NEW TRENDS IN MIGRATING CORROSION INHIBITOR TECHNOLOGY FOR CORROSION PROTECTION OF EMBEDDED STEEL REINFORCEMENT

Transcription

1 NEW TRENDS IN MIGRATING CORROSION INHIBITOR TECHNOLOGY FOR CORROSION PROTECTION OF EMBEDDED STEEL REINFORCEMENT Boris Miksic Cortec Corporation 4119 White Bear Parkway St. Paul, MN USA Jessica Jackson Meyer Cortec Corporation 4119 White Bear Parkway St. Paul, MN USA Alla Furman Cortec Corporation 4119 White Bear Parkway St. Paul, MN USA Marlin Hanson Cortec Corporation 4119 White Bear Parkway St. Paul, MN USA

2 ABSTRACT Lower whole-life costs are achieved by choosing effective protection systems during the design stage of an engineering structure. A belt and braces corrosion protection system will often include Migratory Corrosion Inhibitors (MCI ) and Vapor Phase Corrosion Inhibitors (VpCI ), galvanic protection and life time monitoring of the corrosion process. This paper presents novel MCI (cementitious) and VpCI (epoxy, polyurethane, acrylic) technologies and a new galvanic coating derived from NASA research. The competing mechanisms of corrosion protection are supported by case histories of successful worldwide applications. INTRODUCTION Corrosion of embedded reinforcement has long been recognized as one of the most prevalent durability factors affecting whole life costs of concrete structures such as bridges, parking ramps, buildings worldwide. Similarly, the life of steel structures is shortened by corrosion. A recent report from the Federal Highway Administration estimates the annual cost to replace structurally deficient bridges in the United States to be $3.79 billion. The same report estimated two billion dollars as the annual maintenance and cost of capital for concrete bridge decks, another two billion dollars as the annual cost of capital and maintenance of substructures and superstructures (minus the decks), and a half a billion dollars as the annual cost for maintenance painting of steel bridges (1). Typical coatings simply act as a protective film to isolate the substrate from the environment. Steel coatings are composed of epoxy, vinyl, inorganic zinc, organic zinc, furan, urethane, coal-tar epoxy and chlorinated rubber (2). Concrete coatings are composed of cement, polyurethane, polyurea, and elastomeric compounds. These coatings protect materials only where they contact the substrate. Tears, cracks, holes and holidays defeat the coating and allow corrosion to occur. Vapor Phase Corrosion Inhibitors (VpCI) possess appreciable vapor pressure at atmospheric conditions, allowing significant vapor phase transport of the active inhibitor compound. VpCIs function by forming a bond with the metal surface and creating a barrier layer that helps prevent contact of aggressive ions

3 such as chloride. Besides being used alone for corrosion protection of enclosed spaces, VpCI have a number of other applications. They are incorporated into packaging materials, oils, water treatment and process chemicals, metal working products, coatings, and are able to provide corrosion protection in all three phases(liquid, vapor, and interface) (3) Migratory Corrosion Inhibitors (MCI ) are chemically similar to VpCIs. MCI is one method for controlling corrosion and lowering whole life costs. MCI s are mixed inhibitors (anodic/cathodic) based on amine chemistry which can be added to new concrete as admixtures to the concrete mix, or topically applied to an existing concrete surface. MCI s can also be used as additives to repair mortars as well as injected into cracks or inserted into concrete in the form of emitting devices or gels. Incorporating MCI and especially VpCI into coatings provide them with the ability to protect the coated areas, but also areas adjacent to the coating (scratches, for example). Using active coatings with VpCI on concrete structures, provides a source of VpCI., and also using MCI that can migrate to embedded reinforcement to provide corrosion protection, as well as a physical barrier to eliminate further ingress of corrosive materials such as water and chloride into the concrete itself. This paper will discuss the history of MCI and VpCI technologies, their use in active coatings for corrosion protection, give examples of successful testing and case histories worldwide, and present the use of MCI in conjunction with a newly developed galvanic protection coating system. HISTORY Amine based corrosion inhibitors have been in use since the 1940 s. The United States Navy used them to protect boilers and piping systems. Commercial use of VpCI (emitters, packaging products, coatings, and water treatment and process applications) began in the 1970 s. MCI s are based on the same chemistry as VpCI, and were first introduced for use in reinforced concrete structures in the early 1980 s (4). MCIs are a mixed (anodic/cathodic) amine based inhibitor system. MCI mixed inhibitors contain molecules in which electron density distribution causes the inhibitor to be attracted to

