Evaluation of SD345 Corrosion Resistance in Simulated Concrete Pore Solution
Authors: Yamada Maya , Katayama Hideki , Hoshi Yoshinao , Shitanda Isao , Itagaki Masayuki 
Published: February 2020

Reinforced concrete known as high strength material has been used for social infrastructure for a long time. However, the number of infrastructures used for more than 50 years has been increasing year by year, and deterioration of their durability is concerned. It is well known that corrosion of reinforcing steel leads to concrete cracks. It means that understanding for corrosion behavior of reinforcing steel is important in order to properly maintain the social infrastructures. In this study, corrosion resistance of SD345 steel materials is evaluated under simulated concrete environment by electrochemical methods.

　SD345 steel materials are used as samples, and a saturated Ca(OH)2 solution is used in order to simulate the environment in a concrete. The effect of chloride ion was examined by adding NaCl (0, 0.01, 0.1, 0.2, 0.3, 0.5 M) to the test solution. Electrochemical polarization measurements were conducted after immersing in these solutions for 30 minutes. A three-electrode cell was used for electrochemical measurements. Hg/ HgO/ NaOH and Pt were used as the reference electrode and the counter electrode, respectively. Next, impedance measurements were performed at the open-circuit potential with an AC signal of amplitude 10mV in the frequency range from 105 Hz to 10-2 Hz after immersing electrodes for 1day, 1week and 3weeks, in order to investigate passivation behavior in the test solution.

　The corrosion rate of SD345 was monitored under cyclic wet–dry environments. The wet–dry cyclic corrosion test was conducted by exposure to alternate conditions of 2 minute-immersion in a 0.1 M NaCl solution and 2h-drying or 7 h-drying at 60% RH. 10 mHz and 10 kHz impedances were continuously measured during the wet-dry cycles by corrosion monitor.

The plateau of current density is observed in anodic polarization curve in test solutions, indicating that passive film is formed on the surface. In case of the test solution containing NaCl more than 0.1M, however, the current density rapidly increases, and it occurs at the smaller potential with an increase of amount of Cl-.

The corrosion monitoring of the samples with passive film previously formed in saturated Ca(OH)2 solution was conducted under the alternate condition of 2 minute-immersion in a 0.1 M NaCl solution and 8 h-drying at 60% RH. The passive film formed by immersion for longer time gave the smaller increase of corrosion rate. It means the state of passive film affects the change in corrosion rate in an early stage. The corrosion rates in both conditions did not increase after 11 cycle. On the other hand, in case of shorter drying condition, the change in corrosion rate showed almost the same behavior regardless of immersion times. This means there is no influence of state of passive film on corrosion behavior in shorter drying environment.

The surface of SD345 steel was passivated in a simulaed concrete pore solution, and the resistance of passive film increased with time. The breakdown potential of passive film shifted to less noble with an increase of Cl⁻ concentration. The corrosion performance in cyclic corrosion test with longer drying period depends on the state of passive film previously formed on the surafce. On the other hand, there was no effect of initial state of passive film under cyclic corrosion environment with short drying process.