Electrochemical realkalisation for rehabilitation of reinforced concrete structures

In steel reinforced and prestressed concrete structures depassivation of the reinforcing steel can take place due to carbonation of the concrete cover. Depending on humidity and oxygen availability subsequent corrosion reactions will be initiated. Such conditions require measures to exclude corrosion induced damages during the designed lifetime of the structure. In the last few years an electrochemical realkalisation treatment has been proposed as adequate rehabilitation technique for carbonated concrete. This temporary treatment should increase the pH-value of the concrete pore water solution due to penetration of alkaline electrolyte from the concrete surface as well as repassivate the reinforcement due to electro chemical reactions at the steel surface. In order to clarify the different mechanisms taking place during electrochemical realkalisation laboratory tests have been carried out using carbonated reinforced mortar specimens. The investigations were aimed at checking the influence of various parameters, e.g. treatment time and current density, as well as the efficiency and long-term durability of this rehabilitation method.     

Factors affecting the corrosion rate of steel in carbonated mortars

The corrosion of steel in carbonated mortar has been investigated by monitoring polarization resistance, corrosion potential and mortar resistivity. The results suggest that factors such as relative humidity and chloride contamination affect the corrosion rate via their influence on mortar resistivity. To account for the observed electrochemical relationships, most notably the linear correlation between corrosion rate and conductivity, and the exponential increase in corrosion rate with fall in corrosion potential, it is proposed that the corrosion kinetics of steel in carbonated mortar are subject to anodic control with the anodic reaction rate being limited by the resistance of the mortar. In this model, termed anodic resistance control, resistivity is viewed as a factor which may limit a half reaction rate in a similar way to diffusion and activation polarization. It is also shown that, when such a model is operating, the use of polarization resistance to monitor corrosion rate changes may still be justified provided the cathodic reaction rate is an exponential function of potential.     

Electrochemical corrosion parameters for active and passive reinforcing steel in carbonated and sound concrete

The electrochemical corrosion parameters, such as corrosion potential, corrosion current density, and the Tafel constants are necessary inputs for the corrosion modeling in reinforced concrete. Literature shows large variability in their values, whereas the data are scarce for the carbonated concrete. This paper presents a range of corrosion parameters for the active steel in carbonated and the passive steel in noncarbonated concrete. Forty-eight singly reinforced concrete cylinders were cast, of which 24 were carbonated and the others were sound samples. Potentiodynamic polarization curves were obtained at three different scan rates and extrapolated to extract the corrosion parameters. To validate these parameters, a macrocell corrosion system was simulated using FEM-based Comsol multiphysics® software. The numerical results were compared to two experimental studies. A natural dispersion in the values of corrosion parameters for both active and passive steels was observed. The average Stern–Geary constant was 54 and 47 mV for active and passive steels, respectively. Numerical simulations with the obtained parameters predicted the macrocell corrosion in partially carbonated concrete with a high accuracy. The presented values of corrosion parameters in this study could help researchers and engineers to simulate the corrosion phenomena in concrete accurately.     

Untersuchung über die Auswirkung der Elementbildung zwischen Stahl in Beton und Stahl in Erdboden

Investigation of the effect of galvanic corrosion between steel in concrete and steel in soil The investigation of cathodic polarization of steel in concrete on samples of portland cement and blast furnace cement revealed that oxygen reduction on steel in portland cement is less hindered than on steel in blast furnace cement. As to the magnitude of the cathodic current the aeration of the samples is important. Dry stored samples before measuring case a higher current density than wet stored ones. A significant relationship of the cathodic current depending on the water/cement value and the storage time of concrete could not be ascertained. The polarization resistances for portland cement samples lie within 86 and 3000 kΩ · cm² and for blast furnace cement samples between 430 and 5100 kΩ · cm². The measurements of the anodic current densities on corrosion cells of steel in concrete/steel in solution and steel in wet sand respectively rendered, by varying the cathode/anode aspect ratio from 1000: 1 to 1: 1, values for portland cement samples between 560 and 1,2 μAJcm² (corresponding to an annual corrosion of 6,5 and 0,014 mm respectively), and for blast furnace cement samples values between 730 and 0,28 μA/cm² (corresponding to an annual corrosion rate of 8,5 and 0,003 mm respectively). A decrease of the anodic current density variable with time could not be observed during the 28 days of measurements. On account of the investigations the following conclusions can be made: A formation of a corrosion cell with steel in concrete/steel in soil is particularly then to be expected if the concrete is dry on one side, e.g. the foundation of a building where oxygen diffuses by way of the gas pores in the concrete from the inner side to the cathode. This is less serious if the concrete is fully embedded in soil and thoroughly moistened. In this case the transport of oxygen has to be carried via waterfilled pores.     

Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization

High strength steels used in prestressing concrete structures are not exempt from the effects induced by corrosion on the normal concrete reinforcement. Carbonation of surrounding concrete or mortar is not unlikely for prestressing tendons and strands. Moreover, these steels undergo to brittle fracture as a consequence of stress corrosion cracking phenomena. To evaluate if concrete carbonation can promote this kind of failure, constant load tests in bicarbonate aqueous solutions under anodic polarization were carried out on high strength steel wires. Microscopic examination pointed out that the wires exhibited a brittle fracture mode, while its natural feature is ductile, as indicated by air testing. Failure mechanism was evaluated by a fracture mechanic approach. Cracks initiation was attributed to an anodic dissolution mechanism, while its propagation, interpreted by means of the surface mobility theory, was related to interaction between hydrogen atoms and magnetite at a crack tip.     

Zum Kontaktverhalten von feuerverzinktem und unverzinktem Bewehrungsstahl in Beton bei erhöhter Temperatur

Contact behaviour of galvanized and ungalvanized steel in concrete at higher temperature Electrochemical measurements at 60 °C on steel and galvanized steel embedded in cement mortar with and without chloride show that there is no potential reversion between steel and zinc during the test time of 30 weeks. The contact current between steel and zinc is always anodical related to zinc. The current is continually diminishing with time, its value is slightly higher in mortar containing chloride. Contacted steel and galvanized steel therefore leads to a higher zinc loss at least in the starting time. Without contact there is only a small corrosion attack on zinc in both cases of chloride free and chloride containing mortar.     

Steel corrosion in concrete: Determinist modeling of cathodic reaction as a function of water saturation degree

The prediction of the long-term behavior of reinforced concrete structures involved in the nuclear waste storage requires the assessment and the modeling of the corrosion processes of steel reinforcement. This paper deals with the modeling of the cathodic reaction that is one of the main mechanisms of steel rebar corrosion. This model takes into account oxygen reduction and oxygen diffusion through a diffusion barrier (iron oxide and/or carbonated concrete) as a function of water saturation degree. It is demonstrated that corrosion rate of reinforcement embedded in concrete with water saturation degree as low as 0.9 could be under oxygen diffusion control. Thus, transport properties of concrete (aqueous and gaseous phase, dissolved species) are key parameters that must be taken into account to model electrochemical processes on the reinforcement.     

Electrochemical injection of organic corrosion inhibitors into concrete

Two organic bases (ethanolamine and guanidine) that are known to act as corrosion inhibitors for steel in aqueous media were introduced into saturated specimens of carbonated and non-carbonated concrete from external electrolytes under the influence of an electrical field applied between embedded steel cathodes and external anodes. The cathodic current density was galvanostatically controlled at values in the range 1-5 A/m² for periods of 3-14 days. Control experiments, in which the corrosion inhibitors were applied to similar saturated concrete surfaces from external electrolyte without current, were also conducted. After treatment, the specimens were sectioned and analysed to determine the concentration profiles of the corrosion inhibitors within the concrete. It was found that the efficiency of injection of both ethanolamine and guanidine under the applied field was far higher in carbonated concrete than in non-carbonated concrete and that, in the carbonated specimens, the inhibitors became concentrated near the embedded steel. In non-carbonated concrete, guanidine penetration was accelerated to a modest extent by the applied field but ethanolamine penetration was not significantly enhanced by the field. These findings were explicable in terms of the influence of the pH values of the pore solutions in the various specimens on the degrees of ionisation of the organic bases concerned and hence on their tendencies to migrate and neutralise cathodically-generated hydroxyl ions.     

Cover Picture: Materials and Corrosion. 9/2020

Cover: Corrosion behaviour of carbon steel was evaluated in pore solutions extracted from mortars made of diff erent blended cements. Pictured are corrosion products produced in the following conditions, from left to right, top to bottom: carbonated CEM II mortar with chlorides, non-carbonated CEM II mortar with chlorides, carbonated CEM III mortar with chlorides and noncarbonated CEM III mortar with chlorides. More detailed information can be found in: Miha Hren, Tadeja Kosec, Andraž Legat, Corrosion behavior of steel in pore solutions extracted from diff erent blended cements, Materials and Corrosion 2020 , 71, 759.     

