal test specimens was determined by electrochemical impedance spectroscopy (EIS). After determining the initial corrosion state by evaluating the charge transfer resistance and the polarisation resistance, respectively, the specimens were polarised cathodically. Impedance data were recorded before, during and after polarisation. The impedance data were evaluated by equivalent circuit fitting with special attention to charge transfer resistances and the impact of diffusion on the corrosion and polarisation behaviour. The results indicate that the reduction of oxides and oxygen diffusion during cathodic polarisation has strong impact on the systems behaviour and that repassivation effects occur after switching off the polarisation current and during depolarisation, respectively.     

Numerical evaluation of the capacity of galvanic anode systems for patch repair of reinforced concrete structures

Cathodic protection (CP) is an electrochemical repair or corrosion prevention technique for steel structures exposed to a corrosive environment. For reinforced concrete (RC) usually impressed current CP is used, due to the comparably high resistivity of the concrete, serving as electrolyte. Nevertheless, the market provides a wide range of galvanic anode systems for RC structures. Their most common use is the application within the framework of partial concrete replacement due to chloride-induced corrosion. This patch repair is often accompanied by the so-called anode ring effect, causing accelerated corrosion of the rebar in the substrate concrete in the vicinity of repair patches. This is caused by the cathodic capabilities of the repassivated rebar. Galvanic anodes are reported to prevent this effect. In this paper, a numerical model is proposed, which is capable of determining the effectiveness of the method dependent on, for example, the type and quantity of anodes, rebar content, and geometry or climatic conditions. The method is presented for a specific set of input parameters and the applicability is discussed against the background of different protection criteria.     

Polarisation behaviour of steel bar samples in concrete in seawater. Part 2: A polarisation model for corrosion evaluation of steel in concrete

In the companion paper [Z.T. Chang, B. Cherry, M. Marosszeky, Polarisation behaviour of steel bar samples in concrete in seawater, Part 1: Experimental measurement of polarisation curves of steel in concrete, Corrosion Science 50(2) (2008) 357-364], influences of the experimental procedure on measured polarisation curves of steel in concrete in seawater were investigated. It was found that an undistorted full polarisation curve of a steel sample in concrete can be obtained by the two-test procedure to conduct separate anodic and cathodic polarisation tests and combine the two partial curves into one curve. However, polarisation curves of steel samples in concrete in seawater were found not to fit with the theoretical curves based on the kinetics of charge transfer reactions. This was considered to be due in the main to the influence of a passive film on the steel surface in concrete. This paper proposes an empirical model for the polarisation behaviour of steel in concrete based on the assumption of two major electrochemical processes taking place at the interfaces of steel/passive-film/concrete: one is the active corrosion process and the other is the passive film growth or dissolution process. Typical curve-fit results are presented using the proposed model to simulate the polarisation behaviour and to evaluate the corrosion rate and Tafel parameters of three types of steel corrosion in seawater: steel bars in concrete, new steel bars and corroded steel bars.     

Effect of electrode orientation on linear polarisation measurements using sensor controlled guard ring

Abstract

Use of a sensor controlled guard ring has been developed in recent years to enhance the accuracy of linear polarisation corrosion rate measurements on reinforced concrete structures. The sensors are used to monitor potential differences measured on the concrete surface above the reinforcing steel. These data are then used to confine the corrosion measurement to a known area of reinforcing steel. The role of the sensors is paramount in maintaining adequate confinement of the perturbation applied to the reinforcing steel. Experiments were conducted on reinforced concrete specimens containing both active and passive zones of reinforcing steel. Polarisation resistance measurements were taken using both a potentiostatically controlled guard ring device developed at the University of Liverpool and a galvanostatically controlled commercial device. Both devices indicated that the orientation of the sensor electrodes can affect the polarisation resistance determined when taking measurements on passive steel next to actively corroding areas. The sensor orientation was not observed to affect the polarisation resistance measurements taken on actively corroding steel next to passive steel.     

钢筋混凝土构筑物电化学保护的新进展

综述了钢筋混凝土构筑物电化学保护的发展和现状,包括牺牲阳极系统和外加电流系统。内容涉及对不同类型、不同龄期构筑物的保护标准、保护参数选择、保护效果的评价以及检测、监测等诸方面。认为有必要加强电化学驱氯、电化学再碱化等新技术的研究;根据构筑物的不同使用环境,采用多种有效的、综合的保护措施。     

Microstructural properties of the bulk matrix and the steel/cement paste interface in reinforced concrete,maintained in conditions of corrosion and cathodic protection

