Polarization Behavior of Carbon Fiber as an Anodic Material in Cathodic Protection

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1 ISTANBUL - TURKEY May th, 2008 Polarization Behavior of Carbon Fiber as an Anodic Material in Cathodic Protection Mahdi Chini 1 Roy Antonsen 2 Øystein Vennesland 3 Jon Håvard Mork 4 Bård Arntsen 5 T 18 ABSTRACT In impressed current CP systems, an insoluble anode is used on the surface or within the concrete structure, and in some parts an electrical connection between the anode and rebars are established. The most frequently and widely used anode material is titanium covered with noble metal oxides. Meanwhile much work is carried out to replace the anode material or improve the application technique such as using conductive coatings and paintings. In this regard, carbon fiber meshes have been introduced as an alternative for the last decade. By studying the Pourbaix diagram of carbon, one could conclude that it is a not a stable material in alkaline solution at its own corrosion potential. Unfortunately, scientific work and publications concerning electrochemical properties of carbon embedded in concrete are scarce. Hence, a series of experiments have been designed and carried out in order to improve knowledge of its behavior as anode. As a first step, polarization behavior of carbon was investigated and the results are presented in this paper. The polarization curves, from 20 mv up to 5000 mv polarization, indicate that carbon has a passive-active-passive behavior in alkaline solution. It is in passive condition up to 500 mv polarization and then enters to active area up to 2500 mv. From that point, it starts the second passive condition. The effect of addition of 1% and 3% chloride ions has also been investigated. The addition of chloride ions into the solution would increase the corrosion rate of the carbon but it seems that it is due to the characteristic of chloride. In other words, the amount of chloride has a very slight effect on the corrosion. KEYWORDS Carbon fiber, Corrosion, Cathodic protection, Alkaline solution, Polarization Norut Teknologi, Narvik 8514, Norway, mahdi.chini@tek.norut.no Norut Teknologi, Narvik 8514, Norway, roy.antonsen@tek.norut.no Department of Structural Engineering, NTNU, Trondheim 7491, Norway, oystein.vennesland@ntnu.no Maxit Group AB, Oslo, Norway, jon-havard.mork@maxit-group.com Norut Teknologi, Narvik 8514, Norway, baard.arntsen@tek.norut.no

2 1 INTRODUCTION During the last decades, widespread problems of deterioration of civil engineering structures have been experienced in many countries. The major portion of these structures consists of reinforced concrete and the corrosion of reinforcing steel is the principal cause of deterioration. Therefore the registered need for rehabilitation and repair and the associated costs is a matter of great concern for those responsible for assessment and maintenance of affected structures. Due to this matter, several protection and prevention methods and technique have been developed. Cathodic protection and prevention are one of the effective techniques in this regard. Cathodic protection (CP) of steel reinforcement in concrete structures, as schematically illustrated in Fig.1, is obtained by applying a direct current through the concrete from an anode system which is usually applied on the concrete surface. Anode material is connected with the positive terminal of a low voltage direct voltage source towards reinforcement; in this case rebars are acting as cathode and connected with the negative terminal [Pedeferri 1996]. The anode material is perhaps the most critical component of a CP system, because it serves to distribute the protective current across the structure and provides the locations for anodic reactions to take place in lieu of the reinforcing steel. Therefore, while the system is in service, the anode, instead of the reinforcing steel, will degrade. There are some materials which are used as anode. Although the most frequently used anode material is titanium mesh covered by activated metal oxides, as a more recent anode material the carbon fiber meshes have been used for some years in some countries. Figure 1. Schematic illustration of CP system [Pedeferri 1996]. By studying the Pourbaix diagram of carbon (graphite), which is illustrated in Fig.2, one could conclude that it is a not a stable material in alkaline solution at its own corrosion potential [Pourbaix 1974]. Unfortunately the published papers about this anode material are scarce [Chini 2007]. Hence, a series of experiments have been designed and applied in order to improve knowledge of its behavior as anode. In this paper the first results are presented. Potentiostatic behavior of carbon in simulated concrete pore solution and also in present of different amount of chloride would be illustrated. 2 TEST DESIGN The first property of carbon interested was corrosion current density in different polarization potentials (E-I plot). In order to reach that, the corrosion current density of carbon was measured in different anodic polarization potential from open circuit corrosion potential as 20, 40, 60, 100, 150, 300, 500, 750, 1000, 1500, 2000, 2500, 3500, 5000 mv. These measurements were operated by Electrochemical Interface 1287 from Solartron Company. By using Solartron 1287 the potentials were continuously applied and the data was stored. A titanium mesh was used as counter electrode and the reference electrode was SCE (Hg/Hg 2 Cl 2 in saturated KCl).