Einflußeiner Verzinkung auf die Korrosion von mit Zementmörtel beschichtetem Stahl in NaCl-Lösung

Effect of galvanizing on the corrosion of steel in concrete immersed in NaCl solution Galvanized or pickled steel sheet specimens were embedded in portland cement mortar of various water cement ratios and curing conditions and then wholly or partially immersed in 0.5 M NaCl solution for 1 to 5 yrs. Free corrosion potentials and electrical resistances have been measured. Immersion conditions and the presence of zinc have a significant effect on the corrosion resistance of the embedded steel sheets. The potentials of the wholly immersed specimens are very negative. Thus, these specimens cannot act as cathodes in corrosion cells, and the steel sheets within the mortar do not corrode. The partially immersed specimens, on the other hand, show very noble potentials. Also in the case of galvanized steel sheets the potentials are shifted to the same positive values in the course of exposure time. Thus, all these specimens can act as cathodes in corrosion cells. Localized corrosion generally occurs at the water/air line. In the case of pickled specimens the mortar is cracked due to growing corrosion products. In the case of galvanized steels the corrosion is retarded significantly. The test results are discussed in detail with respect to practical problems of cell formation, internal and external protection of pipes as well as the corrosion resistance of reinforced concrete.     

Experimental investigation of time dependent non-linear 3D relationship between critical carbonation depth and corrosion of steel in carbonated concrete

Abstract

This paper aims at the experimental investigation of time dependent non-linear relationship between critical carbonation depth and corrosion rate of steel in carbonated concrete by laboratory controlled experimentation under severe environmental condition. In this research, three-dimensional experimental observations are taken consecutively involving carbonation depth, half-cell potential and elapsed time as well as the gravimetric corrosion mass loss. The experimental observations revealed an interesting non-linear relationship between the above said measurements due to the varying resistivity of carbonated concrete. It is also found that the carbonation induced corrosion does not start until the carbonation depth reaches a certain critical level from the steel rebar and the half-cell potential values become constant after carbonation reaches the critical depth and then start rising again after carbonation reaches the rebar level.     

Rebar corrosion in carbonated concrete exposed to variable humidity conditions. Interpretation of Tuutti’s curve

This paper deals with the modelling of the rebar corrosion kinetics in unsaturated concrete cover. The concrete is assumed carbonated resulting in an active corrosion of steel. The corrosion kinetics is coupled with the ionic transport processes. Free corrosion in different concretes is studied in terms of ambient relative humidity. The numerical results obtained by the finite volume method are discussed and compared with a reported experiment performed on a carbonated concrete.     

Influence of steel-concrete interface condition on galvanic corrosion currents in carbonated concrete

This paper presents specific experiments which were developed in order to assess galvanic currents in macrocell corrosion specimens involving active steel in carbonated concrete and passive steel in sound concrete. The influence of the steel-concrete interface condition on the galvanic current was also experimentally investigated. To focus on macrocell corrosion rate assessment, the initiation time of the corrosion process (concrete carbonation) was accelerated. FEM simulations were carried out in order to enhance the physical comprehension of these corrosion experiments. It was found that, in realistic condition, the electrical coupling of active and passive steel areas leads to high galvanic currents and consequently high corrosion levels according to RILEM recommendation. Moreover, steel-concrete interfacial defaults significantly increase the macrocell driving potential and, therefore, the galvanic corrosion current.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

Corrosion of Carbon Steel and Corrosion-Resistant Rebars in Concrete Structures Under Chloride Ion Attack

Corrosion of reinforced concrete is the most challenging durability problem that threatens reinforced concrete structures, especially structures that are subject to severe environmental conditions (i.e., highway bridges, marine structures, etc.). Corrosion of reinforcing steel leads to cracking and spalling of the concrete cover and billions of dollars are spent every year on repairing such damaged structures. New types of reinforcements have been developed to avoid these high-cost repairs. Thus, it is important to study the corrosion behavior of these new types of reinforcements and compare them to the traditional carbon steel reinforcements. This study aimed at characterizing the corrosion behavior of three competing reinforcing steels; conventional carbon steel, micro-composite steel (MMFX-2) and 316LN stainless steel, through experiments in carbonated and non-carbonated concrete exposed to chloride-laden environments. Synthetic pore water solutions have been used to simulate both cases of sound and carbonated concrete under chloride ions attack. A three-electrode corrosion cell is used for determining the corrosion characteristics and rates. Multiple electrochemical techniques were applied using a Gamry PC4? potentiostat manufactured by Gamry Instruments (Warminster, PA). DC corrosion measurements were applied on samples subjected to fixed chloride concentration in the solution.     