4 both anodic and cathodic sites. They are secondary electrolyte-layer inhibitors. Substances dispersed or dissolved in the electrolyte-layer cause electrolyte-layer inhibition. MCIs possess appreciable saturated vapor pressures under atmospheric conditions, thus allowing significant vapor phase transport of the inhibitive substance. MCI admixtures migrate to the reinforcing steel through diffusion (both liquid and vapor. The first commercial use of mixed inhibitors in concrete was in 1986 as shown in Table I. Year Type of Chemical Application Inhibitor Composition 1979 Anodic Calcium Admixture Nitrite 1986 Mixed Alkanolamines Admixture 1990 Mixed Water-based amine Surface applied carboxylate 1990 Mixed Water-based, Admixture organic (amines and esters) 1993 Anodic Calcium Nitrite Surface applied 1994 Anodic Organic and Admixture inorganic 1996 Mixed Water based, organic Surface Applied 1997 Mixed Amino Alcohol Admixture, Surface Applied 1997 Anodic Sodium mono fluorophosphat e Surface applied 1998 Mixed Ethanol amine Pellets into predrilled holes Table I: Development of Concrete Reinforcement Corrosion Inhibitor Technology and Application Methods UNIQUE FEATURES OF AMINE CARBOXYLATE BASED CORROSION INHIBITORS First generation MCIs based on amino alcohols were found to extend the time to corrosion initiation, and cut corrosion

5 rates in half compared to controls. Second and third generation migrating corrosion inhibitors are based on amine carboxylate technology and can generally double the time to the onset of corrosion. Once corrosion starts, they can slow the corrosion rate to four times or more less than that of a control. Many laboratory and field studies have been conducted to verify the effects of MCIs on concrete properties and corrosion behavior of embedded rebar. According to published and distributed information, MCIs have the following characteristics when used in concrete for corrosion protection. A) MCIs can be incorporated as admixtures to concrete in small dosages (4,7,8,9). B) MCIs have little to no effect on air content, setting time, slump and slump loss, compressive strength, freeze-thaw resistance, etc, and therefore, there is no need to adjust the mix proportions when MCIs are used (7,8,9). In general it can be said that MCIs are compatible with all additives and ingredients used in concrete mixes. C) MCIs are effective in corrosion protection of steel in concrete when used as an admixture or by surface impregnation (8,9,10). Under some conditions, MCIs provided better protection of rebar than calcium nitrite. The highest efficiency in terms of corrosion current reduction has reached 75% and 83% (10,11) and the extended service life of structures with corrosion damage was estimated to be 26 years. D) The techniques employed to study MCI properties in corrosion protection include electrical and electrochemical measurement as well as weight loss comparisons in both solution and concrete. There is evidence that concrete containing MCIs had shorter initiation times than concrete containing calcium nitrite, but much longer protection times in terms of time to repair, indicating a much lower corrosion rate once started. This reveals the difference in the mechanisms of corrosion protection for these two inhibitors. E) Compared to other corrosion inhibitors, MCIs have several clear-cut characteristics: they do not affect concrete properties, are environmentally friendly, are not reactive and adsorb onto the rebar surface. They have mixed inhibiting actions and pose no risk of

6 misuse, have fast diffusion and do not require direct contact with the rebar. Based on these characteristics, MCIs can be used as a nondestructive measure for corrosion protection both as admixtures and surface treatments to concrete (21). Less salt penetration was seen on an MCI treated bridge section after 7 years of heavy road salt exposure. This effect is achieved due to the amine carboxylate chemistry of Cortec MCIs. When the selected Cortec MCI molecules migrate through the concrete, they react with calcium in the substrate. As a result of this reaction, insoluble salts of carboxylic acids are formed, and the amine portion of the molecules is released. This creates two obvious advantages: 1) The formation of insoluble salts blocks pores making the concrete less permeable. 2) Amine is fast migrating and moves quickly towards embedded reinforcement. 3) The migrating portions of MCI released by the amine carboxylate also have a stronger attraction to the steel reinforcement than amino alcohol based products. The reaction an amine carboxylate based MCI has with the substrate creates true positively and negatively charged molecules that in turn migrate toward embedded reinforcement to form a film. These true charges have a greater affinity for the steel than the _+ and _- charges an amino alcohol based product would have, allowing for even longer corrosion protection. CASE HISTORIES Many successful case histories using MCIs and VpCIs in the field have been completed since their introduction. In repair applications, MCIs are often used in combination. For instance, using an MCI modified repair mortar to patch spalled areas, followed by a surface application of inhibitor to the entire structure, followed with a topcoat of an MCI or VpCI modified coating. The oldest commercial concrete repair case history was a repair done to Randolph Street Bridge over I-35 in St. Paul, Minnesota in It was a patch and overlay of low-slump, dense concrete. The westbound lanes of the bridge were treated with an amino alcohol based MCI admixture, while the