3 Figure 2. Pourbaix diagram of Carbon. Since existence of chloride ion plays an important role in corrosion process, effect of different amount of chloride were also investigated. Therefore measurements were also made in chloride contents of 0, 1 and 3 percent of solution volume. 3 SPECIMENS AND SOLUTION PREPARATION In CP system the carbon is used as mesh. The applied anode material is a continuous, highly flexible mesh woven of carbon fibers. This makes the anode easy to install. The threads may consist of thousands of filaments with a thickness of 7 µm of each filament. In a common industrial product, a thread consists of 6000 or filaments which make a very narrow thread [Vennesland et al. 2005]. Since it was impossible to find a carbon rod from industry, it is very difficult to measure the corrosion current density among longitude of fibers, due to the fact that it is impossible to measure the exact surface area of exposed fibers. Although some techniques have been introduced [Mork et al. 2006], there are still uncertainties on measurement of surface area. Hence, in order to have more accurate active surface area after various experiments, the cross section corrosion monitoring was designed. In this method 10 bundles of filament thread were cast in a non-conductive epoxy. Transparent pipe was used as mold in order to monitor the accuracy of the procedure. After molding, the specimen was cut and ground to have a smooth and clear cross section. This method gives a surface area of around 10 mm 2. A chamber was also designed for this method. It is a rectangular prism with dimensions of 15cm length, 12.5 cm width and 12 cm depth which is filled by 1.75 liter solution. Figure 3 shows the final version of sample. New samples were made by cutting 4 to 5 millimeters from the head of the used specimen. Therefore 20 cm length specimen could be used for several experiments. To reach a stable status in polarization the following procedure was applied. First, system was run in open circuit condition for 2 hours to stabilize the carbon in this alkaline solution. After that anode was

4 polarized between 30 minutes and 150 minutes, depends on polarization value. All procedure was monitored and saved by Solartron A B C D HOLE FOR SPECIMEN E Figure 3. Photos of specimen and chamber. A) Carbon mesh. B) Carbon specimen. C) Chamber. D) Test setup E) Cross section. To simulate the concrete pore solution, a mixture of sodium, potassium and calcium hydroxide was used. ph of the solution was 13.2 and the measurements made in room temperature between 22 and 25 centigrade. Chloride ions were added as NaCl to make 1% and 3% chloride solutions. 4 RESULTS The potentiostatic results were measured for different chloride contents. Figures 4 and 5 show examples of open circuit curve and 750 mv-polarization curve in 3% chloride solution. The E-I plot (potentiodynamic curve) for all 3 different chloride contents is illustrated in Fig. 6. Unfortunately the last read in 3% chloride content was out of range due to destabilization of the system. The result for

