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Effect of Corrosion Inhibitors on Concrete Pore Solution Composition and Corrosion Resistance by M. O’Reilly, D. Darwin, J. Browning, L. Xing, C. E. Locke Jr., and Y. P. Virmani Three commercially available corrosion inhibitors—calcium nitrite, a solution of amines and esters, and an alkenyl-substituted succinic acid salt—are evaluated in conjunction with conventional reinforcement in concrete based on corrosion rate, metal loss, the critical chloride corrosion threshold (CCCT), pore solution analyses, and concrete compressive strength. All three inhibitors increase time to corrosion initiation and decrease corrosion rate, but are less effective in cracked concrete than in uncracked concrete. Of the three inhibitors, the alkenyl-substituted succinic acid salt results in the greatest decrease in corrosion rate, but exhibits the lowest CCCT—below that measured in concrete with no inhibitor. The compressive strengths of concretes containing the amine-ester inhibitor and the alkenyl-substituted succinic acid salt were 15% and 60% lower, respectively, than concrete without an inhibitor. For the latter inhibitor, pore solution analyses indicated elevated sulfate contents at 1 and 7 days, which may explain the low CCCT and strength. Paste containing the amine-ester inhibitor had an elevated sulfate content at 7 days. Keywords: chlorides; corrosion; corrosion inhibitor; cracking; durability; pore solution; steel reinforcement.
concrete with calcium nitrite exhibiting a longer time to corrosion initiation and correspondingly higher chloride content in uncracked concrete (known as the critical chloride corrosion threshold) than reinforcement in concrete without calcium nitrite (Pyc et al. 1999; Bola and Newtson 2005; Xing et al. 2010). In addition to serving as a corrosion inhibitor, calcium nitrite acts as a set accelerator. To counteract this, calcium-nitrite-based inhibitors are often combined with a set retarder. No significant reductions in compressive strength have been observed for concrete with calcium nitrite. The first organic inhibitor in this study is a water-based organic corrosion inhibitor composed of amines and esters (AE). This inhibitor protects steel by adsorption of the amines on the surface of the reinforcement, where they form a protective film. In addition, this inhibitor forms an insoluble salt, blocking the pores in concrete and decreasing permeability. Research involving AE has yielded mixed results. An analysis by Soylev and Richardson (2008) found that while it and other organic inhibitors delayed the onset of corrosion, there was no significant effect on corrosion rate in uncracked concrete; they, however, noted that other studies contradicted this finding (Nmai et al. 1992; Batis et al. 2003). Nmai et al. (1992) observed an approximately 10% reduction in strength for concrete containing this inhibitor compared to concrete with no inhibitor. No other significant adverse effects on material properties were observed. The second organic inhibitor (disodium tetrapropenyl succinate) is a salt of alkenyl-substituted succinic acid (ASSA). The polar end of the molecule binds to the steel, possibly stabilizing the passive layer. The molecule also exhibits hydrophobic properties, decreasing the tendency for moisture to enter concrete (Wojakowski and Distlehorst 2009). Prior research has noted significant reductions in corrosion rate in both uncracked and cracked concrete (Goodwin et al. 2000; Civjan et al. 2003; Gong et al. 2006; Xing et al. 2010). In addition, these studies noted reductions in concrete compressive strength with the use of ASSA, with reductions of between 12 and 52% compared to concrete without an inhibitor. Each of the inhibitors evaluated in this study displays potentially undesirable effects on concrete properties not directly related to corrosion resistance—calcium nitrite acts as a set accelerator, while strength reductions have been observed with the organic inhibitors—and as ACI Materials Journal, V. 110, No. 5, September-October 2013. MS No. M-2012-079.R1 received January 14, 2013, and reviewed under Institute publication policies. Copyright © 2013, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the July-August 2014 ACI Materials Journal if the discussion is received by April 1, 2014.
