A REVIEW OF UTILIZATION OF CARBON FIBERS AS AN ANODIC MATERIAL IN CATHODIC PROTECTION

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1 A REVIEW OF UTILIZATION OF CARBON FIBERS AS AN ANODIC MATERIAL IN CATHODIC PROTECTION Mahdi Chini 1, Roy Antonsen 1, Øystein Vennesland 2, Bård Arntsen 1, Jon Håvard Mork 3 1) Norut teknologi, Narvik, Norway 2) Department of Structural Engineering, NTNU, 7491 Trondheim, Norway 3) maxit Group AB, Oslo, Norway Abstract: A literature review has been made on deterioration of carbon fibers embedded in concrete or subjected to alkaline solutions. The main emphasis of this work is to evaluate carbon used as anode material in cathodic protection systems for reinforced concrete structures. Anodes for impressed current systems may be installed as mesh or by strips embedded in shotcrete on the surface on concrete structures with ongoing reinforcement corrosion. For such systems a widely used anode is titanium covered with metal oxides. However, continuous research are carried out in order to reduce cost of anode systems and to improve application techniques such as using conductive coatings and paintings. In this regard, carbon fiber meshes have been introduced as an alternative anode material and have been used as anode meshes for the last decade. Some practical experiences seem to indicate that carbon is insignificantly affected by degradation. The sustainability and lifetime of CP systems are often dependent upon degradation of the anode material. In concrete the anode material must withstand increased anodic polarization in alkaline media. According to Pourbaix diagram, carbon is not stable in ph values for normal concrete. Thermodynamically carbon will dissolve into either gaseous derivates such as CO 2, CO, CH 4, or into dissolved substances like carbonic acid, bicarbonate and carbonate. Unfortunately, scientific work and publications concerning electrochemical properties of carbon embedded in concrete are scarce. Some studies on carbon fiber behavior on different applied potentials exist which mostly were aimed to study the life span of carbon as anode material. These investigations showed that polarization above a critical potential about 1.8 V vs. corrosion potential leads to accelerated corrosion. Hence, accelerated tests on available industrial carbon fiber meshes indicate proper life time where working potential of the anodes are held below the critical potential limit. Ongoing research sponsored by the Norwegian Research Council and directed by Norut Teknologi was established in order to evaluate the electrochemical properties of carbon fibers in concrete. The work is carried out by studies of thermodynamics and kinetics of carbon in alkaline solution and concrete media. Keywords: Cathodic protection, Carbon fiber, corrosion, concrete Contacts: Mahdi Chini, Roy Antonsen, Bård Arntsen, Norut Tekonologi, Narvik, Norway s: chini@ntnu.no roy@tek.norut.no baard.arntsen@tek.norut.no Øystein Vennesland Department of Structural Engineering, NTNU 7491 Trondheim, Norway Phone: Fax: oystein.vennesland@ntnu.no Jon Håvard Mork Maxit Group AB, Oslo, Norway jon-havard.mork@maxit-group.com