7 eastbound lanes were kept as controls. In the year 2000, fourteen years after the repair, the MCI modified concrete was still showing the benefits. The MCI treated side had lower average copper-copper sulfate half cell potentials, lower total corrosion, and higher resistivity readings than the untreated side of the bridge. It was found that MCI was still present at the depth of the reinforcing steel (7.6 cm) and that chloride levels on the treated side were much lower that chloride levels on the untreated side. Just based on this lowering of chloride levels, it is estimated that the treated side of the bridge will have a seventeen year increase in service life. Commercial MCIs developed by Cortec Corporation have been used by (see table 2). Cortec Corporation, a leader in corrosion prevention technologies, has developed a special approach to achieve compatibility of their molecules with the delivery system. This approach allows them to adjust the properties of existing products, as well as develop new products perfectly suited to the requirements of any specific application. A study on the application of an MCI modified fairing coat to Building Block B located at the Psalter Lane Campus of Sheffield Hallam University was conducted. The study included three test sites representing external exposed, external sheltered and internal exposure environments. The application of the MCI modified fairing coat resulted in inhibitor being detected at significant depths of cover after only a few days, and to be present at the depth of reinforcement after 18 weeks (14). STRUCTURE REHABILITATION PROJECT LIST PROJECT NAME CONTRACTOR APPLIED Charles Wilson Building Leicester University A1 Canal Bridge Ranby Mount Concrete October 2003 Strategic Ltd May 1999 SUPPLYING COMPANY

8 Al Road Over Bridge Blythe Bridge Pleasey Building Collingwood Dock Yard Liverpool Trent Bridge Newark Chartham Court Brixton Strategic Ltd September 1999 Repair Services May 2000 C Probe June 2000 Yoldings May 2001 Floating Dock Davenport Maker Coatings September 2001 Parkside Police Station MSCP Cambridge Oldest known UK Sectional Concrete Toilets Norwich Old Post Office Harlow Bell Court Barnet Talbot Road College Stretford Makers UK Ltd T Gill & Son (Norwich) Ltd Westcrete Ltd September 2002 November 2002 March 2003 Renocon Ltd May 2003 Brookes Northern Ltd July 2003 The Grampians London W4 Yoldings September 2003 Table II: Examples of Cortec MCI Applications in the United Kingdom The most recent case history is the use of migratory inhibitors on the rehabilitation of the Pentagon in Washington, D.C. The amine based corrosion inhibitor being used on the building was chosen because the corrosion on the walls is due to carbonation, not chlorides, which eliminated several other types of corrosion inhibitors. Over 18,560 square meters (200,000 square feet) of hand patch repairs are being done to the exterior walls, followed by the application of an amine-carboxylate based, surface applied migrating corrosion inhibitor to over 92,903 square meters (1,000,000+ square feet) of surface area. The final step is the

9 application of a silicate based coating system. This project began in April 2003 and will continue over the next 8-10 years TESTING Theoretical calculations and experimental studies show that very low concentrations of surface applied migrating corrosion inhibitors provide significant corrosion protection for reinforcing steel. One such research program at the University of California, Northridge, by Dr. Behzad Bavarian and Lisa Reiner (15) studied use of migrating corrosion inhibitors incorporated into a silane/siloxane based water repellent. The concentration of MCI put into the repellent was below 5%, and the repellent was then applied at a rate of 3.68 square meters per liter to the concrete. The study showed that MCI molecules were able to penetrate 3 cm of concrete and formed a monomolecular layer on the embedded rebar. Presence of MCI on the rebar was confirmed using X-ray Photon Spectroscopy (XPS) analysis and corrosion protection was measured using electrochemical impedance spectroscopy (EIS) and half cell evaluation. The ability to migrate to unprotected areas and re-passivate the damaged passivating layer on the steel rebar surface was confirmed in the results of Cracked Beam Corrosion Testing by Wiss, Janney, Elsner and Associates, Inc. (16) and American Engineering and Testing. Both studies show that MCI reduces corrosion rates (by 2-5 times) compared to controls and performs better than traditional corrosion inhibitors such as calcium nitrite and butyl oleate (12). The long-term study conducted by General Building Research Corporation of Japan (11) on reinforcing concrete samples cast with 3 kg/m 3 of chlorides confirms the excellent corrosion protection provided by surface applied amine carboxylate based migrating corrosion inhibitors. Results were obtained using electrochemical potentials, polarization resistance monitoring, and weight loss measurements after terminating the test. It should be noted that the majority of corrosion problems in concrete are caused by chloride ingress, which leads to pitting corrosion, making the use of MCIs even more important. A special study conducted by using cyclic polarization

