Corrosion Inhibition of Steel Rebar in Concrete By Migrating Corrosion Inhibitors

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1 Corrosion Inhibition of Steel Rebar in Concrete By Migrating Corrosion Inhibitors Bezad Bavarian and Lisa Reiner Dept. of Civil and Manufacturing Engineering California State University, Northridge Nordhoff Street Northridge, California Corrosion of embedded steel is one of the major causes of concrete deterioration in reinforced concrete structures. This type of corrosion results when a corrosive species, water and air, penetrate through the concrete pores to the steel s surface. The key to inhibiting rebar corrosion is to restrict the permeability of concrete. Migrating corrosion inhibitors and concrete sealers can reduce the corrosive ion mobility and permeability by decreasing concrete porosity. The effectiveness of several commercially available migrating corrosion inhibitors and concrete sealers for steel rebar (class 60) in concrete was investigated. Concrete specimens were prepared with reinforcement at 1 and 2 inches of concrete coverage. Corrosion inhibitors and sealers were applied to the surfaces of the concrete after they had been cured for 28 days. Specimens were immersed in a 3.5% sodium chloride solution. Using Electrochemical Impedance Spectroscopy, the specimens were tested to determine changes in their resistance polarization, Rp, over a 52-week period. The R p values are significant because they are inversely proportional to the rate of corrosion. The open circuit potential versus a Cu/CuSO 4 reference electrode (ASTM C876) was also monitored as a function of time to ascertain the probability of corrosion activity. MCI 2022 has successfully demonstrated corrosion inhibition of steel rebar in 3.5% NaCl solution. This MCI product has exhibited promising corrosion inhibiting properties by maintaining a high resistance polarization (low corrosion rate) for the rebar in concrete. The reference concrete samples (non-protected), however, have shown a decline in their resistance polarization, indicative of corrosion. INTRODUCTION Corrosion of steel rebar is the major cause of concrete deterioration in reinforced and prestressed concrete structures. Billions of dollars each year are spent to repair the damages resulting from this type of corrosion that initiates when salts, water and air penetrate through the pores of concrete and reach the surface of the steel. In general, embedded steel rebar in concrete is stable; the concrete provides a highly alkaline, protective environment. Problems arise when the embedded concrete structure is exposed to the aforementioned corrosive species. Chemical admixtures have been combined with concrete as a possible means of preventing chloride ions from reaching the steel's surface. Inhibitors have the effect of promoting a passive layer at the steel s surface, which makes it more difficult for the chloride ions to remove electrons. A reduction in the corrosion rate can occur if the permeability of the concrete is restricted. Migrating corrosion inhibitors (MCI) can reduce the corrosive ion mobility and neutralize these corrosive species. Migrating Corrosion Inhibitor (MCI) technology was recently developed to protect the imbedded steel rebar/concrete structure. MCIs are based on amino-carboxylate chemistry. They are very effective

