X.R. Nóvoa

Electrochemical behaviour of steel rebars in concrete: influence of environmental factors and cement chemistry

Four series of reinforced concrete specimens have been studied over 3 years exposure in a 100% relative humidity atmosphere. Addition of CaCl2, NaNO2, and a mixture of CaCl2 and NaNO2 changed the cement chemistry with respect to an ordinary portland cement series of samples used as reference series. The study, based on low-scan rate cyclic voltammetry and electrochemical impedance spectroscopy, confirms previous results obtained in alkaline medium, i.e. the redox activity in the rebar’s oxides layer greatly influences the electrochemical behaviour of rebars in the passivity potential domain. Different redox processes also influence the active and cathodic protection domains that make corrosion rate estimations very difficult. The possibilities of estimating corrosion rate are discussed in terms of cement chemistry and corrosion potential of the system.

Use of EIS, ring-disk electrode, EQCM and Raman spectroscopy to study the film of oxides formed on iron in 1 M NaOH

Abstract Different electrochemical techniques (electrochemical impedance spectroscopy (EIS), ring-disk electrode, electrochemical quartz crystal microbalance (EQCM), and in situ Raman spectroscopy) have been employed to study the behaviour of the passive film formed on iron in alkaline medium simulating pore solution in fresh concrete. The study, based on low scan rate cyclic voltammetry performed over the entire electrolyte stability domain, allows for establishing the influence of the redox activity developing in the oxides layer on the electrochemical behaviour of the system and, thus, to get valuable information on the applicability of classical electrochemical techniques employed to assess corrosion of steel in concrete. The passive film is based on a magnetite-type structure which, in partially reversible processes, can be oxidised and reduced depending on the electrode potential. Those redox processes mask the corrosion process itself.

Electrochemical behaviour of zinc-rich epoxy paints in 3% NaCl solution

Electrochemical impedance spectroscopy (EIS) in the 100kHz–1mHz frequency range was employed as the main electrochemical technique to study the corrosion protection behaviour of a zinc-rich epoxy paint in 3% NaCl solution. The EIS results obtained at the open-circuit corrosion potential have been interpreted using a model involving the impedance of particle to particle contact to account for the increasing resistance between zinc particles with immersion period, in addition to the impedance due to the zinc surface oxide layer and the electrical resistivity of the binder. Galvanic current and dc potential measurements allowed us to conclude that the cathodic protection effect of the paint takes some time to be achieved. The loss of cathodic protection is due to a double effect: the decrease of the ZnFe area ratio due to Zn corrosion and the loss of electric contact between Zn to Zn particles. Even when the cathodic protection effect by Zn dust became weak, the substrate steel is still protected against corrosion due to the barrier nature of the ZRP film reinforced by Zn corrosion products.

Impedance spectroscopy study of hardened Portland cement paste

Abstract In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste–electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.

Impedance measurements on cement paste

Abstract The present paper is devoted to the study of the high frequency region of the impedance spectrum, and focused at determining the origin of the time constants in the impedance and their relation to the dielectric properties of cement paste. The origin of the intermediate frequency capacitive range was investigated by intercalating polyester sheets between the paste sample and the graphite electrodes. It was concluded that the dielectric constant of cement paste can be determined only when impedance measurements are performed to frequencies higher than 10 MHz. Oven dried cement paste presents a single almost perfect capacitive behaviour with a dielectric constant, e, below 10. In partially moistened cement paste the higher frequency range gives a reasonable e (≈80) for awater containing material. It is concluded that the interpretation of the high values of e in terms of the DAF effect does not seem correct.

Characterisation of the barrier properties of different paint systems. Part I. Experimental set-up and ideal Fickian diffusion

The water uptake of a commercial alkyd paint system is studied using the impedance spectroscopy technique. The first part of the paper is devoted to establish the experimental setup to accurately measure capacitance values in the pF range, characteristic of the studied system. It was concluded that electrochemical instrumentation is not adequate to perform this task. The second part of the paper discusses the water uptake of the paint system under two different immersion conditions: deionized water and 5% NaCl solution. This analysis leads to the conclusion that the diffusion process seems to follow the ideal-Fickian behaviour in both systems, the corresponding diffusion coefficients were calculated.

Characterization of porous aluminium oxide films from a.c. impedance measurements

An equivalent circuit (EC) that reproduces the a.c. impedance of porous aluminium oxide films in a highly approximate manner is proposed. The results reveal that electrochemical impedance spectroscopy (EIS) is a powerful tool for obtaining detailed information on the electrochemical properties of both the porous and barrier layer on which the corrosion resistance of aluminium depends. The impedance at a given frequency can be used for accurate calculation of the electrochemical parameter for the oxide film represented by each element of the EC. In this way, the effects of any factor on sealing and ageing of anodized aluminium oxide films can be precisely analysed. The EIS technique provides an effective, advantageous alternative to existing seal quality control tests.

Galvanic coupling between carbon steel and austenitic stainless steel in alkaline media

The galvanic currents between carbon steel and two types of stainless steels have been studied in solutions of pH close to that of concrete. The situations analysed were passivity and active corrosion induced by chlorides. In all cases the galvanic effects recorded were of minor importance indicating that no significant risk of galvanic corrosion exists when carbon steel and stainless steel are electrically coupled in reinforced concrete structures. Oxygen reduction current is lower for stainless steel than for passive carbon steel. The results have been interpreted by means of EIS in terms of the resistivity of the passive layer formed on steels.

Characterisation of the barrier properties of different paint systems: Part II. Non-ideal diffusion and water uptake kinetics

Abstract Three commercial paint systems, based on water-borne acrylic, chlorinated rubber and alkyd resins, were used to evaluate the water uptake in 5% NaCl solution (wt.%) and partially deionised water. Impedance spectroscopy allows following the coating capacitance evolution and, from this, water uptake versus immersion time can be tracked. Its analysis leads to the conclusion that diffusion process seems to be an ideal Fickian one just in one type of paint. The two others have important deviations from this ideal behaviour. In those conditions, an apparent diffusion coefficient can be estimated. The study of the electrolyte flux with exposition time was analysed and fitted to an expression, which includes four exponential terms with four time constants, associated to different processes. Three time constants behave as exponential decay functions and the fourth one has an exponential growth shape. A physical meaning for the expression is suggested. The values of all the studied parameters indicate that the barrier properties of the water-borne acrylic paint are the worst.

A New Approach to the Determination of the Cathodic Protection Period in Zinc-Rich Paints

Abstract The present study examined the behavior of zinc-rich paints (ZRP) based on an epoxy binder in 3% sodium chloride (NaCl) solution. The study focused on the effect of some design parameters ...