Zum rückseitigen kathodischen Korrosionsschutz einseitig korrodierender Stahlbetonbauteile – Modellversuche und numerische Berechnungen

Cathodic corrosion protection of corroding reinforced concrete structures from the opposite side – Model experiments and numerical calculations Cathodic protection is a worldwide approved method to protect the reinforcement of concrete structures against corrosion. For this purpose a voltage is applied between the reinforcement an inert anode or a sacrificial anode which is placed on the concrete surface to shift the electrochemical potential to a more negative range where only cathodic reactions on the surface of the reinforcement take place. In case of concrete structures, that are only accessible from one side, it is often of interest to have only one anode at the opposite side of the endangered reinforcement layer (retaining walls, tunnels, etc.). In this regard investigations at the institute of building materials research Aachen, ibac, are currently carried out to clarify, whether a protection of the reinforcement of the opposite layer is possible or it is prohibited by the nearer layer. First results of these investigations show, that the protection of the opposite reinforcment layer is possible under certain conditions. Furthermore the first attempts to predict the current densities and the potential distributions under variable boundary conditions by the use of numerical calculations are presented.     

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.     

Numerical design for cathodic protection systems for concrete

Cathodic protection (CP) has become a successful method for the rehabilitation of concrete structures affected by chloride-induced corrosion of reinforcing steel. CP involves applying an electrical current from an external anode through the concrete to the reinforcement. The current causes steel polarisation, electrochemical reactions and ion transport. Normally the anode is placed over relatively large surface areas, including those where the steel is passive. Conventional views assume that protection current will not significantly flow outside the anode area. In many cases this results in a conservative design. This paper presents principles and first results of numerical calculations for design of an example CP system by finite element modelling. The final objective is to develop a tool for more economical CP system design. In particular, a CP system for the protection of local damage in bridges (e.g. at leaking joints) has been simulated. The corroding area with respect to the size of the anode is varied. Current and potential distributions and depolarisation values are predicted, both close to and more distant from the anode. It appears that current densities required to achieve sufficient polarisation are much higher than those usually found in the field. Neglecting time-dependent repassivation processes is likely to be the main cause and further work is needed to include them. The present model can be used with reasonable confidence for preventive application to passive steel.     

Measuring the corrosion rate of steel in concrete – effect of measurement technique,polarisation time and current

Both on‐site investigations and laboratory studies have shown that different corrosion rates are obtained when different commercially available corrosion rate instruments are used. The different electrochemical techniques and the measurement parameters used, i.e. polarisation current and time, are in some studies considered the main reasons for the variations. This paper presents an experimental study on the quantitative effect of polarisation time and current on the measured polarisation resistance – and thus the corrosion current density – of passively and actively corroding steel. Two electrochemical techniques often used in instruments for on‐site corrosion rate measurements are investigated. On passively corroding reinforcement the measured polarisation resistance was for both techniques found to be highly affected by the polarisation time and current and no plateaus at either short or long polarisation times, or low or high polarisation currents were identified. On actively corroding reinforcement a large effect of the polarisation time was also found, but only a minor effect of the polarisation current. The effect of the polarisation time was, however, practically independent of the corrosion rate for actively corroding steel. For both techniques guidelines for polarisation times and currents are given for (on‐site) non‐destructive corrosion rate measurements on reinforcement steel in concrete.     

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.     

Determination of corrosion rate of reinforcement with a modulated guard ring electrode; analysis of errors due to lateral current distribution

The main source of errors in measuring the corrosion rate of rebars on site is a non-uniform current distribution between the small counter electrode (CE) on the concrete surface and the large rebar network. Guard ring electrodes (GEs) are used in an attempt to confine the excitation current within a defined area. In order to better understand the functioning of modulated guard ring electrode and to assess its effectiveness in eliminating errors due to lateral spread of current signal from the small CE, measurements of the polarisation resistance performed on a concrete beam have been numerically simulated. Effect of parameters such as rebar corrosion activity, concrete resistivity, concrete cover depth and size of the corroding area on errors in the estimation of polarisation resistance of a single rebar has been examined. The results indicate that modulated GE arrangement fails to confine the lateral spread of the CE current within a constant area. Using the constant diameter of confinement for the calculation of corrosion rate may lead to serious errors when test conditions change. When high corrosion activity of rebar and/or local corrosion occur, the use of the modulated GE confinement may lead to significant underestimation of the corrosion rate.     