ACI member Matthew O’Reilly is an Assistant Professor of civil, environmental, and architectural engineering at the University of Kansas, Lawrence, KS. He received his BS in mechanical engineering from the University of Rochester, Rochester, NY, and his MS and PhD in civil engineering from the University of Kansas. ACI Past President David Darwin, FACI, is the Deane E. Ackers Distinguished Professor and Chair of the Department of Civil, Environmental and Architectural Engineering at the University of Kansas, Lawrence, KS. He is a member of ACI Committees 130, Sustainability of Concrete; 222, Corrosion of Metals in Concrete; 224, Cracking; 318-B, Reinforcement and Development (Structural Concrete Building Code); and Joint ACI-ASCE Committees 408, Development and Splicing of Deformed Bars; 445, Shear and Torsion; and 446, Fracture Mechanics of Concrete. JoAnn Browning, FACI, is a Professor of civil, environmental, and architectural engineering and an Associate Dean of Engineering at the University of Kansas. She is a member of the ACI Technical Activities Committee and ACI Committees 314, Simplified Design of Concrete Buildings; 318-D, Flexure and Axial Loads: Beams, Slabs, and Columns (Structural Concrete Building Code); 341, Earthquake-Resistant Concrete Bridges; 374, Performance-Based Seismic Design of Concrete Buildings; and Joint ACI-ASCE Committee 408, Development and Splicing of Deformed Bars. Lihua Xing is a Structural Engineer with Archer Daniels Midland Company, Decatur, IL. She received her BS in structural engineering from Lanzhou Jiaotong University, Lanzhou, Gansu, China; her MS in mathematics from Marquette University, Milwaukee, WI; and her PhD in civil engineering from the University of Kansas. Carl E. Locke Jr. is Professor Emeritus of chemical and petroleum engineering and former Dean of Engineering at the University of Kansas. His research interests include corrosion of steel in concrete. Y. Paul Virmani is the Corrosion Specialist in the Office of Infrastructure Research and Development of the Federal Highway Administration. His research interests include reinforced concrete, prestressed concrete, and cable-stayed bridges.
al. 2010), was used for all specimens. Specimens are wetcured for 3 days and air cured for 25 days prior to testing. The concrete properties, including the average compressive strength at 28 days for four 102 x 204 mm (4 x 8 in.) cylinders cured in lime-saturated water, are shown in Table 2. Test procedures and measurements—The southern exposure and cracked beam tests take 96 weeks. The tests alternate between two cycles for the duration of the test. On Day 1 of the first cycle, a 15% NaCl solution is ponded on the surface of the specimens. The solution concentration provides for relatively rapid corrosion initiation (McDonald et al. 1998) and has been adopted in ASTM A955/A955M (2010) for the cracked beam test. The specimens are maintained at room temperature (20 ± 2°C [72 ± 3°F]) and readings are taken on Day 4. The voltage drop across the 10 ohm resistor is measured to determine the corrosion macrocell current. Dividing the measured current by the surface area of the test bar gives the average corrosion current density i (traditionally expressed in mA/cm2), which is used to determine corrosion rate R (in mm/yr or mils/yr) using Faraday’s Law, written assuming that corrosion occurs uniformly over the surface area of the bar.
Equivalent dosage for concrete with cement content of 355 kg/m3 (598 lb/yd3) and w/c is 0.45.
Corrosion rate and loss The average corrosion (metal) losses for the southern exposure specimens without and with corrosion inhibitors are shown in Fig. 4 and listed in Table 6. Also listed in Table 6 are the coefficients of variation and average corrosion rates based on the slope of the corrosion loss plots for the individual specimens after initiation of corrosion (Darwin et al. 2011). Table 7 lists the ages at initiation. In the southern exposure specimens, conventional reinforcement with no inhibitor was the first to initiate corrosion (at 20.7 weeks) and exhibited the greatest average corrosion rate (10.1 mm/yr [0.40 mils/yr]) and loss (14.4 mm [0.57 mils]) at 96 weeks. To provide some context, a corrosion loss of approximately 25 mm (1 mil) will cause cracking in concrete in cases where there is approximately 25 mm (1 in.) of cover over an uncoated reinforcing bar; the loss increases to a little more than 50 mm (2 mils) for bars with a cover of approximately 75 mm (3 in.), which is typical of the top cover in bridge decks in many U.S. states (O’Reilly et al. 2011; Darwin et al. 2011). All of the inhibitors tested increased the time to corrosion initiation; concrete containing CN and ASSA increased the time to initiation to 29.3 and 31.3 weeks, respectively, while specimens with AE had the longest time to corrosion initiation—37.3 weeks—which