By different testing methods (electrochemical techniques, potential shift monitoring, and Environmental Scanning Electron Microscope), this research evaluates the stray current corrosion of steel rebar in different layouts. The more significant corrosion state is observed when the steel bar is parallel to stray current flow, compared to the situation as a steel bar is vertical to the stray current. These outcomes are further clarified by the recorded level of stray current picked-up by steel rebar. It is found that the level of current actually picked-up by the steel rebar is decreasing. At the instant when the stray current supply is just turned off, an opposite current flow (back flow) is recorded. Besides an expansion of the database for monitoring stray current interference on reinforced concrete structures, the recorded results can be the basis for better understanding the process of stray current interference.

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Currents flowing along paths not being elements of a purpose-built electric circuit, are called stray currents. Various types of reinforced concrete structures (such as viaducts, bridges and tunnels) in the neighborhoods of railways may be subjected to stray current leaking from the rails. In these cases the concrete pore solution acts as an electrolyte, and the reinforcing rebars (or pre-stressed steel wires) embedded in concrete act as conductors, which can “pick up” the stray current and can corrode. The understanding of stray current-induced corrosion of steel rebar in concrete still remains unclear, as it is challenging to inspect in detail the full scale of steel rebar, as embedded in concrete. Most of previous understanding and preventive measures for stray current corrosion refer to investigations or field tests on pipelines. Besides, it is difficult to rebuild or repair the structures under or near rail transits. All above reasons reflect that stray current corrosion of reinforced concrete structures is in need of more in-depth investigation and understanding. As an expansion of the current body of knowledge of stray current corrosion of steel rebar in cement-based materials, this research aims to be a step forward towards for a better understanding of stray current corrosion mechanisms, a basis of feasible preventive measures for stray current-induced corrosion of reinforced concrete structures.@en

The focus of this work is to present test results on the bond of steel-mortar interface undergoing stray current. The bond strength, derived by pull-out tests, is correlated to the electrochemical response of the steel rebar and the properties of the mortar bulk matrix. The effects of curing regimes (in terms of duration of curing) and starting point of stray current are also investigated. It is found that stray current exerts bond degradation of the steel-mortar interface in all investigated cases, irrespective of the presence or absence of a corrodent (Cl⁻) in the external medium. For the ease of operation in lab tests, the stray current is generally simulated by anodic polarization, although fundamentally, the stray current effect on the steel surface is composed of both anodic and cathodic polarizations. Hence this work also differentiates the effects of stray current on steel-mortar bond, versus the effects of anodic polarization.

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Different concrete structures (viaducts, bridges, or tunnels) in the neighborhoods of railways may be subject to the stray current leaking from the rails. In these cases, the reinforcing rebars embedded in concrete act as conductors, “pick up” the stray current, and can corrode. For simulating the stray current-induced corrosion of metals, most researchers just supplied anodic polarization on samples. However, stray current induces both cathodic polarization and anodic polarization. This work experimentally justifies the different effects of stray current and anodic polarization on reinforcing steel embedded in mortar. A comparison between stray current and anodic polarization effects on the corrosion behavior of embedded steel is performed for both fresh (24 hour-cured) and hardened matrix (28 day-cured) in chloride-free (Cl-free) and chloride-containing (Cl-containing) environments. It is found that in all studied conditions, anodic polarization leads to a significantly different electrochemical performance of the steel rebar compared to the stray current. Hence, anodic polarization cannot reflect all the effects of stray current, and therefore, it has limited significance for simulating stray current. It is also clarified that the curing regimes and starting time of the stray current play significant roles in the formation of a corrosion product layer on the steel surface.@en

Metallic corrosion can cause substantial damage at various levels and in almost all types of infrastructure. For metallic corrosion to occur, a certain external environment and the presence of corrodents are the prerequisites. Stray current-induced corrosion, however, is a rather underestimated issue in the field of corrosion and civil engineering. Stray current arising from power sources and then circulating in metal structures may initiate corrosion or even accelerate existing corrosion processes. The most frequent sources of stray current are light rail transits and subways, which are also main traffic tools with continuously accelerating urbanization all over the world. Stray currents from these systems may easily flow into nearby metallic structures, making stray current-induced corrosion the most severe form of damage of buried structures, such as tunnels, pipelines, and various underground reinforced concrete structures. The objective of this paper is to critically review stray current-induced steel corrosion in infrastructure with regard to sources of stray current and the characteristics and mechanism of stray current corrosion in view of electrochemical aspects. The methods and techniques for the evaluation, monitoring, and control of stray current-induced corrosion for steel and reinforced concrete structures are also presented and discussed.@en

Stray current, arising from direct current electrified traction systems, further circulating in nearby reinforced concrete structures may initiate corrosion or accelerate existing corrosion processes on the steel reinforcement. In some extreme conditions, corrosion of the embedded steel will occur at very early stage. One of the significant consequences is loss of bond strength and premature failure of the steel-matrix interface. This plays an important role for the integrity of a structure during its designed service life.

In this work, the level of stray current was set at 0.3 mA/cm2, applied as an external DC electrical field. This level of stray current was chosen based on preliminary calculations on expected corrosion damage, i.e., in view of material loss at the level of 10% weight loss of the steel rebar (analytically calculated via Faraday’s law). The investigated reinforced mortar specimens were cured for 24 h only and then conditioned in chloride-free and chloride-containing environment. The evolution of steel electrochemical response in rest (no stray current) and under current conditions was monitored for approx. 240 days via OCP (Open Circuit Potential), LPR (Linear Polarization Resistance), EIS (Electrochemical Impedance Spectroscopy) and PDP (Potentio-dynamic Polarization).

