D.A. Koleva

Investigation of Corrosion and Cathodic Protection in Reinforced Concrete I. Application of Electrochemical Techniques

The electrochemical behavior of steel reinforcement in conditions of corrosion and cathodic protection was studied, using electrochemical impedance spectroscopy (EIS) and compared to reference (noncorroding) conditions. Polarization resistance (PR) method and potentiodynamic polarization (PDP) were employed as well, in addition to ac 2 pin electrical resistance monitoring, thus deriving a comparison of the involved parameters, mainly polarization resistance and bulk electrical properties, obtained by all methods. It was found out that EIS is readily applicable for evaluating electrochemical behavior of the steel surface not only for corroding or passive state, but also in conditions of cathodic protection, although the interpretation of derived parameters is not straightforward and is related to the properties of the product layers, formed on the steel surface in the different conditions. For verification of the latter dependence, EIS, PDP, and PR measurements were performed additionally in cement extract solution, using steel samples from the previously embedded rebars in all technical conditions. The bulk matrix properties in passive, corroding, or under-protection conditions can be well defined by EIS. The evaluation of the electrochemical behavior of the steel surface, in terms of deriving polarization resistance, should take into account the crystallinity, morphology, and composition of the surface layers, which were investigated by scanning electron microscopy and energy dispersive X-ray analysis.

Electrochemical Behavior, Microstructural Analysis, and Morphological Observations in Reinforced Mortar Subjected to Chloride Ingress

The behavior of steel reinforcement was studied using electrochemical impedance spectroscopy (EIS) and polarization resistance (PR) techniques in conditions of chloride-induced corrosion in ordinary Portland cement-mortar specimens immersed in 7% NaCl for a test period of 120 days and compared to specimens immersed in demineralized water for the same period as reference specimens. This study was an initial phase of ongoing research on electrochemical methods for corrosion protection in reinforced concrete structures and aimed at investigating the applicability of widely accepted techniques as EIS and PR and their possible correlation with structural observations of the bulk matrix, relevant to cement-based materials science and product-layers distribution, to corrosion and further protection. The results indicate that the concept of EIS modeling and the components used in the latter correspond well to alterations in structural properties of the bulk matrix, while the electrochemical behavior can be additionally supported by morphological observations of the steel/cement paste interface.

Corrosion Performance of Carbon Steel in Simulated Pore Solution in the Presence of Micelles

This study presents the results on the investigation of the corrosion behavior of carbon steel in model alkaline medium in the presence of very low concentration of polymeric nanoaggregates [0.0024 wt % polyethylene oxide (PEO)113-b-PS70 micelles]. The steel electrodes were investigated in chloride free and chloride-containing cement extracts. The electrochemical measurements (electrochemical impedance spectroscopy and potentiodynamic polarization) indicate that the presence of micelles alters the composition of the surface layers (i.e., micelles were indeed absorbed to the steel surface) and influences the electrochemical behavior of the steel, i.e., the micelles lead to an initially increased corrosion resistance of the steel whereas no significant improvement was observed within longer immersion periods. Surface analysis, performed by environmental scanning electronic microscopy, energy-dispersive x-ray analysis, and x-ray photoelectron spectroscopy, supports and elucidates the corrosion performance. The product layers in the micelles-containing specimens are more homogenous and compact, presenting protective ?-Fe2O3 and/or Fe3O4, whereas the product layers in the micelles-free specimens exhibit mainly FeOOH, FeO, and FeCO3, which are prone to chloride attack. Therefore, the increased “barrier effects” along with the layers composition and altered surface morphology denote for the initially increased corrosion resistance of the steel in chloride-containing alkaline medium in the presence of micelles.

Corrosion performance of reinforced mortar in the presence of polymeric nano-aggregates: electrochemical behavior, surface analysis, and properties of the steel/cement paste interface

This study reports on the effect of admixed polyethylene oxide-b-polystyrene (PEO113-b-PS70) micelles on corrosion behavior of reinforced mortar. The electrochemical measurement shows that the corrosion performance of the reinforcing steel was not significantly improved. However, surface analysis and microstructural investigation at the steel/cement paste interface reveal that the admixed micelles lead to a steel surface layer with enhanced barrier properties in terms of morphology and composition. Therefore, the presence of micelles further minimized and halted the corrosion process, despite the very aggressive, chloride-containing environment of 5% NaCl. The reasons and mechanisms behind the hereby observed corrosion behavior of reinforced mortar in the presence of micelles are related to the influence of these nano-aggregates on microstructural properties. These further results in altered water/ion transport and chloride-binding mechanisms in the bulk mortar matrix and thus steel product layer modifications towards enhanced corrosion resistance.

Investigation of Corrosion and Cathodic Protection in Reinforced Concrete II. Properties of Steel Surface Layers

The present study explores the formation of corrosion products on the steel surface (using as-received low carbon construction steel) in reinforced concrete in conditions of corrosion and subsequent transformation of these layers in conditions of cathodic protection (CP).

