2009 Volume 50 Issue 4 Pages 799-805
To understand environmental factors in mortar, the Cl ion concentration and pH were monitored by inserting microelectrodes into artificial pores in the mortar. At the same time, the corrosion behavior of the reinforcing steel was investigated by EIS. In the EIS measurements of the reinforcing steel, Warburg impedance by diffusion was confirmed in the initial period, but it could no longer be observed after 35 days. In comparison with a 10 mm cover thickness, a 20 mm cover thickness showed a higher impedance behavior. The Cl ion concentration in the mortar was obtained using Ag/AgCl microelectrodes, showing that this behavior is generally controlled by diffusion. When the diffusion equation was used in this work, the diffusion coefficient (Dc) showed a high value of Dc=2×10−4 mm2/s. Similarly, the pH in the mortar was obtained using W/WOx microelectrodes. With a 20 mm cover thickness, pH was limited to approximately pH 11, but with a 10 mm cover thickness, pH continued to decrease to around pH 9.5. The latter phenomenon was considered to be the result of neutralization by penetration of the immersion solution from the surface. Based on the results of monitoring with the microelectrodes, solutions simulating those in the pores in mortar were prepared and used in EIS measurements. The charge transfer resistance Rct in the simulated solutions showed good correspondence with the impedance (Z2mHz) in the low frequency region (2 mHz) in the actual mortar. This is attributed to the fact that the corrosion of reinforcing steel was controlled by the solution conditions (mainly Cl concentration and pH) in the pores in mortar. If these solution conditions (Cl concentration, pH) exceed threshold values, it was found that the passivation film is destroyed, resulting in high corrosion.
