Corrosion Protection of Steel Rebar in Concrete By Migrating Corrosion Inhibitors

Transcription

1 Corrosion Protection of Steel Rebar in Concrete By Migrating Corrosion Inhibitors Bezad Bavarian and Lisa Reiner Dept. of Manufacturing Systems Engineering and Management California State University, Northridge Nordhoff Street Northridge, California USA Abstract: Technological advances have provided new ways of combating corrosion. Migrating corrosion inhibitors (MCI) are one means of protection for reinforced concrete structures. The effectiveness of commercially available inhibitors for concrete embedded steel rebar has been investigated, this study focused on the usefulness of inhibitors and means of its application. Ten specimens with varying densities were compared for corrosion inhibition properties and an additional six concrete samples were prepared with corrosion inhibitors employing three different application methods: MCI treated concrete surface, MCI coated rebar cast in concrete, and MCI mortar mixture applied to the concrete surface. All specimens were immersed in a 3.5% sodium chloride solution. Using DC Electrochemical and AC Electrochemical Impedance Spectroscopy, the specimens were evaluated to determine changes in their corrosion potential and resistance polarization (R p ) over a 450 days period. These MCI products have exhibited corrosion inhibition by maintaining a high resistance polarization (low corrosion rate) in each of the applications. The measured corrosion potentials for the MCI treated samples (2.08 g/cm 3 (Low-L) and 2.24 g/cm 3 (Medium-M) concrete densities) were more positive than -200 mv, while the higher density concrete showed poor results with values around -500 mv. Resistance polarization estimates showed a similar trend with MCI 2022L measuring 21,000 Ω, MCI 2021L with 13,000 Ω, untreated L had R p less than 3,000 Ω, and all high density samples R p were around 1,600 Ω. To further evaluate the test data, the rebar surfaces were analyzed using X-ray photoelectron spectroscopy (XPS). XPS analysis of the surface treated concrete confirmed the presence of inhibitor and chloride on the steel surfaces, demonstrating that MCI molecules and chloride ions migrate through the concrete layer to reach the rebar. XPS depth profiling showed about 140 nm layer of amine-rich compound on the rebar surface and the migration of chloride ions into the concrete. The MCI molecules retarded the corrosion process by suppressing the aggressive chloride ions and protecting the steel rebar. Introduction: In March of 2002, the final report of a two year nationwide study was released to congress; it projected that the United States incurs billion of dollars in corrosion costs each year for reinforced concrete structures (highway bridges, waterways, ports, drinking water and sewer systems). In 1998, it was estimated that there were 583,000 bridges in the United States (200,000 steel bridges, 235,000 conventionally reinforced concrete, and about 108,000 pre-stressed concrete bridges). Roughly 87,000 of these bridges were considered structurally deficient, largely because of corroded steel reinforcement. The annual direct cost of corrosion for highway bridges is estimated to be $8.3 billion, consisting of $3.8 billion to replace structurally deficient bridges over the next ten years, $2.0 billion for maintenance and cost of capital for concrete bridge decks, $2.0 billion for maintenance and cost of capital for concrete substructures (not