5 applied potential less than 500 mv are showed in separate plot. As it is observed in Fig. 4, before exposing the carbon to the alkaline solution, it is in immune area. After exposing by passing time, it eventually reaches to its open circuit corrosion potential. The open circuit value for testing carbon material is around -410±10 mv vs. SCE. Moreover during polarization, it took time to stabilize the current and that depended on polarization amount (Fig. 5). Figure 4. Open circuit measurement in 3% chloride content. Figure 5. Potentiostatic measurement in 750 mv polarization and 3% chloride content. As it is showed in Fig. 6, behavior of carbon in different chloride contents was mostly the same. There is a very small shift in curves to right by increasing the chloride content. In all three conditions there is a small area of passivation up to 300 mv polarization. In this area the corrosion current density is low, between 2E-8 ~ 7E-8 A/mm 2. It is more obvious in non chloride contaminated condition. After that carbon has an active behavior with a very rapid slope up to 2500 mv polarization. In this point, corrosion current density reaches to values of 1E-3 A/mm 2. After this point there is another passive

6 area in non-chloride content solution. But is both 1% and 3% chloride content this passive area is started after a jump from 1E-3 to 1E-2 A/mm2 in 2500 to 3500 mv polarization respectively. It should be also mentioned that the oxygen reduction theorically should happen in 220 mv vs. SCE. This potential is equal to 630 mv polarization for the test material. During tests formation of bubbles on the surface of the carbon was observed after 750 mv polarization. Unfortunately monitoring the changes in ph value of the solution due to oxygen reduction could not be possible because of small surface area of working electrode compared with solution volume Cl=0% Cl=1% Cl=3% E E E E(V) E(V) i(a/mm2) Cl=0% Cl=1% Cl=3% E E E E E E E E E+00-1 i(a/mm2) Figure 6. E-i plot for different chloride contents. However the formation of corrosion products was observed on the surface of the samples after 1500 mv polarization, but these products were solid and didn t released to the solution in non-chloride solution case. But in 1% and 3% chloride contents conditions, changing color to brown around the working electrode surface was observed from 3500 mv and 2500 mv polarization respectively by researcher. 5 CONCLUSIONS The results have been presented of carbon fibre material which could be used as an anode material in CP systems. To improve the knowledge about behavior of this material in alkaline media, the polarization tests were fulfilled. The performed investigations have shown that carbon has a passive-active-passive behavior in alkaline solution. It is in passive condition up to 500 mv polarization and then enters to active area up to 2500 mv. From that point, it starts the second passive condition.

7 Corrosion current density is 2E-8 A/mm 2 in low potentials up to 1E-3 A/mm 2 in high polarization potentials. This value would be increased up to 1E-2 A/mm 2 by adding chloride ions in the solution. Addition of chloride into the solution increases the corrosion current densities in high polarization values. But it seems that this effect is slight in low polarization values and amount of chloride ions do not play a great role in that. Oxygen reduction has been observed from 750 mv polarization. It could make some mechanical damage to the surface of the carbon. But the change in solution color to brown was monitored just only in very high polarization potentials. ACKNOWLEDGEMENT A part of this work is financed by the Norwegian Research Council through the strategic research program RECON at Norut Teknologi in Narvik, Norway. Additionally the support from maxit Group AS is highly appreciated both by sharing their scientific and practical experience and financing part of this work. REFERENCES Pedeferri, P., 1996, Cathodic protection and cathodic prevention, Construction and Building Materials, 10[5], Pourbaix, M., 1974, Atlas of electrochemical equilibria in aqueous solutions, National association of corrosion engineering, Houston, Texas. Chini, M., Antonsen, R., Vennesland, Ø., Arntsen, B. & Mork, J. H., 2007, A review of utilization of carbon fibers as an anodic material in cathodic protection, International RILEM Workshop on Integral Service Life Modeling of Concrete Structures, Guimarães, Portugal, 5 6 November Vennesland, Ø., Haug, R. & Mork, J. H., 2005, Cathodic protection of reinforced concrete a system with woven carbon mesh, International Conference on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR), Cape Town, South Africa, November Mork, J. H., Mayer, S., Rosenbom, K., Tuveson-Carlstrom, L., Sederholm, B. & Sandberg, B., 2006, Cathodic protection of concrete structures with a carbon fibre mesh anode, EUROCORR2006, MAASTRICHT, Netherlands, September 2006.