Most transportation infrastructure is built from steel and concrete. The steel may be in structural sections, such as girders, piles or rails, or embedded in concrete to form reinforced or prestressed concrete. Concrete provides excellent protection for embedded steel because Portland cement is very alkaline, forming a passive, 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 admixtures, surface treatments (coatings) 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 surface in about 150 days. The amine-rich compound on the rebar surface improved corrosion protection for the MCI treated steel rebar even in the presence of chloride ions and prevented 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 corrosion damage in new structures.1-3 Outside the laboratory environment, infrastructure may suffer from attack by carbonation and chloride. Chloride ions dissolved in water can permeate 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 12-13 about 7,000 to 8,000 ppm 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 lowered 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 (CaCl2), 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 chlorides from the environment due to direct exposure with marine environment or the use of deicing salts and chemicals. There are four different mechanisms of chloride transport into crack-free concrete, they include: capillary action, diffusion due to the high concentration on the surface, permeation under pressure, and migration due to electrical potential gradients.4,5 Similar to carbonation, the chloride attack process does not directly corrode steel reinforcement, however, it does break down the protective iron oxide film and promote corrosion. Chlorides do play a role as catalysts to corrosion. However, the mechanism of chloride diffusion into concrete is different for carbonation in that it attacks the passive layer without the requirement of pH reduction.