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.     

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.     

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).     

Modeling cathodic shielding of sacrificial anode cathodic protection systems in seawater

Two kinds of cathodic shielding phenomena of the sacrificial anode cathodic protection systems, which are caused by a metal barrier, were investigated by numerical simulations and experiments in this paper. The results show that if the cathode current is completely shielded by the barrier, one side of the barrier would act as the anode leading to the occurrence of serious corrosion, while the protected components and the other side of the barrier would act as cathode being protected. However, if the cathode current is only partially shielded, both the metal components and the barrier would act as cathodes. Furthermore, this non‐full shielding effect of the barrier would be weakening by decreasing the mass transfer resistance between the sacrificial anode and the cathodically protected component.     

Influence of irregularities in the electrolyte on the cathodic protection of steel: A numerical and experimental study

This paper shows how the last algebraic matricial form can be obtained when the finite element method is used to approximate the potential distribution of a cathodic protection system that includes low conductivity irregularities in the electrolyte away from, close to and directly on the cathode. In order to study the influence of the resistivity of these irregularities on the possibilities of steel protection, five conductivities were analysed. The numerical results, validated with COMSOL® Multiphysics, show the importance of considering irregularities in the domain in order to prevent systems from becoming unprotected. The experimental data agrees with the theoretical data.     

Stray current-induced corrosion in cathodic protection installations of steel-reinforced concrete structures: FEM study of the critical parameters

A wide range of parameters was investigated by numerical calculations concerning their impact on DC stray current corrosion of reinforced concrete (RC) structures. A simplified model geometry was used to extract the relevant parameters and their interaction in terms of stray current-affected structures. This study mainly focuses on RC structures that are fitted with cathodic protection installations. The findings reveal a complex interaction between the investigated parameters. The possible relevance of further parameters, which is not the subject of this study, was emphasised.     

Thermal sprayed titanium anode for cathodic protection of reinforced concrete bridges

Stable operation of cobalt catalyzed thermal sprayed titanium anodes for cathodic protection (CP) of bridge reinforcing steel was maintained in accelerated tests for a period equivalent to 23 years service at Oregon Department of Transportation (Oregon DOT) bridge CP conditions with no evidence that operation would degrade with further aging. The cobalt catalyst dispersed into the concrete near the anodeconcrete interface with electrochemical aging to produce a more diffuse anode reaction zone. The titanium anode had a porous heterogeneous structure composed of α-titanium containing interstitial oxygen and nitrogen, and a fee phase thought to be Ti(O,N). Splat cooling rates were 10 to 150 K/s, and microstructures were produced by equilibrium processes at the splat solidification front. Nitrogen gas atomization during thermal spraying produced a coating with more uniform composition, less cracking, and lower resistivity than using air atomization.     

钢筋混凝土结构实施阴极保护的几个问题

1混凝土结构中钢筋腐蚀的原因 钢筋混凝土中钢筋的腐蚀是一个电化学过程,腐蚀的原因有内因和外因.内因:钢筋本身的不均匀性,如成分不均匀、钝化膜不连续、受应力作用不同等.外因:环境因素,如混凝土的物理化学性质不均匀性,介质中含氧、水、二氧化碳、各种盐类及不同pH等会在钢筋表面不同部位形成电位差;混凝土作为介质能在电位不同...     

FEM‐models for the propagation period of chloride induced reinforcement corrosion

The paper reports the results of numerical simulations carried out with FEM and aimed at evaluating the corrosion conditions of steel bars in concrete elements subjected to chlorides. Two case studies were analysed: a reinforced concrete element subjected to de‐icing salt in the presence of a crack and a concrete tunnel in a chloride‐contaminated, water saturated soil. Attention was focused on the selection of proper values of concrete resistivity and of the parameters suitable to describe the electrochemical behaviour of steel in the different conditions of exposure. The results allowed to quantify the effects of the galvanic coupling between active and passive areas on the corrosion rate of steel.     