Although rarely considered, especially within the investigation of steel corrosion phenomena or electrochemical protection techniques in reinforced concrete structures, the concrete bulk matrix has a significant contribution in the global performance of the system “reinforced concrete.” This is especially the case when chloride‐induced corrosion or electrical current flow [as within impressed current cathodic protection (CP)] are involved. In the latter cases, the concrete bulk matrix undergoes significant alterations in chemical composition, electrical properties, and microstructures, thus influencing the overall performance of the system. This work reports on the microstructural investigation of the bulk concrete matrix and the steel/cement paste interface in reinforced concrete, previously subjected to corrosion and CP for 460 days. The emphasis hereby is to evaluate the altered structural properties, i.e., porosity, pore size, permeability of the bulk cement matrix, and the steel/cement paste interface (translated to bond strength) as a result of chloride‐induced corrosion and two types of CP (conventional and pulse), compared to control (non‐corroding, non‐protected) conditions. The research revealed a major contribution and close dependence of all microlevel interfaces on the global performance of reinforced concrete. The electrical current flow (as in CP applications) was found to bring about unfavorable modifications to the material structure, both in the bulk matrix (reducing porosity) and at the steel/cement paste interface (enlarging interfacial gaps). The derived microstructural parameters show that the conventional CP leads to a higher level of structural heterogeneity, whereas the pulse CP exerts minimal or no effects, maintaining the material properties close to the reference (control) conditions, the underlying mechanism being a more homogeneous material microstructure.     

大型海水循环泵腐蚀与防护技术应用

滨海电厂主循环泵的腐蚀研究及阴极保护技术应用推广在国内已有十余年的历史,因其效果显著,已愈来愈得到生产厂家的重视.目前国内滨海电厂循环水泵的保护主要是:泵内采用涂层加外加电流阴极保护技术;泵外水下部分采用涂层加牺牲阳极保护方法.本文从腐蚀原因分析、外加电流阴极保护技术及应用情况等几方面对海水循环泵的腐蚀与防护进行介绍.     

An investigation into the mechanisms of protection afforded by a cathodic current and the implications for advances in the field of cathodic protection

The influence of a cathodic current on the corrosion behaviour of mild steel exposed to both dilute sodium nitrate solutions and chloride contaminated concrete has been examined. In both cases the formation of passive films was promoted by stimulating a cathodic reaction on the metal surface, the cathodic kinetics being under activation control in the near passive region. The sodium nitrate solution was not capable of supporting a passive film on its own and it is suggested that, in this environment, film formation is promoted by the production of inhibitors (hydroxyl ions, free radical intermediates and metal oxide intermediates) resulting from the cathodic reduction reaction. In chloride-contaminated concrete an additional effect is the removal of aggressive anions via the flow of negative ionic current away from the metal surface. These passivating effects of the cathodic current provide persistent protection and may be harnessed to develop intermittent cathodic protection for use in areas such as the tidal zone where an electrolyte is not always present. The improvement in the local environment at the steel surface may facilitate a reduction in the protection current density, a factor which may be used to reduce the risk of overprotection when cathodic protection is applied to prestressing steel. The use of potential depolarisation and current density related criteria may be more appropriate than the achievement of a predetermined absolute potential in cases where the purpose of the cathodic protection system is to induce passivation.     

阴极保护在“引碧入连”供水工程中的应用

介绍了牺牲阳极阴极保护技术在大连市输水钢管上的应用情况,针对钢管管径粗达1820mm,土壤电阻率偏高,杂散电流的影响等具体工况,采取了有针对性的技术措施,电位测量结果证实了此项工程中阴极保护的合理、有效.     

A galvanic sensor system for detecting the corrosion damage of the steel embedded in concrete structures: Laboratory tests to determine the cathodic protection and stray-current

In order to evaluate the possible application of a galvanic sensor for cathodic protection (CP) and straycurrent monitoring of steel embedded in concrete structures, the correlation of sensor current to protection abilities (protection potential/current) and stray-current was investigated by galvanostatic tests that simulated the CP and stray-current. The results revealed a good correlation between the sensor current and the protection abilities or stray-current in terms of the sign as well as magnitude. Thus, the galvanic sensor is proposed as an effective tool for the quantitative determination of the effectiveness of CP or of the interference effect of stray-current.     

循环静水压力对牺牲阳极阴极保护的影响

实验模拟了常压和3.5MPa静水压力循环作用下,牺牲阳极阴极保护系统(CP)中阳极对阴极的保护。利用电化学测试法,结合SEM分析,对CP系统进行了电化学测量和腐蚀形貌观察,并对腐蚀产物进行了XRD成分分析。结果表明：在循环静水压力下,阴极的保护电位升高;牺牲阳极表面形成了一层相对致密的腐蚀产物壳层,导致其工作电位升高,放电能力下降;CP系统中斜率参数增大,牺牲阳极对阴极的保护效果变差。