Zum Einfluss der kathodischen Reaktion auf die chloridinduzierte Bewehrungskorrosion

Investigations on cathodic control of chloride‐induced reinforcement corrosion Regarding the mechanisms of reinforcement corrosion there are still contradictions with respect to the controlling/rate‐determining factors of the corrosion process. It is often discussed, that the electrolytic resistance of the concrete is the controlling factor and that the corrosion rates can subsequently be calculated from concrete resistivity. However, extensive research carried out by the authors has clearly demonstrated, that instead of concrete resistivity the resistance to cathodic polarisation normally is the controlling factor in the case of chloride‐induced macrocell‐corrosion. Generally, cathodic control can be related to restricted oxygen diffusion or activation control. In the present paper, these relationships are discussed in detail by results of numerous tests on the cathodic polarisation behaviour of passive reinforcement. For simple defined geometrical conditions simulating practical cases it is shown by a numerical analysis that the resistance to activation is normally the controlling factor for the corrosion rate and that oxygen diffusion has only to be taken into account, when the concrete is permanently water saturated or extremely dense. To verify, whether it is correct to estimate corrosion rates from resistivity data, tests should be carried out to check the influencing parameters on concrete resistivity and cathodic activation of passive steel surface areas.     

A practical method for calculating the corrosion rate of uniformly depassivated reinforcing bars in concrete

The quantification of active corrosion rate of steel in concrete structures through nondestructive methods is a crucial task for scheduling maintenance/repair operations and for achieving accurate service life predictions. Measuring the polarization resistance of corroding systems and using the Stern‐Geary equation to calculate the corrosion current density of active steel is a widely‐used method for this purpose. However, these measurements are greatly influenced by environmental factors; therefore, accurate monitoring of corrosion requires integrating the instantaneous corrosion rates over time. Although advanced numerical models are helpful in research settings, they remain to be computationally expensive and complex to be adopted by general engineering community. In this paper, a practical numerical model for predicting corrosion rate of uniformly depassivated steel in concrete is developed. The model is built on Stern's earlier work that an optimum anode‐to‐cathode ratio exists for which the corrosion current on the metal surface reaches a maximum value. The developed model, which represents the corrosion rate as a function of concrete resistivity and oxygen concentration, is validated using experimental data obtained from the literature.     

Theoretical considerations on the supposed linear relationship between concrete resistivity and corrosion rate of steel reinforcement

Traditionally, the assessment of service life of steel reinforced concrete structures has been focused on the prediction of the time required to achieve a transition from passive to active corrosion rather than to accurately estimate the subsequent corrosion rates. However, the propagation period, i.e. the time during which the reinforcing steel is actively corroding, may add significantly to the service life. Consequently, ignoring the propagation period may prove to be a conservative approach. On the other hand the prediction of the corrosion rate may result in a very complex task in view of the electrochemical nature of corrosion and the numerous parameters involved. In order to account for the various influences an essentially empirical model has been introduced in which the electrolytic resistivity of the concrete environment serves as the major parameter. This model will be discussed for carbonation‐induced corrosion based on the commonly accepted theory of aqueous corrosion. An alternative model for microcell corrosion is proposed which is based on the commonly accepted view that anodic and cathodic sites are microscopic and their locations change randomly with time. In line with this view electrolytic resistivity can be incorporated and thus may play a significant role in the kinetics of the corrosion process. For a wide range of corrosion current densities the relationship between corrosion current density, log(i_corr), and concrete resistance, log(R_con), can then be approximated by an almost ideal linear relationship. Assuming a fixed geometrical arrangement of anodic and cathodic sites on the steel surface, this linear relationship is also valid for concrete resistivity, ρ_con. However, from the theoretical treatment of the electrochemical processes underlying reinforcement corrosion it becomes evident that a linear relationship between corrosion current density and concrete resistivity does not necessarily imply that concrete resistance is dominating the overall corrosion cell resistance. In most cases a significant portion of the driving voltage of the corrosion cell will be consumed by the transfer of electrical charge involved in cathodic reactions, i.e. cathodic activation control will dominate.     

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.     