2 mixed cathodic and anodic corrosion inhibitors. Under normal conditions these substances enhance the vapor pressure. Increased pressure causes the inhibitor molecules to diffuse through the concrete. This diffusion process requires a period of time to migrate through the concrete s pores. Once the MCIs migrate to the rebar s surface, a protective layer is formed. This suggests that the migratory inhibitors are physically adsorbed onto the metal surfaces 1. MCIs can be incorporated as an admixture or can be used by surface impregnation of existing concrete structures. With surface impregnation, diffusion transports the MCIs into the deeper concrete layers. They will delay and inhibit the onset of corrosion on steel rebar. Bjegovic and Miksic recently demonstrated the effectiveness of MCIs over four years of continuous testing 1. They also showed that the migrating aminebased corrosion-inhibiting admixture can be effective when they are incorporated in the repair process of concrete structures 2. Furthermore, laboratory tests have proven that MCI corrosion inhibitors migrate through the concrete pores and protect internal steel bars against corrosion even in the presence of chlorides 3-4. The objective of this investigation was to study the corrosion inhibition of several commercially available migrating corrosion inhibitors on steel rebar in concrete. Electrochemical monitoring techniques were applied while samples were totally immersed in 3.5% NaCl at ambient temperatures. Due to the low conductivity of concrete, the corrosion behavior of steel rebar had to be monitored using AC electrochemical impedance spectroscopy (EIS). During this investigation, changes in the resistance polarization and the corrosion potential of the rebar were monitored to ascertain the degree of effectiveness for these MCI products. The results were compared with previous investigations conducted on several admixtures and stainless steel rebars. EXPERIMENTAL PROCEDURES In theory, the steel rebar/concrete combination can be treated as a porous solution that can be modeled by a Randles electrical circuit. EIS tests performed on a circuit containing a capacitor and two resistors indicate that this model is an accurate representation of an actual corroding specimen. EIS testing allows for the determination of fundamental parameters relating to the electrochemical kinetics of the corroding system. This is attained through the application of a small amplitude-alternating potential signal of varying frequency to the corroding system. Because processes at the surface absorb electrical energy at discrete frequencies, the time lag and phase angle, theta, can be measured. The values of concern in this study are R p and R. The R p value is a measure of the polarization resistance or the resistance of the surface of the material to corrosion. R is a measure of the solution resistance to the flow of the corrosion current. By monitoring the R p value over time, the relative effectiveness of the sample against corrosion can be determined. If the specimen maintains a high R p value in the presence of chloride, it is considered to be "passivated" or immune to the effects of corrosion. If the specimen displays a decreasing R p value over time, it is corroding and the inhibitor is not providing corrosion resistance. The EG&G Instruments Potentiostat/Galvanostat Model 273A and EG&G M398 Electrochemical Impedance Software were used to conduct these experiments and to record the results. Bode and Nyquist plots were produced from the data obtained using the single sine technique. Potential values were recorded and plotted with respect to time. By comparing the bode plots, changes in the slopes of the curves were monitored as a means of establishing a trend in the R p value over time. To verify this analysis, the R p values were also estimated by using a curve fit algorithm on the Nyquist plots (available in the software). Results from EIS tests were organized into bode and Nyquist plots. Based on these plots, the R p and R combined values are displayed in the low frequency range of the bode plot and the R value can be seen in the high frequency range of the bode plot. The diameter of the Nyquist plot is a measure of the R p value. Ten concrete samples with dimensions 8 x 4 x 4 were prepared. Each sample consisted of one 8 inch steel (class 60) rebar (3/8 diameter) and one 8-inch Inconel metal strip (counter electrode). The rebar prior to being placed in concrete were exposed to 100% RH (relative humidity) to initiate corrosion. Teflon tape was wrapped around the top 2-3 inches of inconel and rebar to prevent corrosion rust at the