2 including the decks), and $0.5 billion for maintenance painting of steel bridges. Indirect costs due to traffic delays and lost productivity were estimated at more than ten times the direct cost of corrosion maintenance, repair, and rehabilitation. 1 Previous studies have verified the benefits of using migrating corrosion inhibitors, the importance of good concrete, and the significance of ingredients used to make the concrete Reinforcing steel embedded in concrete shows a high amount of resistance to corrosion. The cement paste in the concrete provides an alkaline environment that protects steel from corrosion by forming a protective ferrous (FeO,/Fe 3 O 4 /Fe 2 O 3 ) oxide film. The corrosion rate of steel in this state is negligible. Factors influencing the ability of the rebar to remain passivated are the water to cement ratio, permeability and electrical conductivity of concrete. These factors determine whether corrosive species like carbonation and chloride ions can penetrate through the concrete pores to the rebar oxide layer. In highly corrosive environments (coastal beaches and areas where deicing salts are common), the passive layer will break down, leaving the rebar vulnerable to chloride attack. The steel embedded concrete structure will require additional help to prevent corrosion damage. Migrating Corrosion Inhibitor (MCI) technology was developed to protect the embedded steel rebar/concrete structure. Recent MCIs are based on amino-carboxylate chemistry and the most effective types of inhibitor interact at the anode and cathode simultaneously. 2-4 Organic inhibitors utilize compounds that work by forming a monomolecular film between the metal and the water. In the case of film-forming amines, one end of the molecule is hydrophilic and the other hydrophobic. These molecules will arrange themselves parallel to one another and perpendicular to the reinforcement forming a barrier. Migrating corrosion inhibitors are able to penetrate into existing concrete to protect steel from chloride attack. 6 The inhibitor migrates through the concrete capillary structure, first by liquid diffusion via the moisture that is normally present in concrete, then by its high vapor pressure and finally by following hairlines and microcracks. The diffusion process requires time to reach the rebar s surface and to form a protective layer. 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, where they will inhibit the onset of steel rebar corrosion. Bjegovic and Miksic recently demonstrated the effectiveness of MCIs over five years of continuous testing. 2-4 They also showed that the migrating amine-based corrosion-inhibiting admixture can be effective when incorporated in the repair process of concrete structures. 2 Furthermore, laboratory tests have proven that MCIs migrate through the concrete pores to protect rebar against corrosion even in the presence of chlorides. 4, 5 The objective of this investigation was to determine what effect concrete density and method of application had when using migrating corrosion inhibitors. Since the rate of migration is in part dependent on the density and permeability of the concrete, a high density concrete that impedes corrosive species from reaching the surface of the rebar may also prevent the inhibitor from reaching the surface of the concrete. Experimental Procedure: Ten concrete samples with dimensions 20x10x10 cm were prepared with varying density concrete (5 low density-2.08 g/cm 3, 5 high density-2.40 g/cm 3 ). The rebar prior to being placed in concrete were exposed to 100% RH (relative humidity) to initiate corrosion. Concrete was mixed with 0.5 water/cement ratio. All 18 samples had an 8 inch steel (class 60) rebar (1.25 cm diameter), a 20 cm Inconel metal strip (used as the counter electrode) and a Cu/CuS0 4 reference electrode was used. The rebar coverage layer was maintained at one

3 inch of concrete. Compressive strengths ranged between Mpa (2,700 3,000 psi) after 28 days of curing. For eight of the ten low/high density samples, MCI 2022 and MCI 2021 were applied to the concrete samples (2 low density with 2021, 2 low density with 2022, 2 high density with 2021, 2 high density with 2022). Another eight samples (concrete density: 2.24 g/cm 3 ) inhibitors employing three different methods: MCI treated concrete surface (2 samples), MCI coated rebar cast in concrete (2 samples), MCI mortar mixture applied to the concrete surface (2 samples). The remaining two concrete samples were untreated (references). All samples were continuously immersed in 3.5 % NaCl solution (covering roughly 7 inches of the sample). Testing was conducted using the EG&G Potentiostat/Galvanostat (Model 273A with a 5210 Lock-in amplifier), EG&G M398 Electrochemical Impedance Software and additional testing was done on the Gamry PC4-750 Potentiostat. Electrochemical monitoring techniques were applied while samples were immersed at ambient temperatures. In theory, the steel rebar/concrete combination can be treated as a porous solution and modeled by a Randles electrical circuit. Due to the low conductivity of concrete, the corrosion behavior of steel rebar has 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 effectiveness for these MCI products. Each sample was tested once per week monitoring the open circuit potential and the resistance polarization (R p ); data are collected for bode and Nyquist plots to further corroborate the values. After 450 days of immersion in a NaCl solution, several concrete samples were cut open to remove the rebar for examination. A large area surface analysis was performed on several low density samples using a Kratos Axis Ultra XPS in electrostatic lens mode with a resolution pass energy of 160, and a magnesium anode. The depth profiles were conducted using 2 kv Argon ion gun. The etching rate for depth profiling was estimated based on silicon oxide removal rate. Results & Discussion: Assessment of these corrosion inhibitors (MCI 2022 and 2021) for three concrete densities was based on R p values determined from bode plots, curve fitting algorithms in the Nyquist plots and open circuit potentials. 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. These products proved to be effective means of protection for steel rebar in concrete. For purposes of clarity, only one of the samples from each group is compared in the bode plots; either sample (Sample 1 or Sample 2) is representative of the group, so there is no loss of information. Open Circuit Potentials: According to the ASTM (C876) standard, if the open circuit potential (corrosion potential) is -200 mv or higher, 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. There were noticeable problems with the high density concrete samples; high density concrete impedes corrosive species from reaching the surface of the rebar. It may also have prevented the inhibitor from reaching the surface of the concrete. Figure 1 shows that corrosion potentials for all of the high density samples (H2021, H2022, H untreated) were between the range of -400 mv to -600 mv after 128 days of immersion in NaCl. For the untreated control sample (L untreated), the corrosion potential was 295 mv at the end of testing. The corrosion potentials for MCI treated concrete samples (L2022, L2021) were between 120 to -145 mv. For the samples with a density of 2.24 g/cm 3, all corrosion potentials fell within the range of -48 mv to -175 mv during the first130 days of testing, regardless of