The beneficial secondary effects of conventional and pulse cathodic protection for reinforced concrete,evidenced by X‐ray and microscopic analysis of the steel surface and the steel/cement paste interface

The present study reports on the investigation of conventional and pulse cathodic protection (CP), in terms of steel surface analysis and investigation of the product layers at the steel/cement paste interface, after a long term (460 days) of conditioning and monitoring. The techniques used were: X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy‐dispersive X‐ray (EDX). Wet chemical analysis was used as supportive evidence for ion concentrations in the vicinity of the steel re‐bars. Generally, CP promotes beneficial secondary effects i.e. enhanced OH^? concentration and reduced Cl^? concentration near the steel surface. Cathodic polarization also results in additional deposition of portlandite, which stabilizes the protective properties of the product layer on the steel surface. Consequently, the fundamental mechanisms, underlying the efficiency of CP techniques in reinforced concrete, are strongly related to beneficial secondary effects of CP, affecting the morphology and transformations of these product layers. Since the experimental evidences to support the aforementioned beneficial effects are rather limited, this study investigates the morphology and composition of the “naturally” formed steel surface layers, along with the properties of the steel/cement paste interface, on corroding and protected steel reinforcement (in comparison to reference, non‐corroding, non‐protected conditions) after 460 days of conditioning. For the corroding specimens, the formation and substantial deposition of voluminous corrosion products, with low adherence to the steel surface is relevant (low protective ability), whereas in the protected specimens, a compact and adherent product layer of more stable high valent iron oxides, or calcium substituted such, was observed. To this end, the present work provides the experimental evidence for the fundamental mechanisms, related to the otherwise recognized positive secondary effects of CP in reinforced concrete.     

Effect of the calcareous deposits on the cathodic protection potential of 10Ni5CrMoV high-strength steel in a deep-sea environment by using the response surface methodology

The effect of deep-sea environments on the properties of calcareous deposits was studied and we established the mapping relationship between their cathodic protection potential and the temperature (T), pressure (P), dissolved oxygen (DO), and the properties of the calcareous deposits according to the response surface methodology. This revealed that the calcareous deposits mixed with CaCO₃ and Mg(OH)₂ under cathodic protection. The environment including temperature, pressure, and oxygen content had various impacts on the thickness, porosity, and the ratio of Ca and Mg. Moreover, the calcareous deposits slightly decreased the cathodic protection potential. Therefore, it can be believed that the cathodic protection potential for naked high-strength steel is also approximately applicable to that covered with calcareous deposits in the deep-sea environment.     

钢筋混凝土埋入式牺牲阳极阴极保护试验研究

目的 提高现有牺牲阳极阴极保护技术的效果,采用活性阳极包覆砂浆,制备一种埋入式牺牲阳极,并研究其应用特性。方法 采用二电极法测试阳极包覆砂浆的电阻率,通过加速试验、SEM-EDS分析锌腐蚀产物的迁移状况,采取自耦合试验测定埋入式牺牲阳极下钢筋的电位等参数;在此基础上,研究埋入式牺牲阳极的特性及其阴极保护范围。结果 活性阳极包覆砂浆的电阻率仅为18.48 Ω.m。闭路电位、瞬间断电电位测试显示钢筋的稳定保护电位为−400~ −440 mV,断电电位为−218 mV,满足NACE标准对衰减电位的最低要求(200 mV)。电流密度结果表明,埋入式阳极可提供的保护电流密度为6.1~7.7 mA/m²,符合EN 12696要求。通过网格法测量的结果显示,在钢筋密度比为0.20,以及高腐蚀环境条件下,埋入式牺牲阳极的最大有效保护距离可达到700 mm。SEM-EDS分析结果表明,锌阳极发生反应,生成了可溶性锌酸盐(ZnO2²⁻),且会由锌阳极表面向砂浆内部迁移,最终逐渐分散到砂浆孔隙中,可有效解决因锌阳极表面腐蚀产物聚集而影响活性的问题,并消除腐蚀产物体积增大造成的膨胀应力。工程应用结果表明,各测试点钢筋的保护电位均负于−400 mV,满足保护要求。结论 埋入式牺牲阳极对钢筋有较好的保护效果,能够保持电位、电流输出稳定,不会影响阳极的活性,也不会给混凝土结构带来膨胀应力。     

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.     

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.