Corrosion Analysis of Reinforcing Steels in Mortar for Nuclear Facilities

It is important to estimate the corrosion of reinforcing steel in mortar facilities, because the nuclear plants are located near the sea side. In the case of 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 (electrochemical impedance spectroscopy). In the EIS measurements of the reinforcing steel, diffusion behavior was confirmed in the initial period, but diffusion could no longer be observed after 35 day. In comparison with a 10mm cover thickness, a 20mm 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 (D_c) showed a high value of Dc = 2×10⁻⁴ mm²/sec. Similarly, the pH in the mortar was obtained using W/WO_x microelectrodes. With a 20mm cover thickness, pH was limited to approximately pH11, but with a 10mm cover thickness, pH continued to decrease to around pH9.5. 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 in the low frequency region (2mHz) 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.