2 A REVIEW OF UTILIZATION OF CARBON FIBERS AS AN ANODIC MATERIAL IN CATHODIC PROTECTION MAHDI CHINI 1, ROY ANTONSEN 1, ØYSTEIN VENNESLAND 2 BÅRD ARNTSEN 1, JON HÅVARD MORK 3 1) Norut teknologi, Narvik, Norway 2) Department of Structural Engineering, NTNU, 7491 Trondheim, Norway 3) maxit Group AB, Oslo, Norway Abstract: A literature review has been made on deterioration of carbon fibers embedded in concrete or subjected to alkaline solutions. The main emphasis of this work is to evaluate carbon used as anode material in cathodic protection systems for reinforced concrete structures. Anodes for impressed current systems may be installed as mesh or by strips embedded in shotcrete on the surface on concrete structures with ongoing reinforcement corrosion. For such systems a widely used anode is titanium covered with metal oxides. However, continuous research are carried out in order to reduce cost of anode systems and to improve application techniques such as using conductive coatings and paintings. In this regard, carbon fiber meshes have been introduced as an alternative anode material and have been used as anode meshes for the last decade. Some practical experiences seem to indicate that carbon is insignificantly affected by degradation. The sustainability and lifetime of CP systems are often dependent upon degradation of the anode material. In concrete the anode material must withstand increased anodic polarization in alkaline media. According to Pourbaix diagram, carbon is not stable in ph values for normal concrete. Thermodynamically carbon will dissolve into either gaseous derivates such as CO 2, CO, CH 4, or into dissolved substances like carbonic acid, bicarbonate and carbonate. Unfortunately, scientific work and publications concerning electrochemical properties of carbon embedded in concrete are scarce. Some studies on carbon fiber behavior on different applied potentials exist which mostly were aimed to study the life span of carbon as anode material. These investigations showed that polarization above a critical potential about 1.8 V vs. corrosion potential leads to accelerated corrosion. Hence, accelerated tests on available industrial carbon fiber meshes indicate proper life time where working potential of the anodes are held below the critical potential limit. Ongoing research sponsored by the Norwegian Research Council and directed by Norut Teknologi was established in order to evaluate the electrochemical properties of carbon fibers in concrete. The work is carried out by studies of thermodynamics and kinetics of carbon in alkaline solution and concrete media. Keywords: Cathodic protection, Carbon fiber, corrosion, concrete

3 1. INTRODUCTION Cathodic protection (CP) of concrete structures has been successful in stopping corrosion in reinforcement for about 20 years in Europe 1. This is the only method recognized, by several investigators, to be able to mitigate ongoing corrosion 2. CP involves the application of a voltage to force electrons to go to the steel reinforcing bar, thereby making the steel a cathode. The primary effect of CP is the negative shift of potential where it reduces the driving force for the anodic process. The secondary effect is that CP reduces oxygen content and produces alkalinity on the reinforcement surface. These effects are beneficial because they widen the passive region. On the other hand, in chloride contaminated concrete, the current circulation produces a flux of chlorides from the cathode to anode. The resulting reduction of chloride content on the rebar surface and reduction of the chloride ingress forms a sort of barrier of imperviousness increasing with the current itself (chloride barrier effect). However one should be aware of the negative effects of CP which could be expressed by its effect on adhesion between concrete and rebars and by hydrogen embrittlement. In addition, on the anode surface, oxygen or chloride evolution can take place. Both reactions may lead to conditions which may destroy the concrete and/or anodic material. For this reason the anodic current density have to be limited 3,4. The efficiency of a cathodic protection system depends not only on the electrochemical behavior of the anode but also on how easily it can be installed and its compatibility with other materials, such as cementitious guniting mortars for embedment 8. The anode 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. Consequently, for a CP system to be effective and durable, it is very important that the anode is sufficiently chemically inert and dimensionally stable for use in concrete structures 5. Briefly, the general requirements of an anode system can be summarized as follow: it has to adhere to the concrete surface; it should be applied to any kind of concrete surface (top, bottom, horizontal, vertical, flat, and curved) and exhibit mechanical properties suitable for installation and fixing, long duration combined with low installation cost; it should produce acceptable weight addition and change in structure appearances and dimensions. In this regard, the anode can be: a mesh shaped to fit the structure s surface to which it is fixed and subsequently covered with a cementitious overlay which acts as an electrolyte; a conductive and electroactive layer applied directly to completely cover the concrete surface; wire or strip placed in holes or slots and backfilled with either cementitious or electroactive material or conductive tiles placed on the concrete surface 3. 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. Carbon is a less noble material compared with titanium. By studying the Pourbaix diagram of carbon (graphite) one could conclude that it is a not a stable material in alkaline solution at its own corrosion potential. Hence, knowledge about the consumption of carbon fibers as function of operation parameters (e.g. potential, current density, ph, humidity, etc.) is of major importance in order to design reliable carbon anode systems. This paper is based on a literature survey and some information from industrial research about usage of carbon fibers in CP systems. The results of this survey were used as a base for

