Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 1: calcium nitrite

Laboratory investigations were performed to assess the efficacy of calcium nitrite as an inhibitor when used in surface treatments applied to 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 18 months thereafter. On completion of the exposure tests, measurements of corrosion weight losses and their distribution on the steel surfaces were also made. In non-carbonated specimens with high levels of internal chloride and carbonated specimens with even low levels of internal chloride, the surface-applied inhibitor treatment appeared to be ineffective under the conditions of the experiments and enhancement of local corrosion rates was observed in some specimens.     

Corrosion inhibitor systems for remedial treatment of reinforced concrete. Part 2: sodium monofluorophosphate

Sodium monofluorophosphate (MFP) has been applied in the form of concentrated aqueous solutions to the surfaces of concrete structures with the aim of inhibiting corrosion of embedded reinforcing steel which has become depassivated as a consequence of carbonation and/or chloride contamination. To evaluate the effectiveness of such treatments, a series of laboratory investigations was undertaken with 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 18 months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. It has been found that there were no marked reductions in the corrosion rates of the steel under the conditions investigated. Analysis of aqueous extracts from the treated concrete specimens by means of ion chromatography revealed that negligible penetration of soluble MFP ions had occurred into any of the specimens. Hydrolysis products of MFP (phosphate and fluoride) were present at significant depths in aqueous extracts of the carbonated concrete specimens but only fluoride was detectable in similarly obtained aqueous extracts of non-carbonated specimens.     

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.     

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.     

Development of corrosion sensors for monitoring steel‐corroding agents in reinforced concrete structures

Corrosion sensors were devised to develop a system whereby the depth of chloride permeation from concrete surfaces can be monitored non‐destructively on a real‐time basis using such sensors embedded in cover concrete of reinforced concrete structures. The proposed corrosion sensors were subjected to accelerated corrosion in NaCl solutions, mortar specimens intrinsically containing chlorides, and mortar specimens impregnated with chloride solutions, while recording the changes in the resistance readings. The resistance of sensors increased as the degree of corrosion increased. The time to the first change in the resistance decreased and the corrosion degree and resistance increased as the chloride concentration increased and as the distance from mortar surfaces decreased. It was therefore confirmed that the corrosion sensors are capable of monitoring the depth of chloride permeation with sufficient accuracy.     

Initiation of chloride‐induced corrosion of steel in concrete: role of the interfacial zone

This paper provides a brief review of research aimed at characterising the steel–concrete interfacial zone (SCIZ) and its influence on the susceptibility of the metal to pitting corrosion when concrete is exposed to environments that cause ingress of chloride ions accompanied by leaching of hydroxyl ions. For reinforced concrete made from Portland cements, exposed to aqueous solutions of sodium chloride, the buffering effect of solid calcium hydroxide (portlandite) at pH ～12.6 has been shown to restrain the gradual decline in the hydroxyl ion concentration of the concrete pore solution phase at depths corresponding to the embedded steel. When the concrete is produced under laboratory conditions that are carefully controlled to exclude macroscopic defects from the SCIZ and the steel is cleaned before being embedded, this can lead to observed chloride threshold levels being consistently greater than 1% chloride by mass of cement. The buffering action of cement hydration products formed in the SCIZ is believed to be partly responsible for this high tolerance to chloride‐induced corrosion because it counters the generation of ‘anodic acidity’ that is a necessary condition for stable growth of pits to occur. Translating this behaviour of laboratory specimens to the performance of full‐scale reinforced concrete structures has often proved difficult in the past and there is a need for further research in this area, particularly in relation to the role of non‐traditional cements.     

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.     

Monitoring of corrosion processes in chloride contaminated mortar by electrochemical measurements and X‐ray tomography

Corrosion of steel reinforcement in concrete exposed to chloride containing environments is a serious problem in civil engineering practice. Electrochemical methods, e.g., potential mapping, provide information whether the steel reinforcement is still passive or depassivation has been initiated. By applying such techniques no information on the type of corrosion, its extent and distribution of corrosion products is available. Particular the corrosion progress is a significant problem. Especially in the case of macrocell corrosion in reinforced concrete structures, the development at the anode cannot be separated into corrosion damage resulting from macrocell corrosion or self‐corrosion. Until now also in laboratory tests it is impossible to collect such information without destroying specimens after electrochemical testing was performed. To overcome this problem it was tried to study the steel surface within the mortar specimens by X‐ray tomography (CT). Within the scope of these investigations it could be shown, that X‐ray tomography is suitable to make corrosion pits and their development visible which are embedded in a mortar with a cover thickness of about 35 mm. In this publication the time‐dependent corrosion damage of reinforced steel is documented by X‐ray tomography.     

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.     

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.     

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.     

Corrosion testing on steel reinforced XD3 concrete samples prepared with a green inhibitor and two different superplasticizers

In recent years, the use of corrosion inhibitors in producing high‐performance steel reinforced concrete structures has increased significantly to minimize the chloride and sulfate attacks. However, most inhibitors available in the market are toxic to the environment. Hence, one objective of the present investigation was to test a novel, eco‐friendly, so‐called green inhibitor extracted from a fruit waste (orange peel), and its effects were studied on the compression strength of the XD3 type concrete samples. The inhibitor was added to the concrete mix in concentrations of 1% and 3% by weight of cement in addition to two different superplasticizers (Mapei Dynamon SR 31, Budapest, Hungary and Oxydtron, Hungary). The test results on steel reinforced samples immersed in 3.5 wt% NaCl aqueous solutions at room temperature showed promising corrosion mitigating effects just after 6 months testing period. The lower corrosion currents (i.e., better corrosion resistance) after 6 months immersion were observed when the samples contained both green inhibitor and Oxydtron superplasticizer, especially with sample C2 (in this case 3% green inhibitor was added to the mixture of cement + Oxydtron superplasticizer).     

