Organic substances as inhibitors for chloride‐induced corrosion in reinforced concrete

Corrosion inhibitors are used to prevent chloride‐induced corrosion in reinforced concrete structures. Since performance of commercial organic inhibitors is only partially satisfactory, a 7‐year long research has been carried out in order to set‐up a new organic inhibitive mixture, able to prevent chlorides‐induced corrosion. A first screening, by means of potentiodynamic polarisation test in alkaline synthetic pore solution, was performed on 80 organic compounds, mainly primary and tertiary amines, aminoalcohols, carboxylates compounds and aminoacids, in order to select the best inhibiting substances. The nine best inhibitive organic substances were selected for long‐term tests: 2 amines (dimethylethanolamine and triethylentetramine), 4 aminoacids (aspartate, asparagine, glutamate and glutamine) and 3 carboxylates compounds (tartrate, benzoate and EDTA). Potentiostatic polarisation and free corrosion tests in synthetic pore solution were performed, as well as tests in concrete exposed to accelerated chlorides penetration. Five years of tests allow estimating the efficiency of the substances in preventing chlorides‐induced corrosion, in term of influence on chlorides penetration and on critical chlorides threshold.     

Depassivation of steel reinforcement in case of pitting corrosion: detection techniques for laboratory studies

Methods for service life prediction of reinforced concrete structures exposed to chloride‐bearing environments require, amongst other parameters, the knowledge of the chloride threshold for pitting corrosion initiation (Cl_th). Nowadays, although the main factors influencing the chloride threshold are well known, it is often difficult to quantify a value of the chloride threshold, partly because of its intrinsic high variability, and partly because of the different test methods that have been used to measure it. All the experimental tests rely on the detection of steel depassivation and simultaneous measurement of chloride content or steel potential. This paper deals with the methods that can be used to detect steel depassivation in relation with the determination of the chloride threshold. Tests in concrete‐pore‐simulating solutions as well as tests in concrete will be considered, and advantages and limitations will be discussed.     

Characterisation of the steel concrete interface submitted to chloride‐induced‐corrosion

This paper deals with the characterisation by means of electrochemical, gravimetric and analytical methods of chloride‐induced‐corrosion behaviour of steel coupons embedded in chloride‐containing‐cement pastes. Corrosion rates were estimated from electrochemical measurements as well as gravimetric ones. They vary from 2.6 to 5.7 µm/year for 5 and 10 g/L chloride‐containing‐cement pastes. Analytical characterisations (including optical and electronical microscopy and Raman micro‐spectroscopy) showed that corrosion patterns are not depending on the chloride content of the cement paste (5 and 10 g/L chloride in the interstitial solution). A localised corrosion pattern composed of pits growing inside the metallic substratum, a corrosion products layer (CPL) and a transformed medium (TM) was pointed out. CPL can be divided into two sub‐layers (CPL1 and CPL2), characterised by the presence or absence of calcium coming from the cement matrix.     

1,3‐Bis‐dibutylaminopropan‐2‐ol as inhibitor for reinforcement steel in chloride‐contaminated simulated concrete pore solution

An organic compound, 1,3‐bis‐dibutylaminopropan‐2‐ol (BDAP) was prepared and tested experimentally as inhibitor for reinforcement steel in simulated concrete pore solution. The electrochemical behavior of carbon steel electrodes immersed in simulated pore solutions with different concentration of BDAP was investigated by electrochemical measurements. Adsorption isotherm was determined based on the inhibition efficiency. The surface composition was evaluated via X‐ray photoelectron spectroscopy (XPS) as well in order to verify the presence and adsorption property of BDAP. The results showed that BDAP could effectively suppress the anodic process of carbon steel corrosion and the inhibition efficiency was improving with the increase of inhibitor concentration. BDAP could adsorb on carbon steel surface according to Langmuir adsorption isotherm. XPS spectrum certified the existence of BDAP on the steel surface.     