10 techniques show that use of MCIs prevented pitting corrosion caused by chlorides. Such an ability should make MCI a preferred technology for corrosion protection of steel reinforcement because pitting corrosion is usually associated with initiation of stress corrosion cracking which could lead to structural failure of the reinforcement. Positive effects of utilizing MCIs in repair situations were confirmed by corrosion monitoring of the Vachon Bridge in Louval, Quebec, Canada, report of Concrete Bridge Protection and Rehabilitation by the Strategic Highway Research Program, as well as many others (7,8,13). VAPOR PHASE CORROSION INHIBITORS The main and most valuable ability of MCIs is to migrate/diffuse towards metal surfaces and protect them from corrosion by forming an adsorbed layer(12,15). This ability is also realized in VpCI protective coatings technology (7). Incorporation of VpCIs in resins at sufficient concentration levels not only improves the corrosion protection under coated areas, but also prevents corrosion of surrounding areas. A laboratory study done in 1996 compared a VpCI Aluminum coating with a competitor Aluminum coating. The coatings were applied to cold rolled steel and allowed to cure before testing. Tables III and IV show the results. Sample Film Thickness Corrosion Comments (mils) Rating* VpCI Aluminum Coating Scribe rust + scatters rust Competitor Non-VpCI Al. Coating Many large blisters on most of surface *10 = no corrosion, 1 = total corrosion Table II Salt Spray Resistance (1,000 hours) Sample VpCI Aluminum Coating Competitor Non-VpCI Al. Coating Film Thickness (mils) Corrosion Rating* Comments Coating is shinier and lighter Initial pinholes corroding, lighter

11 Coating *10 = no corrosion, 1 = total corrosion Table III QUV Testing (1,100 hours) corroding, lighter Considering that VpCI Coatings would not cause dangerous localized (pitting) corrosion when partially destroyed should make them a preferred choice for protection of metal. GALVANIC PROTECTION COATINGS A new liquid applied galvanic protection coating researched and developed by NASA is now being commercially marketed. This galvanic coating for concrete is revolutionary in its ease of application. It can be spray applied with conventional equipment or even hand painted onto the concrete surface. The well known galvanic technology suppresses corrosion in steel and iron materials by providing a current that eliminates electrochemical attack, and it can be used in conjunction with migrating and vapor phase corrosion inhibitors. Steel reinforcement in concrete structures is subject to rapid electrochemical attack when chloride contamination reaches it. The bars in the area of contamination become anodic (Slide I) (17). This means they are creating weak electrical currents and are being converted into iron oxide or rust. In time, the rust formation expansion creates enough pressure on surrounding concrete, causing it to spall and rebar to fail. The rate of corrosion degradation can be tested using electrochemical techniques. Slide I

12 With the new liquid applied galvanic coating system, thin wires are attached to the concrete surface. The galvanic coating is then applied over the collection wires. The coating is easily applied to a large area of the structure. This large anodic coating area assures that the embedded reinforcement will have galvanic protection. Corrosion rate potential readings of the rebar are taken (Slide II) (17). The coating is then electrically connected to the rebar. The corrosion suppression begins immediately. The current from the coating forces the reinforcement to become cathodic, meaning they are accepting electrons rather than creating them. In time, the reinforcement becomes passive and stops corroding completely (Slide III) (17,18,19,20). Slide II