The results show that the effect of stray current on concrete bulk matrix properties, together with steel corrosion response, is significantly determined by the external environment, as well as by the level of maturity of the cement-based bulk matrix.

For chloride-free environment the effect of the chosen stray current level was not significant, although lower corrosion resistance of the steel rebars was recorded after longer exposure of ~240 days, compared to control conditions. In fact, even positive effects of the stray current were observed in the early stages, i.e., until 28 days of age: stray current flow through a fresh (non-mature) cement matrix led to enhanced water and ion transport due to migration. The result was enhanced cement hydration, consequently environment, assisting a more rapid stabilization of pore solution and steel/cement paste interface. In chloride-containing external medium, steel corrosion was a synergetic effect of both de-passivation due to chloride ions in the medium and stray current effects. Corrosion acceleration solely due to the stray current flow in chloride-containing medium cannot be claimed for the chosen current density levels and the duration and conditions of the experiment.

What can be concluded is that the effect of stray current for both chloride-free and chloride-containing conditions is predominantly positive in the initial stages of this test. The expected negative influence towards corrosion acceleration was observed after a prolonged treatment, when a stable maturity level of the cement-based matrix was at hand. This also means that the properties of the cementitious material in reinforced cement-based system are of significant importance and largely determine the electrochemical state of the steel reinforcement.
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Stray current circulating in reinforced concrete structures may initiate corrosion or accelerate existing corrosion processes on embedded reinforcement. In some cases, the range of dangerous or unwanted interactions of stray currents under favorable conditions (environment), is much broader than is generally recognized. All these show that investigation of the effects of stray current on the corrosion behavior of steel is necessary and significant. In this work, the tested level of stray current was 3 mA/cm2, and the type of samples were reinforced mortar cubes (40 mm×40 mm×40 mm). To investigate the corrosion behavior of embedded steel undergoing stray current, the evaluation indicators adopted were OCP (Open Circuit Potential), Polarization resistance (Rp) derived through LPR (Linear Polarisation Resistance), and electrochemical parameters recorded through EIS (Electrochemical Impedance Spectroscopy) and PDP (Potentio-Dynamic Polarization). The recorded electrochemical response aimed to elucidate the importance of cell geometry i.e. the effect of steel orientation with respect to the electrical field (placed parallel or orthogonal to the current direction). It was found that the geometrical position of the steel bar is of significant importance and determines the level of stray current-induced degradation. @en

Stray current arising from direct current electrified traction systems and then circulat-ing in reinforced concrete structures may initiate corrosion or even accelerate existing corrosion processes on embedded reinforcement. Therefore, stray-current induced corrosion of nearby reinforced concrete structures deserves more attention in view of maintaining structures‘ integrity. The current state-of-the-art generally reports on sim-ulating stray current corrosion through anodic polarization, rather than stray current effects on the corrosion behaviour of steel in reinforced concrete. This work presents the comparison of effects of stray current (through the application of external DC electrical field) and direct anodic polarization on the corrosion behaviour of steel, embedded in mortar specimens. The level of stray current and anodic polarization were set at 0.3 mA/cm2 and evolution of electrochemical response over time was monitored via OCP (Open Circuit Potential) and EIS (Electrochemical Impedance Spectroscopy). The steel response was recorded as a result from “under polarization” conditions in both anodic and stray current regimes and compared to “rest” (no polar-ization) conditions.@en

In practice, reinforced concrete structures may encounter different kinds of curing conditions due to the weather (sun radiation, air moisture and wind), nearby external environment (seawater, attack of stray current arising from nearby railways), or human factors (insufficient curing period). In some extreme conditions, the corrosion of embedded steel will occur at early age, and the pore structure of cementitious material matrix can also be influenced. One of the significant consequences is the premature failure of steel- matrix interface, which plays important role for the integrity of a structure during the subsequent service life. This phenomenon leads frequently to early deterioration and eventually to risky situations for the stability of structures. In any case, the economic costs inherent to reparation works are considerable. Therefore, the state and properties of the steel/cement-based material interface is to be monitored from early ages, especially under extreme operating conditions. As one of the nondestructive testing methods, electrochemical impedance spectroscopy (EIS) can be employed to account for properties at the steel/cement paste interface. EIS measurements are very useful since they provide the possibility of correlating the dielectric properties and the microstructure of a cementitious material (derived from the high frequency region), with the corrosion behavior of the embedded steel (derived from the low frequency region). In this work, to simulate different curing conditions, standard curing for 28d and “on-air” curing after 24h were employed. Different groups of reinforced mortar specimens of the same mixture were designed according to the existence or coexistence of chloride, stray current, and anodic polarization. The steel-mortar interface properties with age and under different curing conditions was monitored by EIS measurement. The paper presents the evolution of the fitting parameters, corresponding to the cementitious material matrix, embedded steel and interaction between them are explained and compared in terms of different curing conditions. This research is expected to clarify that the, curing conditions in unusual work environment (as for example stray current conditions) must be taken into account regarding the behavior of steel-matrix interface at early age, and some curing methods must be put into practice in order to obtain satisfactory performance of the reinforced structure before the structure is used at full operational capacity.@en