DESIGN OF A CATHODIC PROTECTION SYSTEM TO OVERCOME A GALVANIC DISSIMILARITY ON ICONIC MONUMENT FOR MEXICO’S 200 YEAR INDEPENDENCE CELEBRATION Jorge Cantó Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. Luis Enrique Arvizu i.i.i. Servicios, S.A. de C.V. Jaime Balmes No. 11, Torre C, Piso 2. Colonia Los Morales Polanco, C.P. 11510, México, D.F. Lorenzo M. Martinez-dela-Escalera Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290.
José Alberto Padilla Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. Leonardo De Silva-Munoz Instituto de Investigaciones Eléctricas Reforma 113, Cuernavaca, Morelos, Mexico 62490. Lorenzo Martínez Gómez Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. Edgar Maya Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290.
Angel C. Sanchez Alamina Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290. Maritza Paola Jimenez Corrosión y Protección Ingeniería, S.C. Rio Nazas 6. Cuernavaca, Morelos. Mexico. 62290.
ABSTRACT
Mexico is celebrating two hundred years of independence and in commemoration; the Mexican government is building an iconic monument in the most important avenue in Mexico City. The Estela de Luz monument, which is set to last 200 years free of corrosion while serving as a reminder to the Mexican people of their struggle for independence, is already facing its first challenge. The artist and architects designed the structure using two different metals: Stainless Steel and Carbon Steel, therefore generating a galvanic pair which, if not mitigated, will induce a shortening of the life span of the structure. A combined solution using coatings and a hybrid cathodic protection system was proposed for the structure in order to minimize the net corrosion current. Numerical modeling and analytical calculations are presented demonstrating the effectiveness of a hybrid cathodic protection system using galvanic pairs and an impressed. Current system. In addition, laboratory tests were performed for the study of the stainless steel - carbon steel galvanic pair embedded in concrete, and for a comparison of the performance of different coatings. The design of a corrosion monitoring system is also presented. Keywords: Cathodic protection systems, concrete corrosion, galvanic corrosion
INTRODUCTION Commemorating the two hundred years of independence the Mexican government decided to build a structure, a monument by the name of Estela de Luz, which in essence represents the fight for freedom, the fallen brethren, the born of a new nation and hope for future generations. The monument is being built in the most important avenue in Mexico City. It was designed by the architect Cesar Perez Becerril, graduated from the Universidad Autonoma Nacional de Mexico (UNAM). The Estela de Luz will emerge from an underground chamber representing all the Mexican people that died fighting for independence and national sovereignty. The monument was selected among many for its simple and esthetic design and was conceived in order to last more than 200 years. Nevertheless, its design and construction materials bring forward some important aspects to be considered in order to avoid corrosion of crucial structural elements. The foundation, for example, needs to be protected against corrosion; hence a cathodic protection system [1,2] under ASTM standards [3] was needed.
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Figure 1: a) Graphical representation of the Estela de Luz, b) Construction site. CONSTRUCTION CHARACTERISTICS The Estela de Luz will stand 104 meters tall and have 2 parallel planes constructed with quartz panels with dimensions of 0.865 x 1.78 meters and 3 inches thick. Eight stainless steel columns, 91 centimeters in diameter, will support each plane. The foundation of the structure is 50 meters deep and is being constructed with a network of carbon steel reinforcing bars and carbon steel anchors installed at three different depths, 3.75, 2.7, and 3.20 meters. Each column will be bolted to the foundation through 24 anchors, which brings a total of 192 foundation anchors. All the reinforcing bars and anchors will be embedded in concrete. The contact between the carbon steel anchors and the stainless steel columns forms a galvanic pair that can trigger corrosion of the foundation metal structures. In addition, environmental conditions like rain, wind and moisture can affect the durability of the exposed steel elements. Variable mechanical stress due to wind forces acting upon the monument could also increase corrosion risks on the foundation. In order to avoid corrosion of the foundation, a hybrid system using both sacrificial anodes and an impressed current system was proposed. The sacrificial anodes will protect the carbon steel elements inside the concrete structure while the impressed current system will address the galvanic pair between the stainless steel of the columns and the carbon steel of the bolting anchors. In addition, the application of a special coating on the carbon steel bolts will allow the protection of the bolts from atmospheric corrosion.
