Several migrating corrosion inhibitors for reinforced concrete were investigated using ASTM G109 and Modified G109. XPS depth profiling showed a 90-100 nm amine-rich layer and chloride ions on the steel rebar surfaces indicating that the modified corrosion inhibitor molecules had suppressed chloride ion interaction and protected the steel rebar passivation layer. There is no indication of corrosion after 400 days of testing. The inhibitor treated samples showed improved resistance polarization (Rp was in the 50 kohm to 60 kohm range, corrosion potentials ranged between -64 mV and -103 mV) and potentiostatic tests showed a significantly lowered corrosion rate. These findings led to investigating the adsorption mechanism where comprehensive testing established a Langmuir adsorption isotherm and verified that chemisorption was responsible for the strong bonding between the inhibitor monolayer and the steel surface.
The US infrastructure is heavily reliant upon construction materials that are vulnerable to corrosion. Conventional materials like reinforced concrete, pre-stressed concrete and steel that are used in bridges, highways and building foundations. Reinforcing steel embedded in concrete, however, shows a high amount of resistance to corrosion. Concrete is a highly alkaline material with a pH near 12 that provides a protective oxide layer for steel when embedded. Under corrosive conditions where oxygen and moisture can ingress through the pores of the concrete to reach the rebar surface, the alkalinity can change and cause the passive layer to breakdown. Carbonation of the concrete, where carbon dioxide reacts with calcium hydroxide or other cement hydrates, and ingress of chlorides are major causes of steel rebar corrosion in concrete. In the case of reinforced concrete samples placed in contact with a 3% NaCl solution, the stages for rebar corrosion deterioration would begin with chloride penetration into the concrete, then corrosion initiation or passive layer breakdown, more progressed corrosion, micro cracking and eventual spalling. Corrosion is a complex phenomenon with many interactions to consider (structural, physical, chemical and environmental considerations), for steel in rebar, as the passive film degrades by chloride ions or the pH drops due to carbonation, the metallic iron at the anode is oxidized to form ferrous ions. It is this simplified reaction that has to be prevented or at least mitigated. Much effort has been focused on the design of new structures to reduce or eliminate corrosion through increased concrete coverage using reduced permeability concrete, admixtures, migrating corrosion inhibitors and replacing the steel reinforcement with alternative materials.
The commercially available migrating corrosion inhibitors used to surface impregnate the concrete samples in this investigation are based on amino carboxylate chemistry. They are designed to provide both anodic and cathodic protection with transport to the rebar surface in roughly 100 days. Transport begins through the concrete pores and proceeds through the capillary structure by liquid diffusion, high vapor pressure and by following microcracks to the rebar surface. At the rebar surface, the inhibitor forms a monomolecular layer of protection against corrosive species. To better understand how the inhibitor works or by what mechanism it is able to adsorb to the surface, a surface adsorption isotherm has to be determined. Many models for adsorption isotherms have been defined (Temkin, Freundlich, Langmuir and Frumkin) and each explains a different relationship between concentration and inhibitor surface coverage on a metal or alloy surface. By measuring the corrosion current density o