is significantly higher than observed in the corrosion initiation tests. All of the inhibitors tested also decreased the corrosion rate in uncracked concrete; reinforcement in concrete containing CN and AE had average corrosion rates of 6.67 and 2.91 mm/yr (0.26 and 0.11 mils/yr), while reinforcement in specimens containing ASSA exhibited the lowest average corrosion rate—1.25 mm/yr (0.05 mils/yr). The average corrosion losses for the cracked beam specimens without and with corrosion inhibitors are shown in Fig. 5. Table 8 lists individual losses at 96 weeks, as well as the average losses and coefficients of variation for the cracked beam specimens. Corrosion initiated in all specimens during the first week of the test. The average corrosion rates through 96 weeks are also listed in Table 8. Conventional reinforcement with no inhibitor exhibited the greatest average corrosion rate—16.3 mm/yr (0.64 mils/yr). Although the inhibitors reduced the corrosion rate in cracked concrete compared to that observed for conventional steel without a corrosion inhibitor, they were not as effective as in uncracked concrete. The specimens containing AE and CN exhibited average corrosion rates of 11.9 and 14.5 mm/ yr (0.47 and 0.57 mils/yr), respectively, while the specimens with concrete containing ASSA exhibited an average rate of 4.17 mm/yr (0.16 mils/yr). As shown in Fig. 4 and 5, the order of the corrosion losses, from high to low, is the same for the southern exposure and cracked beam specimens.
After corrosion initiation. Notes: 1 mm = 0.0394 mil; 1 mm/yr = 0.0394 mil/yr.
After corrosion initiation. Notes: 1 mm = 0.0394 mils; 1 mm/yr = 0.0394 mils/yr.
Note: 1 mL = 0.0338 fl. oz.
pore solution. The specimens containing AE also exhibited increased sulfate levels at 7 days; however, sulfate levels at 1 day were only slightly higher than that of the control specimens. An analysis of AE also showed low sulfate levels (3 ppm). The analysis of the corrosion inhibitors is presented in Table 11. Nitrate levels in all specimens were low with the exception of specimens containing CN, which showed levels of approximately 800 ppm both 1 and 7 days after casting. This is likely due to the oxidation of some of the added nitrite. No significant phosphate levels were detected in any of the pore solutions.
elevated sulfate levels at both 1 and 7 days. The elevated sulfate levels may be due to a change in the hydration process caused by the use of these inhibitors that ultimately impacts compressive strength, perhaps in a way similar to that caused by sulfate attack (Mindess et al. 2003). One positive difference compared to sulfate attack, however, is that the source of sulfates is limited and the concentration should decrease over time as the sulfates react with the other constituents of cement paste. The critical chloride corrosion threshold of conventional steel is increased by the use of AE and CN; however, the use of ASSA significantly lowers the CCCT of steel. This again may be due to the presence of sulfates, which not only have been shown to reduce the time to corrosion initiation by working in tandem with chlorides to destabilize the passive layer on iron and steel (Rasheeduzzafar et al. 1994; Al-Amoudi 2007; Shi and Sun 2011), but which have been observed to alone depassivate steel (Somuah et al. 1991; Turkman and Gavgali 2003). The CCCT values obtained for concrete containing CN and AE in this study are approximately 60% lower than those obtained in other studies (Ann and Buenfeld 2007; Ormellese et al. 2008; Ormellese et al. 2011). In this study, watersoluble chloride content was measured, whereas in other studies, acid-soluble chloride content was measured. Watersoluble chloride content was chosen, as any chlorides that bind to the cement matrix are insoluble in water and not free to attack the passive layer of reinforcement. In addition, the referenced studies obtained chloride samples directly above the reinforcement, whereas in this study, chlorides were sampled away from but at the same depth as the reinforcement. This will again result in a lower CCCT value, but is more representative of values that are obtained from bridge decks, where samples are obtained away from reinforcement, as opposed to at the bar surface.
Parts per million. † Extrapolated.
Support for the research described in this paper was provided by the U.S. Department of Transportation Federal Highway Administration under Contract No. DTFH61-03-C-00131 and the Kansas Department of Transportation under Contract No. C1131 and C1281. Material support was provided by the Concrete Steel Reinforcing Institute, DuPont Powder Coatings, 3M Corporation, Valspar Corporation, BASF Construction Chemicals, Grace Construction Products, Hycrete Technologies, Western Coating, Inc., and LRM Industries.
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