Materials Performance Supplements

CORTEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Migrating Corrosion Inhibitors to Protect Steel Reinforced Concrete Structures Behzad Bavarian, a kin B osede o luwaseye a la B a, and l isa r einer, California State University, Northridge, California, USA Jessica Meyer, Cortec Corp., St. Paul, Minnesota, USA MIGRATING CORROSION INHIBITORS Most transportation infrastructure is built from steel and concrete. The steel may be in structural sections, such as girders, piles, or rails, or em- bedded in concrete to form reinforced or prestressed concrete. Concrete provides excellent protection for em- bedded steel because Portland ce- ment is very alkaline, forming a pas- sive, protective layer on the steel surface. Concrete is also permeable, and even good-quality concrete can be penetrated by aggressive chemical ions that may initiate steel corrosion. Migrating corrosion inhibitors (MCIs), a blend of amine carboxylates and amino alcohols, show versatility as ad- mixtures, surface treatments (coat- ings), and in rehabilitation programs. Examination of the embedded steel rebar after corrosion tests showed no corrosion attack for the MCI-treated concrete samples, while non-treated concrete showed localized corrosion. X-ray photoelectron spectroscopy and depth profiling confirmed that the inhibitor had reached the rebar sur- face in about 150 days. The amine-rich compound on the rebar surface im- proved corrosion protection for the MCI-treated steel rebar even in the presence of chloride ions and pre- vented red rust formation. Corrosion is one of the primary concerns in the durability of materials and structures. Research efforts have been made to find a corrosion inhibition process to prolong the life of existing structures and minimize cor- rosion damage in new structures. 1-3 Outside the laboratory environment, infrastructure may suffer from attack by carbonation, and chloride ions dissolved in water can per- meate through the concrete pores, then penetrate the protective oxide film on the steel surface. Carbonation of concrete can lower the amount of chloride ions needed to promote corrosion. In new concrete with a pH of 12 to 13, about 7,000 to 8,000 ppm by weight of cement chloride is required to initiate steel corrosion. If, however, the pH is lowered to a range of 10 to 11, the chloride threshold for corrosion is significantly low- ered to roughly 100 ppm. 4 Chlorides in the concrete can come from several sources. They can be cast into the structure by the use of deliberate admixtures (calcium chloride [CaCl 2 ]), or the chloride ions can appear in the mix (mixing water, aggregates) unknowingly. However, the major cause of chloride-induced corrosion in most structures is the diffusion of chlo- rides from the environment due to direct exposure with a marine environment or the use of deicing salts and chemicals. There are four different mechanisms of chloride trans- port into crack-free concrete. They include capillary action, diffusion due to the high concentration on the surface, permeation under pressure, and migration due to electri- cal potential gradients. 4-5 Similar to carbon- ation, the chloride attack process does not directly corrode steel reinforcement; how- ever, it does break down the protective iron oxide film and promote corrosion. Chlorides do play a role as catalysts to corrosion. How- ever, the mechanism of chloride diffusion into concrete is different for carbonation in that it attacks the passive layer without the requirement of pH reduction. Corrosion inhibitor technolog y was developed to protect the embedded steel rebar/concrete structure. These inhibi- tors use 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 hydro- philic and the other hydrophobic. Most current migrating inhibitor admixtures are based on amino carboxylate chemistry and the most effective types of inhibitor interact at the anode and cathode simultaneously. 2 These molecules will arrange themselves parallel to one another and perpendicular to the reinforcement, forming a barrier. 3,6-11 Migrating corrosion inhibitors (MCIs) are able to penetrate into existing concrete to protect steel from chloride attack. 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 micro- cracks. The diffusion process requires time to reach the rebar surface and to form a pro- tective layer. These corrosion inhibitors can be incorporated as an admixture or can be surface impregnated on existing concrete 20 JUNE 2018 MATERIALS PERFORMANCE CORTEC SUPPLEMENT TO MP

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