Electrochemical Performance of Low-carbon Steel in Alkaline Model Solutions Containing Hybrid Aggregates

This work reports on the electrochemical performance of low-carbon steel electrodes in model alkaline solutions in the presence of 4.9.10-4 g/l hybrid aggregates i.e. cement extract, containing PDADMAC (poly (diallyl, dimethyl ammonium chloride) / PAA (Poly (acrylic acid)/ PDADMAC over a CaO core. The main objective was to determine if the addition of hybrid aggregates will lead to increased corrosion resistance of the steel surface layers, generally formed in the hereby investigated environmental medium. Further, it was expected that when chlorides are involved, as corrosion accelerating factor, the presence of hybrid aggregates will delay corrosion initiation and therefore lead to increased corrosion resistance. This investigation forms part of a novel approach to control steel corrosion in reinforced concrete, using self-healing mechanisms. The results from this study denote for indeed superior corrosion performance of steel in chloride-free and chloride containing alkaline solution, when hybrid aggregates are involved. The mechanisms are related to increased barrier effects of the formed layer and CaO release from the core of the aggregates.

Electrochemical and Microstructural Studies in Reinforced Mortar, Modified with Core-Shell Micelles

This work reports on monitoring chloride-induced corrosion in reinforced mortar specimens, with and without addition of polymeric nano-aggregates in the mortar mixture. The investigation is a novel approach to control steel corrosion in reinforced concrete, hereby reporting the preliminary results, related to one of the main objectives: studying the influence of admixed polymer nano-aggregates (in the form of PEO113-b-PS218 core-shell micelles with a very low concentration of 0.006 wt.% per mortar weight) on the corrosion behavior of the steel reinforcement, compared to reference, micelles-free mixtures.

Composition and Morphology of Product Layers in the Steel/Cement Paste Interface in Conditions of Corrosion and Cathodic Protection in Reinforced Concrete

The present study explores the formation of corrosion products on the steel surface in reinforced concrete in conditions of corrosion and subsequent transformation of these layers in conditions of cathodic protection (CP). Of particular interest was to investigate if the introduced pulse CP (as cost- effective alternative of CP) will lead to similar (or even better) transformation of the product layers on the steel surface, compared to conventional techniques. Qualification and quantification of the studied layers was performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDAX), visualization of morphology and products distribution was achieved using environmental SEM (ESEM).

Zinc Composite Layers, Incorporating Polymeric Nano-aggregates: Surface Analysis and Electrochemical Behavior

This study reports on a comparative investigation of the corrosion behavior of zinc (Zn) and nano-composite zinc (ZnC) galvanic layers in 5% NaCl solution. The metallic matrix of the ZnC layers incorporates nano-sized, stabilized polymeric aggregates, formed from the amphiphilic tri-block co-polymer: poly(2-hydroxyethyl methacrylate) - poly (propylene oxide - poly (2-hydroxyethyl methacrylate) (PHEMA15PPO34PHEMA15). The main objective was to evaluate the electrochemical properties and surface characteristics of both coatings, thus further to investigate if the nano-composite layers will have better corrosion resistance, compared to pure galvanic zinc. The electrochemical behavior, investigated by Impedance spectroscopy (EIS) and Scanning vibrating electrode technique (SVET), supported by surface analysis, using Atomic-force microscopy (AFM) and Scanning electron microscopy (SEM), reveals higher corrosion resistance and consequently better performance of the nano-composite layers, compared to pure galvanic zinc. The mechanism of incorporation of the polymeric nano-aggregates in the coating and their influence on the barrier properties of the composite layers are also briefly discussed.

The Synergy of Electrochemistry and Concrete Material Science in Evaluating Corrosion Resistance of Wastes-Containing Reinforced Cement-Based Systems

Next to water, concrete is the most used material on earth. Reinforced concrete has the potential to be durable and capable of withstanding a variety of adverse environmental conditions. One of the major difficulties in the engineering practice, however, is the multidimensional nature of reinforcement corrosion related issues. Nevertheless, materials and processes involved in the service life of a civil structure and their interaction are independently weighed and are seldom brought together. Very often the design of concrete mix proportions and the design of protective measures (such as the application of coatings or electrochemical techniques) are made in separate steps and (positive or negative) interactions among themselves are neglected. The most original aspect of this work is the integration of fundamental electrochemical techniques and concrete material science within monitoring and assessment of the corrosion performance of reinforced cement-based materials. Further, hereby discussed is the implementation of an eco-friendly approach of waste utilization for corrosion control and/or achieving superior properties and performance. Reinforced cement-based systems (e.g. reinforced mortar and concrete) are multi-phase composite materials at different levels of aggregation. Hence, multi-phase interfaces are involved in their structural performance and material behaviour. The steel reinforcement is embedded (on meso-level, generally relevant to mm dimension) in supposedly homogeneous cement-based bulk matrix. However, on a lower structural level (micro-level, expressed in μm), the bulk material consists of cement paste and aggregate particles, with air voids and macro pores dispersed in the cement paste matrix. Further, the cement paste can be decomposed into un-hydrated cement, hydration products, and capillary pore structure. The latter is generally assumed to have significant relevance to permeability and other transport phenomena in concrete technology. Both the reinforcement corrosion processes (i.e. electrochemical phenomena) and the development of the cement-based microstructure are influencing the material structure in reinforced cement-based materials (as mortar or concrete) on macro and micro/nano level. The corrosion process on one hand and hydration mechanisms on the other, determine changes in microstructural properties and ion transport, compared to rest conditions (in