LABORATORY TESTING OF THE CATHODIC PROTECTION SYSTEM In order to determine if the cathodic protection will be enough for preventing the corrosion caused by the galvanic pair between carbon and stainless steel [4], a laboratory test was performed on a stainless steel plate and a carbon steel bolt embedded in concrete. Cables were welded on the plate and the screw in order to measure the corrosion potentials of both elements using a reference cell of CuSO4. In addition an anode was installed inside the concrete for the impressed current tests.
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Figure 2: a) Stainless steel plate and carbon steel screw in concrete with a cable welded on them in order to measure the potentials and corrosion current, b) Experimental setup.
Table 1 Open circuit potentials of Stainless Steel and Carbon Steel embedded in concrete (CuSO4) Material OCP Stainless Steel -0.340 V Carbon Steel -0.515 V
Table 1 presents the open circuit potentials of the stainless steel plate and the carbon steel bolt. The potential difference between the materials induced a corrosion current that would be also present in the foundation of the Estela de Luz, if not properly protected. By applying current to the galvanic pair the potential of the stainless steel is expected to shift in the electronegative direction reaching a value close to the open circuit potential of the carbon steel and therefore reducing the corrosion current as observed in Table 2. The impressed current system showed to be effective for the reduction of the corrosion current below the detectable limits of the equipment used. Table 2 Potentials and corrosion current with increased cathodic protection Potential Stainless steel vs Potential Carbon Steel vs Corrosion Current CSE CSE -0.309 -0.356 30 mA -0.329 -0.356 10 mA -0.331 -0.353 08 mA -0.342 -0.362 03 mA -0.383 -0.402 00 mA
LABORATORY TESTING OF THE PERFORMANCE OF PROTECTIVE COATINGS In order for galvanic corrosion to occur, three elements are required. 1) Dissimilar metals 2) Metal-to-metal contact 3) Metals in the same conduction solution (usually called an electrolyte) For areas with dissimilar metals in atmospheric contact, galvanic corrosion is expected due to electrolyte presence by rain. In this case cathodic protection is not an option so a coating solution is presented. Three different coatings for atmospheric corrosion protection of the carbon steel bolts were tested (Figure 5). The experiment consisted in exposing to a corrosive environment 8 stainless steel plates with two carbon steel bolts fixed on each plate. Four of the plates were exposed during 1000 hours to a mist of a saline solution made of sodium chloride (50 g/L) and acetic acid (pH between 3.1 and 3.3) inside a controlled temperature chamber. The rest of the plates were immersed in water in order to test the water-resistance properties of the coatings. A 6 cm incision was made on the protective coatings simulating a mechanical damage of the coatings. The three coatings that were tested are presented in table 3. After the saline mist and water immersion tests the protective coatings were removed from the samples in order to visually examine for signs of corrosion. Results of the tests are summarized in table 4.
A a
c
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d
Figure 5: Plates with different coatings. a) Visco-elastic coating protected by PVC tape, b) Elastomeric coating based on liquid rubber, c) Elastomeric coating based on polysulfide, d) Non-coated plate.
Table 3 Type of coatings that were tested Plate Tag
Coating
A
Visco-elastic coating protected by PVC tape.
B
Elastomeric coating based on liquid rubber.
C
Elastomeric coating based on polysulfide.
D
No coating, control exposed plate and bolts.
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Figure 6: Saline mist. a) and water immersion, b) tests. Table 4 Results from the coating performance tests in the saline mist chamber Plate Tag
Result summary
A
Adhesion of the coating was lost in some parts with evidence of electrolyte entrance into the coating-metal interface. Corrosion was found at the bimetallic contact zone.
B
Adhesion was lost around the coating border and around the incision made on the coating. Corrosion was found at the bimetallic contact zone and between the bolt nut and the bolt washers, as well as at the top of the bolts. Galvanic corrosion has occurred.
C
Poor adhesion of the coating was detected around the border of the coating and around the incision made on the coating. Evidence of electrolyte penetration and corrosion was found under the coating. No galvanic corrosion signs were found. Some pores on the coating were detected with signs of corrosion on the carbon steel bolts.
D
Severe corrosion of the carbon steel bolts. No corrosion in stainless steel plate.
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b
c
d
Figure 7: Corrosion evidence on the samples exposed to the saline mist with different coatings. a) Visco-elastic coating protected by PVC tape, b) Elastomeric coating based on liquid rubber, c) Elastomeric coating based on polysulfide, d) Non-coated plate. Table 5 Results from the coating performance tests after sample immersion in water Plate Tag
Result summary
A
Adhesion of the coating was excellent. No corrosion evidence was found. No water penetration was detected through the border of the coating neither through the incision.