4 ongoing investigations on electrochemical behavior of industrial carbon fibers in alkaline solution and evaluation of the criteria for their usage. 2. ANODE MATERIALS Anodes are a critical component of cathodic protection systems. various research was being aimed at searching for the most suitable anode for use in cathodic protection of the various types of concrete in different applications and structures components. The most common anode systems in CP are platinized titanium or titanium covered by activated metal oxides (iridium, ruthenium, cobalt, etc) and conductive coatings and recently carbon fibers meshes. A brief introduction of these systems is illustrated following. Titanium expanded mesh activated with mixed metal oxides (iridium, ruthenium, cobalt, etc.) is the most widely used and successful type of anode. It has good mechanical properties and can be easily fixed to vertical surfaces, and it normally requires an overlay incorporated in the concrete (as in new constructions). Field experiences as well as laboratory tests make it possible to foresee a very long service life for this type of anode (in the range of 20 to in excess of 100 years), provided the quality and strength of the concrete are adequate. The application of titanium as meshes makes it possible to achieve a uniform distribution of current into the concrete. Meshes of different diamond size type are available providing typical current densities of 15, 20 and 30 ma/m 2 with respect to concrete surface area. Moreover if there is a demand of non-uniform current densities, it would be fulfilled by using overlapping of several meshes 3. Also there are several investigations concerning conductive coating systems to apply with titanium meshes. The conventional polymer based coatings were replaced by concrete mortars. Recently carbon fibers have been utilized in the mortar which are introduced more conductive and durable coating system in this industry 5. (Fig. 1-A) Application of conductive coatings as anodes has the following advantages: ease of installation; possibility of protection of places not easily accessible and it enables a large anodic surface allowing uniform distribution of the protective current. Also cathodic protection system installation costs are greatly reduced 6. This system can be easily and economically applied by spraying 10 to 20mm thick layer of a polymer-modified mortar on the surface of the concrete. Moreover, addition of carbon fibers, coated with a thin and homogeneous layer of corrosion resistant metal, are common in order to provide a continuous network with electronic conductivity (resistivity is lower than 0.1 Ω.m). Activated titanium strips are usually embedded in the overlay as primary anodes with the purpose of distributing the current along the conductive overlay. Parallel strips are normally placed at distance of (1 2 m) 4. It should be noticed that the flow of the current through the conductive mortar generates an ohmic drop along the anode that increases as the distance from the primary anode increases. It has been shown that the conductive mortar is reliable up to current densities of 20mA/m 2. At higher current densities, around 50~100 ma/m 2, the cementitious matrix of the anode was destroyed in the vicinity of the primary anode due to acidification after a few months of operation. The limitation in applied current makes design of the current distribution system and primary anodes an important issue together with development and usage of low resistivity coatings. Research shows that the amount of carbon fibers in the mortar plays a great role in functioning of the CP system 6,7.

5 Fig.1 A) titanium mesh B) Carbon fibers mesh Carbon has been introduced as anode material based on woven carbon fibers as a mesh which is embedded in cement based mortars or conductive polymers coatings 9,10,11. Also recently a nickel-based coating has been used on the carbon fibers in order to increase the durability of carbon fibers in concrete media 4,10. (Fig 1-B) Following, a review of previous research which has been done on this novel anode material is presented. 3. CARBON FIBER AS ANODE MATERIAL As a novel and newly innovated anode material, there are scarce published papers on this subject. There are some recent research project sponsored by the industry but unfortunately the results are rarely published till now. Moreover most of previous studies were around the application of this kind of anode and rarely studied its properties thoroughly. Hence, there is a great need to clarify the electrochemical behavior of utilized carbon in this respect. Brief introduction of this method which is mostly used in Nordic countries is presented in follow. 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 a few µm of each filament. In a common industrial product, a thread consists of 6000 filaments which make a very narrow thread 8. Usually thicker threads would be used for the edge of meshes to assure a good distribution of the current. In practice it could be around filaments in a thread 8. The resistivity of the fibers is 15 µωm 8. These meshes can be placed on the surface layer of concrete cover in new structures or installed on the surface of an existing structure and covered with a conductive coating. The coating can be selected as mortar or a conductive polymer based mortar. Fig. 2 partly illustrates the installation procedure in practice 10,12,16. However according to Pourbaix diagram of carbon (Fig. 3), carbon is an active material in concrete media with ph around 13. Hence the lifetime of the fibers in concrete media plays a fundamental role in efficiency of this system. There some recent investigation on this issue to find out the effective parameters on this matter and the criteria of them.