4 application method. Overall, the low density samples had significantly higher corrosion potentials, indicating a greater likelihood of good passivation. Resistance Polarization: Figure 2 shows that MCI treated concrete samples are increasing in their R p values compared with the control samples that appear to have a decreasing trend. The high density concrete sample results were an exception; they suffered rapid chemical deterioration. From the graph in Figure 2, the resistance polarization values at the end of testing were between 13,000 and 22,000 ohms for the low density concrete samples with MCI. The high density concrete showed much less corrosion inhibition with R p values in the 1000 to 2000 ohm range. For non-treated samples (controls), the R p value ended at 3170 ohms for low density samples and 1200 ohms for high density concrete. Changes in the R p value were not immediately observed, indicating that corrosive species and/or Migrating Corrosion Inhibitors (MCIs) require an induction period (about 120 days) for diffusion into the concrete. Figures 3 and 4 show the experimental results for low density (2.08 g/cm 3 ), medium density (2.24 g/cm 3 ), high density (2.40 g/cm 3 ) and untreated concrete samples. For each treatment, both samples from a given group show consistent results; in viewing the graphs, the curves are tightly spaced. In Figure 3, it is evident that there is a substantial difference between the low density and high density concrete samples. XPS Analysis: The interior of the cut open sample showed well formed concrete and no presence of corrosion. XPS analysis was conducted to verify inhibitor migration through the concrete structure and its adherence to the rebar surface. Figure 5 shows the XPS spectrums for two rebars removed from the MCI treated samples after 450 days. The MCIs had penetrated through the concrete coverage layer to reach the rebar (nitrogen content is attributed to the Amine-based MCI) and slow down corrosion. Depth profiling (using 2 kv Argon ion gun) of steel rebars removed from MCI treated concretes are shown in Figure 6; a 140 nm layer of amine-rich compound on the rebars surface were detected. Chloride was also found on the rebar surface with deposits at about 0.99 wt % concentration for MCI 2022 and 0.84 for MCI Table 1 shows the XPS results had organic compound carboxylate chemistry. The XPS results demonstrate that both MCI and corrosive species had migrated in through the concrete capillary system, but MCI had managed to neutralize corrosive species and protect the steel rebar. Table 1. XPS Results of Steel Rebar removed from MCI treated concrete after 450 days. MCI 2022 MCI 2021 peak position FWHM atomic Mass atomic Mass BE (ev) (ev) conc % conc % conc % conc % Fe 2p O 1s Si 2p Ca 2p N 1s Cl 2p C 1s Conclusion: 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 migrating