3 concrete/metal interface. Concrete was mixed with one-half gallon water per 60-lb. bag (.4-.5 cement to water ratio) in a mechanical mixer. Inconel and rebar were placed one inch apart and two inches from the bottom of the wood container before concrete was poured. Five rebars were placed one inch from the side of the box and five were placed two inches. The concrete was vibrated by machine after being poured into the boxes. This was done to minimize bubbles and slurry. The concrete was air cured for 24 hours, then removed from the wooden crates and placed in 8 inches of water to continue the curing process. The concrete specimens were removed from water and set out to dry (approx. 28 days; the attained compressive strength of concrete was about 2,800 3,200 psi). The concrete blocks were sandblasted to remove loose particles, debris, and rust deposited on the metal. This process left the concrete with a marginally smoother surface. Red shrink-wrap was placed on each of the exposed rebars to prevent additional corrosion. CORTEC MCI 2021 BASE SEALER was applied on two concrete samples. CORTEC MCI 2021 INHIBITOR & BASE SEALER were applied on two concrete samples. CORTEC MCI 2022 BASE SEALER was applied on two concrete samples. CORTEC MCI 2022 INHIBITOR & BASE SEALER were applied on two samples. The remaining two concrete blocks were used as control references. These samples were immersed in 3.5 % NaCl solution (roughly 6-7 inches of each sample was immersed in the solution continuously). EIS (Electrochemical AC Impedance Spectroscopy) testing was started after 24 hours of immersion using a Cu/CuS0 4 electrode. MCI 2021, the thicker of the two inhibitors, did not adhere to the concrete and fell to the bottom of the container after ten hours of immersion in solution. This resulted in a heavy white blanket of residue. Two coats of base and/or inhibitor were applied to each sample. There was nothing notable about the MCI The 2022 inhibitor and base sealer resembled a dilute (thin) milky white fluid. RESULTS and DISCUSSION The corrosion inhibition of two commercially available migrating corrosion inhibitors (Cortec MCI 2022 and 2021) was investigated over a period of 400 days using AC electrochemical impedance spectroscopy (EIS). Throughout this investigation, changes in the resistance polarization and the corrosion potential of the rebar were monitored to determine the degree of effectiveness for the Cortec MCI 2021 & 2022 products. According to the ASTM (C876) standard, if the open circuit potential (corrosion potential) is between 0 and -200 mv, this indicates a 90% probability that no reinforcing steel has corroded. Corrosion potentials more negative than -350 mv are assumed to have a greater than 90% likelihood of corrosion. Figure 1 shows corrosion potentials for the MCI 2021 and untreated control samples dropped from 200 mv to 600 mv, which indicates a 90% probability of corrosion attack on the reinforcing steel. Figure 2 shows that the corrosion potentials for MCI 2022 without inhibitor were 200 mv; for samples with inhibitor, the corrosion potentials were 120 mv. All of the MCI 2022 samples had corrosion potentials in the non-corroding range per ASTM C876. AC electrochemical impedance spectroscopy (EIS) showed that MCI 2021 and reference samples had a gradual reduction in their resistance polarization, from about ohms to less than 600 ohms, which is similar to the drop in their corrosion potentials and strongly indicative of steel rebar corrosion (Figures 3-8). Figures 3-8 show that the resistance polarization for MCI 2022 without inhibitor (samples) gradually increased from 10,000 to 100,000 ohms; for samples with inhibitor, the R p was 10,000 at the beginning of the experiment and ended at close to 200,000 ohms. An increase in the Rp value was observed after about 200 days, indicating that Migrating Corrosion Inhibitors (MCIs) require an induction period for diffusion into the concrete to reach the metal rebar. The samples coated with the MCI 2022 Inhibitor and Base Sealer showed the greatest amount of corrosion resistance; the corrosion resistant behavior was similar to stainless steel rebar (investigated in a subsequent project) 5-6. These results are extremely promising for the MCI 2022 product in its ability to protect steel rebar in concrete in aggressive environments.

4 CONCLUSION Corrosion inhibition of two commercially available migrating corrosion inhibitors (Cortec MCI 2022 and 2021) on steel rebar in concrete was investigated while the concrete was totally immersed in 3.5% NaCl at ambient temperatures using electrochemical monitoring techniques. The MCI 2022 products have successfully inhibited corrosion of the rebar in a 3.5% NaCl solution for 400 days. Steel rebar corrosion potentials were maintained at approximately -120 mv, and rebar resistance polarization reached as high as 200,000 ohms. Both results indicate excellent corrosion resistance performance. The MCI 2021 and untreated samples have shown a decline in their resistance polarization and corrosion potentials, indicative of corrosion. The lack of corrosion inhibition of MCI 2021 could be due to the lack of its adhesion to the concrete surface, thereby leaving the concrete with no protection. In summary, the experimental results demonstrate that the MCI 2022 products offer an excellent inhibiting system for protecting reinforced concrete in an aggressive 3.5% NaCl solution. These results are extremely promising for the protection of steel rebar in concrete in aggressive environments. REFERENCE 1. D. Bjegovic and B. Miksic, Migrating Corrosion Inhibitor Protection of Concrete, MP, NACE International, Nov D. Bjegovic and V. Ukrainczyk, Computability of Repair Mortar with Migrating Corrosion Inhibiting Admixtures, CORROSION/97, paper no. 183 (Houston, TX: NACE, D. Rosignoli, L.Gelner, and D. Bjegovic, Anticorrosion Systems in the Maintenance, Repair and Restoration of Structures in Reinforced Concrete, International Conference Corrosion in Natural and Industrial Environments: Problems and Solutions, Grado, Italy, May 23-25, R. Martinez, A. Petrossian,, and B. Bavarian, Corrosion of Steel Rebar in Concrete, presented at the 12 th NCUR, April R. Martinez, A. Petrossian, B. Bavarian, Investigation of the Corrosion Behavior of Steel Rebar in Concrete in High Chloride Environments Through the Use of Electrochemical Impedance Spectroscopy, June, L. Reiner and B. Bavarian, Corrosion of Steel Rebar in Concrete, presented at the 14 th NCUR, Missoula, Montana, April 2000.

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