B
Poor adhesion was observed around the border of the coating and around the incision made on the coating. Water penetration and galvanic corrosion was detected. Ampoules were found on the coating over the bolts and washers, although no corrosion evidence was found under the ampoules.
C
Poor adhesion of the coating was detected. Electrolyte penetration was found causing small corrosion spots on carbon steel. No galvanic corrosion evidence was found. Pores on the coating were detected but caused neither observable corrosion nor water penetration. Ampoules were found but corrosion products were detected only under breached ampoules.
D
Moderate corrosion of carbon steel parts, especially at the bimetallic contact
Plate Tag
Result summary zones. The stainless steel surface remained intact.
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b
c
d
Figure 8: Corrosion evidence on the samples immersed in water with different coatings. a) Visco-elastic coating protected by PVC tape, b) Elastomeric coating based on liquid rubber, c) Elastomeric coating based on polysulfide, d) Non-coated plate. Besides the experimental results, a more extensive analysis was made considering various factors like ease of application, costs, appearance, etc. The analysis indicated that coatings A and C would be the most suitable options for the protection of steel bolts in the Estela de Luz monument, leaving the final decision on availability of the product and on the characteristics of the electrolyte that will be in contact with the foundation bolts of the monument.
CATHODIC PROTECTION SYSTEM CONSTRUCTION As mentioned above, in order to guarantee a 200 year lifetime of the monument without corrosion, a hybrid cathodic protection system was proposed using sacrificial anodes and an impressed current system. On each foundation anchor from 4 to 6 zinc sacrificial anodes were installed. The impressed current anode bed is made of Titanium electrodes encapsulated in slotted PVC casings. As an additional protection measure, the anodes were installed at a distance of 30 cm from the metallic structures of the foundation to avoid a short circuit. A cathodic protection monitoring system was constructed as well using embedded reference electrodes.
The main challenge in the design was achieving adequate current distribution; novel technics including finite element numerical modeling were used. As showed in the next figure the modeling results predicted a reasonable current distribution reaching polarized potentials between -0.948 and -1.098 volts CuSO4.
Figure 9: Numerical modeling of the cathodic protection system. Anodes placing were decided based on the numerical modeling results. The amount of anodes considered a design current density of 20 mA/m2 for 148.4 m2 resulting in a total of 98 anodes at a current rating of 30.92 mA per anode. Voltage drops were considered including those on the wiring.
Figure 10: Sacrificial Anodes in Anchors. A selected area of the structure was instrumented so the cathodic protection system can be turned off and the effectiveness can be tested. Polarization curves were performed. As seen in figure 11, the polarization indicates that the system is working as intended and the structure is being fully protected against corrosion.
Figure 11: Polarization Curve. CONCLUSIONS The Estela de Luz monument needs to last 200 years without corrosion problems. The different evaluation methods showed the efficiency for parts that were planned to be protected, the exposed steel and the submerged steel. The galvanic dissimilarity probed to be a potential problem with particular preference in the foundations of the monument. In order to provide protection to the monument foundations sacrificial anodes of Zinc were implemented in each of the anchors, and a system of impressed current, in which titanium anodes were used as an anodic bed for the structure. Laboratory testing was efficient to provide the simulation needed for the design. With the hybrid system, corrosion problems due to the galvanic dissimilarity will be mitigated at least 200 years as specified.
REFERENCES 1. D.W. Whitmore, J.C. Ball, “Galvanic Corrosion Protection Systems for Concrete Bridge Structures,” Concrete Repair Bulletin (2005): p. 20. 2. JIN Wei-liang, ZHAO Yu-xi, “Effect of Corrosion in Bond Behavior and Bending Strength of Reinforced Concrete Beams”, Journal of Zhejiang University 2, 3 (2001): P 298. 3. ASTM STP 906, “The Determination of the Corrosion Rate in Steel Embedded in Concrete by the Polarization Resistance and A. C. Impedance”, Philadelphia ASTM. 4. Ha-Won Song, V. Saraswathy, “Corrosion Monitoring of Reinforced Concrete Structure – A Review” Int. J. Electrochem. Sci., 2 (2007): P 1.