Corrosion of reinforcing steel in simulated concrete pore solutions: Effect of carbonation and chloride content

The corrosion susceptibility of as-received reinforcing steel bars (rebars) in solutions simulating the pore liquid of alkaline and carbonated concrete has been studied by means of potentiodynamic polarisation tests and polarisation resistance measurements. The effect of different degrees of carbonation and the presence of several chloride contents in the simulated pore solutions was investigated. Results show the beneficial effect of high alkalinity on the localised corrosion of steel caused by chloride ions. From the results of the potentiodynamic tests a critical chloride concentration above which pitting could take place was evaluated for each solution. The chloride threshold values here found are of the same order than those previously reported in the literature for film-free steel. The results obtained in solutions simulating carbonated concrete showed that under weak carbonation conditions carbon steel does not passivate while in the presence of high levels of carbonate and bicarbonate the resistance to localised corrosion is improved.     

Repassivation of steel in carbonated concrete induced by cathodic protection

The paper discusses the peculiarities of cathodic protection applied to steel in carbonated concrete, which are strictly connected to the production of alkalinity at the steel surface. Results of a research on the application of cathodic protection to specimens with steel bars in carbonated concrete are discussed. A thin layer of concrete was realkalized within a period of 4–5 months by applying a current density of 10 mA/m² and steel could repassivate even in the presence of small contents of chlorides. A lower current density of 5 mA/m² could only maintain steel passive when the concrete in the vicinity of the steel had been previously realkalized through the application of a start‐up current density of 70 mA/m² for one month. Protection mechanisms are investigated and design of cathodic protection of steel in carbonated concrete is outlined. The possibility of obtaining protection on deeper rebars is also considered.     

Investigations of an ethanolamine‐based corrosion inhibitor system for surface treatment of reinforced concrete

Laboratory investigations were performed to assess the efficacy of a proprietary ethanolamine‐based corrosion inhibitor system when applied to the surface of reinforced concrete specimens that were chloride‐contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately eighteen months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. Analysis of aqueous extracts from treated concrete revealed that the ethanolamine component of the inhibitor system penetrated to depths of more than 15 mm within the concrete. It was found that, for inhibitor‐treated specimens, there was some reduction in the corrosion rate of pre‐corroding steel at low cover depths in non‐carbonated concrete with modest levels of chloride contamination. At higher levels of chloride contamination and in carbonated specimens, however, the ethanolamine‐based inhibitor was apparently ineffective under the conditions investigated.     

Einflüsse von Werkstoff und Verarbeitung auf die Spannungsrißkorrosion von Spannstählen

Influence of material and processing on stress corrosion cracking of prestressing steel In prestressed concrete constructions the highstrength prestressing steels perform essential bearing effects. The alkaline layer of concrete or mortar protects the steels against corrosion and guarantees a permanent load capacity. If the corrosion protection as a result of poor workmanship is not guaranteed from the beginning, or is lost because of lacks of construction in the course of time, or the steels are predamaged during handling, stress corrosion cracking and failure of steel and construction may occur. Also an application of unsuitable materials (prestressing steel, injection mortar, concrete) can alone or in combination with the before mentioned influences favour stress corrosion cracking. In the contribution the correlations and typical failures are discussed.     

Corrosion Behavior of Epoxy-Coated Rebar with Pinhole Defect in Seawater Concrete

Experiments were carried out to investigate the corrosion behavior of epoxy-coated rebar(ECR) with pinhole defect(diameter in hundreds of microns) immersed in the uncarbonated/carbonated simulated pore solution(SPS) of seawater concrete. Corrosion behavior was analyzed by electrochemical impedance spectroscopy. The composition and morphology of corrosion products were characterized by X-ray diffraction, energy-dispersive spectrometry and scanning electron microscopy. Meanwhile, oxide film produced by preheating before spray coating was investigated by X-ray photoelectron spectroscopy and Mott–Schottky technology. Results indicated that corrosion behavior of ECR with pinhole defect exhibited three stages when immersed in the uncarbonated/carbonated SPS. In the initial stage, steel in defect was passivated when exposed in the uncarbonated SPS and corroded when exposed in the carbonated SPS, due to competitive adsorption between chloride and hydroxyl ions. In the second stage, the oxide film under coating reconstituted(the thickness and defects density decreasing) in the uncarbonated SPS, which was caused by the synergy between high hydroxide and chloride activity, while in the carbonated SPS, crevice corrosion happened under the coating around pinhole,because of the different oxygen concentrations cell at the coating/steel interface. In the third stage, localized corrosion occurred under the coating around the pinhole in the uncarbonated SPS, which was probably induced by ion diffusion at the nano-scale coating/steel interface. The corrosion products adjacent to the defects were re-oxidized from FeCl_2·4H_2O and Fe_2(OH)_3Cl to Fe_2O_3·H_2O, and the corrosion area was expanded outward in the carbonated SPS.