6 Carbon Cabel Secondary cable Connector Fig. 2- Installation net on the surface of concrete structure Research about carbon fibers as anode material was first published in 2004 in CONSEC 04 in South Korea. Nerland and Mork 11 introduced the usage of carbon net in a CPre project on a coastal bridge in Norway. They investigated the functioning of the CPre system on the bridge which was fulfilled in The system was operating properly after installation. An applied voltage of 1.8 V was used and the 100 mv decay was chosen to monitor operation of the system. All reference electrodes which had been installed in different parts of the bridge, showed decay values of more than 130 mv (over 24 hours). No visible cracks resulting from shrinkage of the mortar was observed. Another investigation was carried out by Vennesland, Haug and Mork 9. In this work concrete samples were taken from Ullasund Bridge. Ullasund Bridge is located on the west coast of Norway. The first bridge was built in 1969 but it was demolished and replaced by a new bridge in In this investigation, the criterion given by EN standard was used to control the efficiency of the CP system which was applied on the specimens from the old bridge which has prestressing strands. It was found that all specimens fulfilled the criterion for CP with a feeding voltage of 1.0 volts. But it should be mentioned that due to worry about hydrogen embrittlement and reduce bond strength, this system was never installed on the new bridge. In a recent study by Mork et al. 8, they tried to estimate the expected lifetime of carbon net anode systems. Different applied potential from 1 to 6 volts were used to evaluate the carbon fiber behavior. Also the system was monitored in a test area of 11m 2 on Oland Bridge in Sweden. Mork et al. found that changes in the dominating electrochemical reactions seemed to occur at approximately 2 V above which deterioration of the material took place. Determinations of the weight of the exposed carbon fiber mesh showed that the voltage applied on the anode is crucial considering degradation. The weight was reduced 3% after 168 h for sample at 2 V, but 44% at 3V. No defects were observed in experiments using up to 1.8V polarization. Also EN standard was used on monitoring the field test area during 3 years operation. The results showed that the standard requirements were fulfilled. The protection current was 1-2 ma/m 2.

7 Fig. 3- Pourbaix diagram of Carbon 14 In a monitoring report on a harbor structure in Honningsvåg, Mork et al. 13,15 investigated the performance of the applied CP system. The anode consisted of a highly flexible and conductive woven carbon mesh and a special designed cement based anode sealant used to fix the net to the concrete surface in approximate 3-4mm thickness. The anode sealant was even used as topping in the ceiling, on the columns and on the walls in thickness of 3-4mm. Preparations before installation consisted of manual removal of loose and damaged concrete, sand blasting and open flame on oil spots. Connectors between the carbon net and secondary copper cable was established. The area of cathodic protection of the harbor was 1457 m 2 and on Jetty was approximately 700 m 2. The system was designed to achieve a current density of 2-5 ma/m 2 and a voltage of maximum 1.8V. The system operates well today, with fulfillment of the demands according to EN ,15. However, some researcher tried to achieve a more durable carbon anode material by applying the nickel as a coating on the fibers 4,10. In a high alkaline solution, nickel acts as a corrosion protection for the formation of a passive layer (Ni(OH) 2 ). Toshiaki et al. 10 investigated the operation of this system by the accelerated tests and long term tests on durability of anodes and systems. It was found that 20 ma/m 2 of anode surface was enough to get protection and operational life is expected more than 5 years. In higher current density (110 ma/m 2 of anode surface), a change of color with solution to brown and dark brown was occurred. The cause of color change of solution was assumed by dissolving carbon. This changing of color of the solution at high applied potential and current densities was reported by several researchers 17,18,19,20.