General corrosion of carbon steel in a synthetic concrete pore solution

In this study, we reviewed our recent work on the general corrosion of carbon steel (P355QL2) overpack material for the isolation of high-level nuclear waste in Belgium's supercontainer concept. By using electrochemical impedance spectroscopy and by optimizing the mixed potential model, which incorporates quantum mechanical tunneling of charge carriers across the barrier layer to describe the kinetics of the partial cathodic process, we evaluated all parameters in the model as a function of independent variables such as voltage, temperature, and pH. By delineating the partial anodic and cathodic processes, we found that the corrosion rate (CR) is independent of voltage over the voltage range from 0.2 to −1.0 V_SHE, which is predicted to be experienced in the repository. Furthermore, the CR is found to increase strongly with decreasing pH and increasing temperature.     

Pitting of carbon steel in the synthetic concrete pore solution

Pitting corrosion is a possible mode of failure of the carbon steel overpack of the Belgian supercontainer concept for the isolation of high-level nuclear waste (HLNW). However, no firm experimental data are currently available to estimate the probability of failure over the extended storage time (100,000 years). Extensive work shows that passivity breakdown results from the condensation of cation vacancies (CVs) at the metal/barrier layer (m/bl) interface, in response to the absorption of Cl⁻ into oxygen vacancies at the surface of the barrier oxide layer. The CVs migrate across the bl to the m/bl interface where they condense, leading to the separation of the bl from the metal. The resulting blister prevents the growth of bl into the metal and dissolution results in blister rupture, marking a passivity breakdown event. Stabilization via differential aeration produces a potentially damaging, stable pit. We review our work on passivity breakdown and the nucleation of pits on P355 QL2 carbon steel in high-pH aqueous media typical of concrete pore solution, with emphasis on the mechanistic aspects. We conclude that failure of the carbon steel overpack containing the HLNW over a storage horizon of 100,000 years is improbable.     

Chloride-induced steel corrosion in alkali-activated fly ash mortar: Increased propensity for corrosion initiation at defects

Chloride contents at the steel–mortar interface that initiate steel corrosion were determined for carbon steel in alkali-activated fly ash mortar for three different exposure conditions: exposure to 1 M NaCl solution; leaching in deionized water and then exposure to 1 M NaCl solution; and leaching in deionized water, aging in air at 20°C and natural CO₂ concentration, and then exposure to 1 M NaCl solution. For comparison, a Portland cement mortar, exposed to 1 M NaCl solution, was studied. The median values of the corrosion-initiating chloride contents (average over the full length of the rebar) in the alkali-activated fly ash mortar varied between 0.35 and 1.05 wt% Cl with respect to binder, consistently lower than what was obtained for the Portland cement mortar, but with no clear trend regarding the exposure conditions. For most of the alkali-activated fly ash mortar specimens, preferential corrosion at the connection between the working electrode and the external measurement setup was observed, while preferential corrosion did not occur for the Portland cement mortar. Scanning electron microscopy and auxiliary experiments in synthetic solutions indicated that this behavior was caused by inhomogeneities at the steel–mortar interface in the alkali-activated mortar, likely due to its peculiar rheological properties in the fresh state.     

Influence of chloride and moisture content on steel rebar corrosion in concrete

Reinforced mortar samples were exposed in humidity chambers with different relative humidity or exposed in cyclic moisture conditions. The rebars were in an “as received” condition meaning that the preexisting oxide scale were intact. The lowest chloride concentration that initiated corrosion was 1% Cl^? by mass of cement, corrosion was then observed for samples exposed at 97% relative humidity. It is suggested that the corrosion rate decreases when samples are exposed to a relative humidity lower than 97%. The results indicate that threshold levels should be evaluated at rather humid conditions (97%) despite the fact that the maximum corrosion rate at higher chloride levels is observed in the interval 91–94%. For samples exposed to cyclic moisture conditions, a lower chloride concentration was needed to initiate corrosion compared to samples exposed in static moisture conditions.     

An embedded multi‐parameter corrosion sensor for reinforced concrete structures

A prototype of an embedded corrosion sensor has been developed for assessing the corrosion status of reinforcing steel bar (rebar) in concrete. The integrated sensor unit includes an Ag/AgCl probe, a metallic oxide probe, a multi‐electrode array sensor (MAS), and a four‐pin (Wenner) array stainless steel electrode for chloride content, pH, microcell corrosion current, and localized concrete resistivity measurements, respectively. A stable solid probe was used as the reference probe in this unit to express the potentiometric measurement of chloride content and pH probes. In this study, the chloride and pH probes were calibrated in simulated pore solutions (SPSs) regarding temperature and pH fluctuations. The corrosion current results of the MAS probe in SPSs matched very well with those obtained by the linear polarization resistance technique, which was conducted on companion carbon steel specimens. A sensor prototype was embedded into a paste cylinder for long‐term performance evaluation. Up‐to‐date results show that the sensor probes exhibit excellent sensitivity and reliability through 1 year of monitoring. Continuous monitoring in the laboratory for extended periods is underway.     

