Plan for 6 months tests

Plan for 6 months tests

The three months experimental part of this probation can be divided in two parts: electrochemical and surface analysis of the specimens. There can be a comparative study for the corrosion of steel in concrete in order to point out the differences between passive and corroded specimens. Previous study [1] in simulated pore solution examined samples: i) control ii) containing polyethylenoxide-block-polystyrene (PEO-b-PS) micelles iii) NaCl iv) NaCl + PEO-b-PS micelles. The next step is to study the properties of the self-healing cement based layer containing CaO core micelles, in similar samples. Hereby are suggested some of the techniques for detecting and measuring corrosion that will provide data on the causes or rate of corrosion.

Electrochemical analysis

1.      Open Circuit Potential (OCP)

Concrete acts as an electrolyte and the reinforcement will develop a potential depending on the concrete environment. Potential mapping techniques can be very helpful in order to determine the time to corrosion initiation. OCP provides measurement of the corrosion potential of rebar with respect to a standard reference electrode (calomel SCE or Ag|AgCl) provided the reinforcing bars are exposed to the environment. When the corrosion potential becomes more negative than -270 mV vs. SCE then there’s a 90% propability of corrosion according to ASTM C876. This is useful to find out the anodic and cathodic sites of reinforced concrete. However potential readings are not sufficient as a criterion because they include polarization by limited diffusion of oxygen and concrete porosity [2].

2.      Electrochemical Impedance Spectroscopy (EIS)

EIS is a useful non-destructive technique for quantifying corrosion of steel rebars that measures the current and phase angle for various frequencies. It is also a powerful technique suitable for characterizing the electrochemical processes in inhomogenous materials [3, 4]. The response to AC input is an impedance that has both real and imaginary (capacitive) component Z and Z’. The study of impedance on the variation of frequency we are able to obtain an equivalent electrical circuit describing the behavior of the steel electrodes and has the same response as the corrosion system. The experimental response is then fitted to the equivalent circuit adding time constants with a clear physical meaning. Each time constant is then related to certain charge transfer that occurs in double layer regions.

The plot of the imaginary impedance against the real gives a semicircle with a diameter equal to the charge transfer resistance R_ct. If the semicircle has an offset from the origin then this value is equal to R_s, which is the ohmic resistance of the concrete cover zone between the reference cell and the reinforcing bar that is being measured. At the highest point on the semicircle the frequency f is found and thus the double-layer capacitance can be extracted.

EIS provides detailed information about the mechanism, the kinetics of the electrochemical reactions and R_ct and R_p values related to corrosion rate through the Stern-Geary formula [4].

3.      Potentiodynamic Polarization (PDP)

The evolution of R_p values can also be derived from PDP tests, although with a different trent but similar to those of EIS measurements. Therefore PDP is useful to support OCP findings as well. High R_p values indicates passivity and this behavior is expected for control specimens. The presence of PEO-b-PS nano-aggregates can raise further the corrosion resistance of the steel electrodes.

In an Evans diagram the absolute values of current density is plotted vs. the potential. The kinetic phenomena related with the corrosion rate are the charge transfer and the mass transfer through cement to the electrode surface. In both cases as the driving force increases so does the reaction rate. Since the reaction rate is controlled by diffusion the current will reach a maximum and thus delay steel corrosion [1]. Corrosion current density values for the specimens can be calculated using Tafel and Butler-Volmer equation fit.

4.      Cyclic Voltammetry (CVA)

This is a useful qualitative analysis technique on the investigation of the behavior of steel in the cement environment and how the properties of the layers formed on the steel surface will influence the electrochemical behavior in the environment of cement extract. Consequently it measures the corrosion all over the metals surface (uniform) and corrosion at discrete sites on the surface (localized).

Passive steel samples are expected to have lower currents of the anodic, cathodic peaks and in the passive region. Cathodic peaks are related with the deposition or reduction of hydrous oxide film. When the thickness of the oxide film reaches a steady state this can be attributed to diffusion limitations of the transport of water molecules and hydroxide ions through the oxide layer which means the film permeability will decrease [5]. Anodic shift of the peak potential corresponds to the low corrosion resistance of the corroded steel surface.

In this case and for the purposes of this project the combination of EIS, PDP and CVA measurements can provide information for the strength of the anode in the altered anode/cement based layers interface.

Surface analysis

5.      X-ray Photoelectron Spectroscopy (XPS)

The combination of electrochemical techniques and XPS data can lead to an extensive study of oxidation/reduction reaction and the deposition of oxide layers. XPS results will indicate the types of oxide/hydroxide layers that are formed on the steel surface. After curve fitting of the recorded spectra the relevant binding energy for characteristic peaks are accounted to the corresponding bonds. As already being studied [1] the presence of the PEO-b-PS vesicles exhibit a more homogenous and protective α-Fe₂O₃ and Fe₃O₄. The adsorption of the micelles on the steel surface is expected to delay the breakdown of the passive film and impede the chloride ions penetration.

6.      Energy Dispersive X-ray (EDX)

EDX technique is useful for quantitative analysis and evaluating the chemical composition of corrosion and cement hydration products when combined with scanning   electron microscopy (ESEM) observations. This technique will reveal the surface properties of the modified layer containing nano-aggregates. Strongly adhered to the steel surface cement paste layers enriched of C–S–H and portlandite, were confirmed by EDX observations, denoting lack of corrosion products, that would cause microcracking and increase the bond degradation process [6].

References

1. J. Hu, D. A. Koleva, J. H. W. de Wit, H. Kolev, and K. van Breugel, “Corrosion Performance of Carbon Steel in Simulated Pore Solution in the Presence of Micelles”, Journal of The Electrochemical Society, 158 (3), 76-87, (2011)

2. H. W. Song, V. Saraswathy, “Corrosion Monitoring of Reinforced Concrete Structures – A Review”, Int. J. Electrochem. Sci., 2, 1-28, (2007)

3. D.A. Koleva, J.H.W. de Wit, K. van Breugel, L.P. Veleva, E. van Westing, O. Copuroglu, A.L.A. Fraaij, “Correlation of microstructure, electrical properties and electrochemical phenomena in reinforced mortar. Breakdown to multi-phase interface structures. Part I: Microstructural observations and electrical properties”, Materials Characterization, 59, 290-300, (2008)

4. D.A. Koleva, J.H.W. de Wit, K. van Breugel, L.P. Veleva, E. van Westing, O. Copuroglu, A.L.A. Fraaij, “Correlation of microstructure, electrical properties and electrochemical phenomena in reinforced mortar. Breakdown to multi-phase interface structures. Part II: Pore network, electrical properties and electrochemical response”, Materials Characterization, 59, 1550-1558, (2008)

5. D.A. Koleva, “Electrochemical behavior of corroded and protected construction steel in cement extract”, Materials and Corrosion, 62, (3), 240-251, (2011)

6. D. A. Koleva, Z. Guo, K., van Breugel and J. H. W. de Wit, “The beneficial secondary effects of conventional and pulse cathodic protection for reinforced concrete, evidenced by X-ray and microscopic analysis of the steel surface and the steel/cement paste interface”, Materials and Corrosion, 60, (9), 704-715, (2009)