13 Slide III The future corrosion condition of the reinforcement can be monitored by electrical readings. The coating can be disconnected and a depolarization test conducted to assess the progress. This type of liquid applied galvanic coating is not suitable for vehicle traffic areas. It can be applied to the sides and underside of bridges, balconies, and parking garages. Columns can be coated all the way around. For the top surfaces of balconies and walkways, outdoor carpeting can be applied over the galvanic coating. Since the coating is not suitable for heavy traffic, it can be combined with the surface applied migrating corrosion inhibitor products to produce a two front attack on severe corrosion. Surface applied MCI should be applied to the top of the slab, and the galvanic coating should be applied to the bottom. CONCLUSIONS MCI and VpCI technologies provide dependable corrosion protection to reinforcing steel embedded in concrete, as shown in the multitude of case histories available. MCIs and VpCIs used in repair applications give the advantage of preventing pitting corrosion and providing protection to surrounding untreated areas which they are not in contact with. They can be used to mitigate corrosion in chloride environments, carbonation, as well as in other corrosive environments, unlike nitrite based inhibitors, which are generally only effective against chloride induced corrosion. Migrating

14 corrosion inhibitors and vapor phase corrosion inhibitors are economical to use, are environmentally friendly, and can be formatted into a variety of product types, ensuring the most suitable product for the application can be used. REFERENCES 1. Corrosion Cost and Preventive Strategies in the United States, U.S. Department of Transportation, Federal Highway Administration, FHWA-RD , March Weismantel, G.E., Paint Handbook, McGraw-Hill, Inc., Materials Performance Supplement, Corrosion Prevention and Control Worldwide, NACE International, January B. Miksic, Use of vapor phase inhibitor for the protection of metal parts. Corrosion 83, paper no. 308, (Anaheim California, NACE International, April 18-22, Berke, N., Corrosion Inhibitors in Concrete, Concrete International, July 1991, pg Lafave, J., Pfeifer, D., Sund, D., Lovett, D., and Civjan, S., Using Mineral and Chemical Durability- Enhancing Admixtures in Structural Concrete, Concrete International, August 2002, pg Eydelnant, A., Miksic, B.A., Gelner, L., NACE Corrosion Association of Singapore, Migrating Corrosion Inhibitors for Reinforced Concrete October 94 / New Orleans. 8. Rosignoli, D., Gelner, L., Bjegovic, D., Anticorrosion Systems in the Maintenance, Repair, and Restoration of Structures in Reinforced Concrete, May 1995, Italy. 9. Bjegovic, D., Ukrainczyk, V., Ukrainczyk, B., Miksic, B.A., NACE Corrosion 97, Paper #183 Compatibility of Repair Mortar with Migrating Corrosion Inhibiting Admixtures. 10. Bjegovic, D., Miksic, B.A., Materials Performance Magazine, Migrating Corrosion Inhibitor Protection of Concrete November Nagayama, M., Tamura, H., Shimozawa, K., General Building Research Corporation of Japan, Corrosion Monitoring using Embedded Minisensors on Rebars in Concrete Rehabilitated with a VCI March 97/Japan 12. Stehly, R., Vruno, D., American Engineering Testing, Inc. report April 25, Cherry, B., Forsyth, M., Phanasgaonkar, A., Organic Corrosion Inhibitors: How Do They Inhibit and Can They

15 Really Migrate Through Concrete? Monash University, MCI Modified Fairing Coating. Site Evaluation Report. Oct B. Bavarian, L. Reiner, C. Y. Kim Corrosion protection of steel rebar in concrete by migrating corrosion inhibitors. Corrosion NACE international, paper 16. Cracked-Beam Corrosion Tests on Concrete Treated with MCI 2000 and MCI 2020 Corrosion Inhibitors. Wiss, Janney, Elstner Associates, Inc. 17. Curran, Joeseph, Curran, Jerome, Galvanic Liquid Applied Coating System for Protection of Embedded Steel Reinforcement. Dynacs, Inc., Chemical Instrumentation and Processing Lab, NASA, Kennedy Space Center, FL, May Bertolini, L., Pedeferri, P., Redaelli, E., and Pastore, T., Materials and Corrosion 54, pg. 163 to 175 (2003). 19. Perez-Quiroz, J., Effect of Humectant Agent on Cathodic Protection., Mexican Transportation Institute. 20. Daily, S., Understanding Corrosion and Cathodic Protection of Reinforced Concrete Structures, Corrpro Companies, Inc., Yongmo, X., Hailong, S., Comparative Study of Inhibitors MCI and Sodium Nitrite in Carbonation and Chloride Induced Corrosion of Steel in Concrete, August 2003.