Microstructural properties of the bulk matrix and the steel/cement paste interface in reinforced concrete,maintained in conditions of corrosion and cathodic protection

Although rarely considered, especially within the investigation of steel corrosion phenomena or electrochemical protection techniques in reinforced concrete structures, the concrete bulk matrix has a significant contribution in the global performance of the system “reinforced concrete.” This is especially the case when chloride‐induced corrosion or electrical current flow [as within impressed current cathodic protection (CP)] are involved. In the latter cases, the concrete bulk matrix undergoes significant alterations in chemical composition, electrical properties, and microstructures, thus influencing the overall performance of the system. This work reports on the microstructural investigation of the bulk concrete matrix and the steel/cement paste interface in reinforced concrete, previously subjected to corrosion and CP for 460 days. The emphasis hereby is to evaluate the altered structural properties, i.e., porosity, pore size, permeability of the bulk cement matrix, and the steel/cement paste interface (translated to bond strength) as a result of chloride‐induced corrosion and two types of CP (conventional and pulse), compared to control (non‐corroding, non‐protected) conditions. The research revealed a major contribution and close dependence of all microlevel interfaces on the global performance of reinforced concrete. The electrical current flow (as in CP applications) was found to bring about unfavorable modifications to the material structure, both in the bulk matrix (reducing porosity) and at the steel/cement paste interface (enlarging interfacial gaps). The derived microstructural parameters show that the conventional CP leads to a higher level of structural heterogeneity, whereas the pulse CP exerts minimal or no effects, maintaining the material properties close to the reference (control) conditions, the underlying mechanism being a more homogeneous material microstructure.     

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.     

Performance of galvanized and stainless steel rebars in concrete under macrocell corrosion conditions

In the recent past, the damage caused by rebar corrosion in concrete structures has been considered as one of the major durability problems affecting the service life of concrete structures. In order to prevent corrosion of steel reinforcement, various types of protective methods have been adopted. One of the protective measures is using galvanized reinforcement in concrete to enhance the service life. In the present study various types of galvanized rebars namely bare – Cold Twisted Deformed (CTD), bare – Thermo Mechanically Treated (TMT), galvanized – Cold Twisted Deformed, Galvanized – Thermo Mechanically Treated, galvanized and chromated – Cold Twisted Deformed, galvanized and chromated – Thermo Mechanically Treated and stainless steel rebars have been evaluated for their corrosion resistance in M30 grade concrete under macro cell corrosion condition over a period of one year. From the studies it has been observed that TMT bars performed better when compared to CTD bars. Among the rebars tested, stainless steel rebar has shown negligible corrosion in chloride contaminated concrete.     

Corrosion resistance of carbon steel in simulated concrete pore solution in presence of 1-dihydroxyethylamino-3-dipropylamino-2-propanol as corrosion inhibitor

The inhibition behaviour of 1-dihydroxyethylamino-3-dipropylamino-2-propanol (HPP) as an environment friendly corrosion inhibitor for reinforcing steel was investigated in simulated concrete pore solution contaminated by 0·1 mol L^?1 Cl^? by means of linear polarisation resistance, electrochemical impedance spectroscopy and cyclic voltammetry (CV). The surface morphology and corrosion products were also examined by scanning electron microscopy and X-ray diffraction (XRD). The results show that HPP can effectively inhibit the corrosion of reinforcing steel. The CV interpreted the corrosion inhibitor by restraining the reaction of cathodic reduction and anodic oxide. The XRD shows that the corrosion inhibitor reduces Cl^? adsorption on oxide film and reduces the production of corrosion products. In the conditions of this investigation, HPP behaves as an anodic corrosion inhibitor, protecting steel against corrosion in chloride contaminated environments.