The use of pumice as a coating for the reinforcement of steel against corrosion and concrete abrasions

This study presents the results of corrosion resistance for pumice collected from pyroclastic exposure in the Van, Kayseri, Nevsehir and Osmaniye regions of Turkey. The corrosion resistance of reinforced steel (RS) and mass losses of concrete specimens were investigated. The specimens were exposed to 5 and 10% sodium sulphate solutions. Reinforced steel mass losses and compressive strengths of concrete were measured. Improvements of the compressive strength and corrosion resistance were observed for specimens with the increasing layers of coating. The corrosion rates of pumice coated specimens were lower than the control specimens. There was a close relationship between type of pumice coating and reinforcement corrosion. Corrosion ratio decreased with increasing amounts of reactive SiO₂. The alkalinity of the concrete, permeability of the concrete cover, the quality of the concrete and the corrosion environment were crucial factors in influencing the effectiveness of the concrete cover for corrosion protection of reinforced steel. All pumice concretes offered excellent resistance to corrosion, where Osmaniye pumice (OP) best increased corrosion resistance of reinforced steel.     

Electrochemical injection of organic corrosion inhibitors into carbonated cementitious materials: Part 1. Effects on pore solution chemistry

This series of investigations was intended to clarify phenomena associated with electrochemical injection of the organic base corrosion inhibitors, ethanolamine and guanidine, into carbonated concrete. In Part 1, experiments were conducted with laminated specimens of carbonated cement paste, that were specially designed to facilitate analysis with adequate spatial resolution to assess changes in their pore solution phase chemistry after they had been subjected to constant current electrolysis between embedded cathodes and external anodes. The anolyte solutions provided sources of ethanolamine or guanidine in contact with the exterior specimen surfaces. Effects of variations in the applied current density and duration of electrolysis on the concentration profiles of the two inhibitors and the other main constituents of the pore solution phase were determined. The results have been used to underpin the development of a mathematical model, which is described in Part 2.     

Karbonatisierungszellen – Ein Beitrag zur Korrosion von Stahl in karbonatisiertem Beton

Carbonation cells – On the corrosion of steel in carbonated concrete Corrosion of steel in carbonated concrete involves the activity of active-passive galvanic couples (carbonation cells). Carbonation cells consisting of mortar electrodes in wet condition and subjected to wet-dry cycles have been investigated by means of current and potential measurements. The corrosion current can be explained in terms of oxygen diffusion through the mortar cover. The stationary current gives realistic values for the diffusion coefficient of oxygen in concrete. Changes from wet to dry and vice versa always increase the corrosion rate. Electrolytically and gravimetrically determined weight loss are in good agreement only for constant moisture conditions. In case of wetdry cycles oxidation and reduction reactions within the corrosion products are assumed to contribute to the total corrosion loss.     

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.     

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 on the effect of chloride on the general corrosion behavior of low‐alloy steels in oxygenated high‐temperature water

This project focused on investigations on the effect of chloride contaminations on the general corrosion and crack initiation behavior of low‐alloy steel (German reactor pressure vessel steel 22NiMoCr3 7) in oxygenated high‐temperature water (HTW). Therefore, tests were performed in oxygenated HTW without chloride and at different chloride contamination levels up to 50 ppb. Chloride was added either permanently or temporarily to simulate a chloride transient during plant operation. During these tests, electrochemical noise (EN) and electrochemical impedance spectroscopy (EIS) measurements were performed to monitor the electrochemical behavior depending on the adjusted environment conditions, especially the effect of chloride on the degradation of low‐alloy steel. After the tests, the specimens were examined macroscopically and microscopically. In addition, the oxide layer thickness was determined using the focused ion beam (FIB) technique and different surface analysis techniques as, e.g., TOF‐SIMS were performed to analyze the composition of the oxide layer. A change of the corrosion behavior of the tested specimens was revealed by the applied electrochemical methods EN and EIS during high‐temperature testing. In addition, the applied post‐test investigations showed a decrease in the oxide layer thickness due to permanently increased chloride concentrations in the HTW. Temporary transients, however, did not cause a long‐term memory effect as shown by both, the electrochemical and the metallographic post‐test investigations.     

Influence of N,N′‐dimethylaminoethanol as an inhibitor on the chloride threshold level for corrosion of steel reinforcement

The aim of this study is to examine the influence of N,N′‐dimethylaminoethanol (DMEA) as an inhibitor on the chloride threshold level for corrosion of steel in a concrete contaminated by chlorides. The experiment has been carried out in a saturated Ca(OH)₂ solution and chloride contaminated concrete containing different chloride and DMEA level. The critical point of corrosion onset is concluded by combining the open‐circuit potential (E_corr) with corrosion current (I_corr), which is decided by electrochemical impedance spectra (EIS) in the solution. Besides, the EIS has also been applied to determinate the chloride threshold level in the chloride contaminated concrete. It has been found that the presence of DMEA represented as an amino‐alcohol inhibitor, exerts little influence on the chloride threshold level for corrosion of steel in the solution. Similarly, the effect of the DMEA on the chloride threshold level in the chloride contaminated concrete, is also negligible.