Corrosion of steel bars in saturated Ca(OH)2 and concrete pore solution

Testing steel in solution has the advantage of avoiding the long time necessary for chlorides to penetrate the concrete cover.  It is well known that steel in high alkaline environments is passive and the protective capability of the passive film increases with pH.  The pH of saturated calcium hydroxide solution is lower than concrete pore solution which does induce passivation but not to the degree encountered by steel in good quality concrete.  Nevertheless, saturated calcium hydroxide has been used in many studies of rebar corrosion as a substitute for pore solution.  This paper discusses the electrochemical behavior of low carbon steel bars is chloride free and chloride contaminated pore solution and saturated calcium hydroxide solution.  Results show that the passive film on the steel immersed in pore solution and saturated Ca(OH)2 have similar composition.  However, as a result of lower pH in saturated Ca(OH)2 solution, the passive layer formed in this solution is less protective and does not offer enough passivity to steel to simulate a realistic concrete environment

Finite Element Modeling of Concrete Based on Quantitative Computed Tomography (QCT)

Models have been used before to predict the mechanical and transport behavior of concrete. In most of these studies, aggregates were considered either circle or sphere and the impact of the aggregates geometry and in-homogeneities in concrete structure is ignored. The objective of this study is to develop a novel method for accurate prediction of the mechanical behavior of concrete using quantitative computed tomography (QCT)-based finite element analysis. Concrete cylinders were cast and cured for 28 days. The QCT scans were carried out on the samples using a clinical CT scanner. An image processing method was applied to detect aggregates, paste content and the air voids. The distribution of each phase then calculated in each image slice (2D) and in the bulk material (3D). The processed QCT images were directly converted into voxel-based 3D FE models for linear and nonlinear analyses. The FE models were generated by conversion of each voxel into an 8-noded brick element. Air void content of the cylinders (2D and 3D) was determined. In addition, the aggregates content was estimated using the image analysis. In both cases, the results obtained by the image analysis and the actual measurement and ASTM method are in very good agreement

The influence of incorporation of Mn on the pitting corrosion performance of CrFeCoNi High Entropy Alloy at different temperatures

The electrochemical behavior and susceptibility to pitting corrosion of CrFeCoNi and CrMnFeCoNi high entropy alloys were studied in a 0.1 M NaCl solution at temperatures ranging from 25 to 75 °C. Electrochemical measurements revealed that CrMnFeCoNi is more susceptible to oxide film breakdown and localized corrosion compared to CrFeCoNi. Post corrosion microscopic observations showed severe pitting corrosion for CrMnFeCoNi in higher temperatures compared to CrFeCoNi. Based on in-depth XPS profile measurements on the remaining oxide films, this behavior was attributed to the depletion of Cr in the oxide film and detrimental presence of Mn in the matrix solid solution of CrMnFeCoNi

Surface and Uniaxial Electrical Measurements on Layered Cementitious Composites having Cylindrical and Prismatic Geometries

Electrical measurements are becoming a common method to assess the transport properties of concrete. For a saturated homogenous system, the surface resistance and the uniaxial resistance measurements provide equivalent measures of resistivity once geometry is appropriately taken into account. However, cementitious systems are not always homogenous. This article compares bulk and surface resistance measurements in cementitious materials intentionally composed of layered materials (i.e., layers with different resistivities). For this study, layered systems were composed of paste and mortar layers, representing the heterogeneity that can exist in the surface layers of field applications as a result of differences in moisture content, segregation, ionic ingress, carbonation, finishing operations, or ionic leaching. The objective of this article is to illustrate that these electrical measures can differ in layered systems (with sharp layer boundaries) and to demonstrate the impact of the surface layer properties on the estimation for the underlying material properties, for both cylindrical and prismatic specimens. Accounting for the effects of a surface layer requires a separate correction in addition to the overall specimen geometry corrections

Corrosion-Inhibitor Efficiency Control: Comparison by Means of Different Portable Corrosion Rate Meters

Corrosion-inhibiting substances have been applied to suppress corrosion mainly on bare steel, but when corrosion is progressing, suppression can be achieved if anodic and cathodic reactions are avoided, which is not an easy objective, particularly if the bare metal is surrounded by concrete. In the present article, several corrosion inhibitors are studied to identify their inhibition efficiency in concrete. The percentage of reduction of the corrosion rate without and with inhibitor is named “inhibition efficiency.” This definition calls for the measurement of the corrosion rate and makes its measurement a must when studying corrosion-inhibiting substances. The most extended technique used for the corrosion rate quantification is based on the calculation of the polarization resistance, Rp. For the study presented in this article, several portable corrosion rate meters based on the Rp technique have been used. These equipment are needed for the evaluation of large structures and are able to confine the current in small areas or to determine the steel area affected by the signal for a correct corrosion rate calculation. A comparison among the different corrosion rate meters used for the inhibitor's efficiency has been done. This comparison has indicated that the corrosion rate meter with nonefficient confinement of the current is not able to detect clearly the passivation state

Characterization of electrochemical systems using potential step voltammetry. Part II: Modeling of reversible systems

[EN] This study was carried out to compare the results obtained using potential step voltammetry and linear sweep voltammetry with a rotating gold disc electrode (RDE), when models based on equivalent circuits (EC) were used. The results lead to an equivalent circuit model that allows us to interpret the electrochemical behavior of aqueous solutions containing Fe(CN)(6)(-4) and Fe(CN)(6)(-3). With this model, we determined the values of the electrical resistance of the medium (R-s) as well as its polarization resistance (R-p), and established correlations between these values and the kinetic parameters of the system. The proposal highlights the need to introduce a new component for modeling using EC, which we have called the electrochemical diode. (C) 2019 Elsevier Ltd. All rights reserved.The authors gratefully acknowledge the financial support of BIA2016-78460-C3-3-R, MAT2015-64139-C4-3-R and RTI2018-100910-B-C43 (MINECO/FEDER) projects. We would also like to extend our appreciation for the pre-doctoral FPU scholarships (University Teacher Training scholarship) granted to Ana Martinez Ibernon (FPU 16/00723) and Jose Enrique Ramon Zamora (FPU13/00911) by the Spanish Ministry of Science and Innovation.Martínez-Ibernón, A.; Ramón Zamora, JE.; Gandía-Romero, JM.; Gasch, I.; Valcuende Payá, MO.; Alcañiz Fillol, M.; Soto Camino, J. (2019). Characterization of electrochemical systems using potential step voltammetry. Part II: Modeling of reversible systems. Electrochimica Acta. 328:1-10. https://doi.org/10.1016/j.electacta.2019.135111S11032

The steel–concrete interface

Although the steel–concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made

FEA of the galvanostatic technique with guard ring in cracked concrete

Uncertainty of the polarized area of reinforcing steel is one of the major sources of error when measuring corrosion rate of steel in concrete.  To overcome this problem, instruments equipped with the guard ring, aimed at limiting the polarized area, have been introduced and are available commercially.  However, some limitations and disadvantages of the guard ring equipment have been reported. The function of this system on measuring the corrosion in sound concrete was analyzed experimentally and mathematically [1].  This paper explains the function the guard ring equipment in cracked concrete by using finite element analysis (FEA)