Impedance characteristics of reinforcing steel in mortar and corrosion monitoring

Abstract

The purposes of this study are to clarify fundamental aspect of the corrosion behaviour of reinforcing steel in concrete and to develop an accurate monitoring method. In order to simulate the corrosion of concrete structures in a marine environment, carbon steel rods embedded in mortar were exposed to alternate conditions of 8 h-immersion in 3 mass%NaCl solution and 16 h-drying in room temperatures. The carbon steel rods were embedded at the different positions in the thickness of the mortar (mortar thickness φ: 5, 10, 20, 40 mm). The wet-dry cycles were carried out for the period of 80 days. The impedance was measured in the frequency range from 10 kHz to 1 mHz when the mortar was submerged in 3 mass%NaCl solution. The carbon steel and mortar interface is explained by an equivalent circuit consisting of a solution resistance, a charge transfer resistance and a CPE (Constant Phase Element). As the corrosion progresses, the diffusion impedance appears in the low frequency region. At the initial stage of the wet-dry cycles, the corrosion rate is accelerated due to the enhancement of the anodic dissolution. This enhancement is attributed to introduction of chloride ions into the mortar by the wet-dry cycles. In the middle and final stages, the carbon steel shows higher corrosion rate and less noble corrosion potential as the mortar thickness decreases, indicating the acceleration of anodic process due to neutralization of solution and concentration of chloride ions in the mortar by the wet-dry cycles.