Corrosion mechanisms of steel reinforcement in concrete

Two models are discussed in which chloride ion concentration gradients form in steel reinforced concrete structures. Electrochemical testing to compare the models was conducted. Plain carbon steel specimens were exposed in a simulated concrete environment of saturated calcium hydroxide solution with different but uniform concentrations of sodium chloride. The different increasing chloride ion concentrations promoted open circuit potential decreases with reference to a saturated calomel electrode. These potential shifts may lead to galvanic corrosion effects depending on both the average chloride ion concentration and the presence of chloride ion concentration gradients. Calicium nitrite, a corrosion inhibitor was added to simulated concrete solution. It was found the presence of calcium nitrite decreased, or in some cases eliminated the potential shifts associated with different chloride ion concentrations in solution, thereby, greatly reducing or eliminating the possibility of galvanic corrosion effects. Calcium nitrite was shown to be an anodic inhibitor due to the shift of open circuit potentials to more noble values. Plain carbon steel specimens were exposed to chloride ion concentration gradients by two chloride solutions on the same piece of steel connected by a salt bridge. The galvanic current between the different chloride ion concentrations was measured periodically without interrupting the current. Electrochemical impedance spectroscopy was conducted in each chloride solution along with potential measurements. Data from concentration gradients were compared to steel exposed only to a uniform chloride ion concentration. The EIS data was fitted to an equivalent circuit model of the electrochemical interface. The resulting models for steel specimens exposed to chloride ion concentration gradients were at varying times indicative of a diffusion controlled reaction, while the models for steel exposed to uniform chloride concentration represented an activation controlled reaction. The measured galvanic currents upon initiation of visible corrosion were very similar for coupled specimens indicating an external control, probably oxygen limited diffusion controlled reaction