5 corrosion inhibitor products. MCIs travel through the concrete by means of liquid and vapor diffusion to reach the surface of the steel rebar. Therefore, lower density concrete samples provide an easier path for the MCI inward diffusion, and faster corrosion retardation. The results were compared with previous investigations conducted on several admixtures and stainless steel rebar. X-ray photoelectron spectroscopy (XPS) and depth profiling were done to confirm that the inhibitors had reached the rebar surface. The experimental results demonstrate that the MCI products offer protection for the steel rebar and are capable of providing an inhibiting system in aggressive environments like seawater. The migrating corrosion inhibitors proved to be more effective in the low-density concrete samples and MCI 2022 had better corrosion protection than MCI X-ray photoelectron spectroscopy (XPS) analysis demonstrated the presence of inhibitor on the steel rebar surface, indicating that the MCI migrated through the concrete layer. The XPS depth profiling showed the presence of a 140 nm layer amine-rich compound on the rebar surface while chloride ions were also present; this corresponded with the increase in the R p values and improved corrosion protection for the steel rebar treated with migrating corrosion inhibitors. References 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, & D. Bjegovic, Anticorrosion Systems in the Maintenance, Repair & Restoration of Structures in Reinforced Concrete, International Conf. Corrosion in Natural & Industrial Environments: Problems and Solutions, Italy, May 23-25, D. Darling and R. Ram "Green Chemistry Applied to Corrosion and Scale Inhibitors." Materials Performance (1998): D. Stark Influence of Design and Materials on Corrosion Resistance of Steel in Concrete." R and D Bulletin RD098.01T. Skokie, Illinois: Portland Cement Association, B. Bavarian, and Lisa Reiner, Corrosion Protection of Steel Rebar in Concrete using MCI Inhibitors, EUROCORR 2001, Italy, October B. Bavarian, D. Bjegovic, and M. Nagayama Surface Applied Migrating Inhibitors for Protection of Concrete Structures, CONSEC 01, Vancouver, June B. Bavarian, and Lisa Reiner, Corrosion Protection of Steel Rebar in Concrete using MCI Inhibitors, NACE 2001, Houston, March B. Bavarian, and Lisa Reiner, Corrosion Inhibition of Steel Rebar in Concrete using MCI Inhibitors, EUROCORR 2000, London UK, September 2000.

6 0 Figure 1. Corrosion Potential vs Time, ASTM C876-91, Cortec MCI 2022 & 2021 Compared with Unprotected Concrete (Various Concrete Densities) L 2022 Potential, (mv) L 2021 L untreated H 2022 H untreated H 2021 M untreated M Rebar 2022 M Surface 2022 M Mortar Time of Submersion (Days) Figure 2. Comparison of Resistance Polarization (R p ) for MCI Treated Concrete (various densities) with Untreated Samples L 2022 L 2021 L untreated H 2022 H 2021 H untreated Rp (OHMS) TIME (Days)

7 Figure 3. Comparison of AC Impedance Spectroscopy, Bode Plot Results for Low (L) & High (H) Density Concrete L untreated-day 1 L untreated-day 380 H2021-Day 1 H2021-Day H2022-Day 1 H2022-Day 327 L 2021-Day 1 L 2021-Day 381 L 2022-Day 1 L 2022-Day 383 l Z l (Ohms) FREQUENCY (Hz) Figure 4. Comparison of AC Impedance Spectroscopy, Bode Plot Results for Various MCI Applications, Concrete Density = 2.24g/cm 3 l Z l (Ohms) Untreated-Day 1 Untreated-Day 105 Mortar 2022-Day 1 Mortar 2022-Day 107 Surface 2022-Day 1 Surface 2022-Day 107 Rebar 2022-Day 1 Rebar 2022-Day FREQUENCY (Hz)

8 Figure 5. XPS Spectrum of Steel Rebars removed from MCI treated concrete after 450 days of submersion. Large area (1000 x 800 mm) survey scan. Lens mode electrostatic; Resolution pass energy-160; anode: Mg (150 W). MCI 2022 MCI Figure 6. XPS Depth Profiles of Steel Rebar removed from MCI treated Concrete Samples after 450 days of submersion (Etched using 2 kv Argon Ions) 2022 N (1s) 2022 Cl (2p) 2021 N (1s) 2021 Cl (2p) 1.4 Concentration, atm % Etch Time (Seconds)