8 4. CONCLUSION Carbon fiber meshes have been used in Scandinavian countries for the last decade as anode material in cathodic protection and prevention. Although it has beneficial aspects compared with other cathodic protection anode materials, the rate of degradation of carbon at different potential/current densities are still unclear. The literature review presented in this paper terms the basis of ongoing research at Norut Teknologi, where the electrochemical properties of carbon embedded in concrete are studied with variation in applied potential/current density and concrete properties. 5. 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. 6. REFERENCES 1. R. B. Polder, A. Krom, W. Peelen, J. Wessels, J. Leggedoor Service life aspects of CP of concrete structures, Proceedings of the 2nd international conference of Concrete Solution, St. Malo, France, June I. Martinez, C. Andrade, M. Castellote, P. Garcia de Veidma Advance in the practical application of electrochemical repair techniques for the control of their efficiency, Proceedings of the 2nd international conference of Concrete Solution, St. Malo, France, June P. Pedeferri, CP and cathodic prevention, Construction and Building Materials, V.10 No.5, pp , L. Bertolini, F. Bolzoni, T. Pastore, P. Pedeferri Effectiveness of a conductive cementitious mortar anode for CP of steel in concrete, Cement and Concrete Research, 34 (2004) G. G. Clemena, D. R. Jackson, Cathodic protection of concrete bridge decks using titaniummesh anodes Final report, VTRC 00-R14, Virginia Transportation Research council, January K. Darowicki, J. Orlikowski, S. Cebulski, S. Krakowiak Conducting coatings as anode in CP, Progress in organic coatings 46 (2003) J. Orikowski, S. Cebulski, K. Darowicki, Electrochemical investigations of conductive coatings applied as anodes in CP of reinforced concrete, Cement and Concrete Research 26 (2004) Ø. Vennesland, R. Haug, J. H. Mork, 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 J. H. Mork, S. Mayer, K. Rosenbom, L. Tuveson-Carlstrom, B. Sederholm, B. Sandberg, Cathodic protection of concrete structures with a carbon fibre mesh anode, EUROCORR2006, MAASTRICHT, Netherlands, September Toshiaki, Kobayashi, Ch. Wu, New cathodic protection system for concrete structures using nickel-coated carbon fibers sheet anodes 21st US-Japan Bridge Engineering Workshop, Japan, October O. C. Nerland, J. H. Mork, Cathodic prevention of reinforced concrete structures Forth International Conference on Concrete under Severe Conditions Environment and Loading (CONSEC 04), Seoul, Korea, June maxit CarboCath general information Brochure 13. K. Byfors, NORECON network on repair and maintenance of concrete structures NORECON Task T2: Repair methods a review, Norway, April 2004

9 14. Marcel Pourbaix, Atlas of electrochemical equilibria in aqueous solutions National association of corrosion engineering, Houston, Texas, USA, Cebelcor 15. Installation of maxit CarboCath on Honningsvåg Harbor Brochure 16. maxit CarboCath reference projects catalogue 17. Consumption rate of CarboCath anode in concrete pore-solution Korrosionsinstitutet, Swedish corrosion institute, November Polarization curves for a carbon fibre anode in simulated concrete pore solution Technical work report, KiMab, December Characterization of carbon fibre anode CarboCath 605 after accelerated testing in the cement-based anode grout maxit CarboCath 680 Technical work report, KiMab, February Characterization of CarboCath anode in concrete pore-solution at two different circuit voltages 1.8 V and 3.0 V, Korrosionsinstitutet, Swedish corrosion institute, December 2005