Numerical model of Ca(OH)2 transport in concrete due to electrical currents

A mathematical model is being developed to describe a repair method in concrete, called cathodic protection (CP). The model is in principle also useful to describe electrodeposition in concrete, e.g. the process of re‐precipitation of Ca(OH)₂ invoked by an electrical current. In CP, the current is sent from an external anode to the reinforcement inside the concrete. This model is implemented using the numerical software package Comsol Multiphysics. The model is based on the Nernst–Planck equations and the electroneutrality condition considering the ionic species Na⁺, OH^? and Ca²⁺ and the solid Ca(OH)₂. The mathematical model makes it possible to predict the location where Ca(OH)₂ precipitates when a certain current density is used. This could be of great use for controlled crack repair in concrete and for electrochemical re‐alkalisation. This paper presents the qualitative behaviour of dissolution and re‐precipitation of Ca(OH)₂ in CP. It discusses model calculations and preliminary experimental results. Experiments for a more complete validation of the model are in process.     

Ten‐year results of galvanic sacrificial anodes in steel reinforced concrete

Zinc sacrificial anodes have been included in patch repairs to steel reinforced concrete structural elements suffering from corrosion since the mid‐1990s. A number of these anode‐containing repairs have been monitored with time. One of the first monitored sites was of a locally repaired cross beam of a bridge structure in Leicester, UK, which has now completed 10 years since its original repair and anode installation. This paper reviews the performance of the anodes installed at the Leicester site in terms of anode current output and steel reinforcement polarisation and corrosion rate over the period. It also presents results of analysis of recovered anodes exposed for 10 years which still show electrolyte continuity, uniform consumption of the zinc and coherent encasing mortar. The knowledge gained from the 10 year results has enabled the development of new, higher current output anodes, which are now trialled in this and other sites.     

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.     

Influence of conductivity on cathodic protection of reinforced alkali-activated slag mortar using the finite element method

Cathodic protection (CP) is considered to be the only rehabilitation method for chloride-induced rebar corrosion in reinforced concrete structures. The protection current distribution depends on several parameters, such as the geometry and number of rebars and the concrete resistivity. In order to investigate the influence of concrete resistivity on the possibilities and limitations of rebar protection, this paper presents a numerical approach based on the finite element method (FEM) in conjunction with laboratory results to determine its impact on the CP of alkali-activated slag mortar. An ordinary Portland cement was also tested for comparative purposes.     

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.     

The effect of temperature on the corrosion of steel in concrete. Part 1: Simulated polarization resistance tests and model development

The effect of temperature on the corrosion rate of steel corrosion in concrete is investigated through simulated polarization resistance experiments. The simulated experiments are based on the numerical solution of the Laplace’s equation with predefined boundary conditions of the problem and have been designed to establish independent correlations among corrosion rate, temperature, kinetic parameters, concrete resistivity and limiting current density for a wide range of possible anode/cathode (A/C) distributions on the reinforcement. The results, which successfully capture the resistance and diffusion control mechanisms of corrosion as well as the effect of temperature on the kinetic parameters and concrete/pore solution properties, have been used to develop a closed-form regression model for the prediction of the corrosion rate of steel in concrete.     

Kelvin Probe electrode for contactless potential measurement on concrete – Properties and corrosion profiling application

The practical feasibility of Kelvin Probe measurement of potential of the concrete surface was demonstrated. The measurements require no contact between the reference element and the concrete. Potential readings when placing the probe on dry concrete were nearly instantaneous and highly stable, in contrast with considerable potential drift with a conventional wet-tip electrode. The probe output was only modestly sensitive to the reference element working distance. The shape and range of potential profiles measured with the probe on concrete with locally corroding reinforcement were consistent with those using a conventional wet-tip reference electrode, both identifying the anode location.     

Throwing power of cathodic prevention applied by means of sacrificial anodes to partially submerged marine reinforced concrete piles: Results of numerical simulations

The paper deals with the determination of current and potential distribution in reinforced concrete elements partially submerged in seawater aimed at predicting the throwing power of cathodic prevention applied by means of sacrificial anodes. Experimental results from previous laboratory tests showed that the throwing power of cathodic prevention is higher compared to that of cathodic protection [1]. In order to extend the results obtained on small-scale specimens to elements of higher dimensions, FEM numerical simulations of potential distribution were carried out. Several cases were considered, representative of conditions differing in electrochemical behaviour of steel bars, geometry of the pile and of sacrificial anodes, concrete resistivity. The results allowed to discuss the role of different factors on the throwing power that can be reached by using sacrificial anodes immersed in the seawater to protect reinforcing steel bars in the emerged part of a pile.     

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.