Buried steel structures generally deteriorate due to soil corrosion. The rate of the deterioration depends on soil environment factors such as the water content, temperature, pH value, resistivity, and oxygen concentration. Therefore, preventing the deterioration of buried steel structures or predicting their lifetime requires clarification of the quantitative relationship between these soil environment factors and the corrosion rate. However, it is difficult to determine the relationship because soil is a complex environment constituted by solid, liquid, and gas phases. Therefore, we are investigating the influence of the soil environment factors on the corrosion rate by using electrochemical measurements. In this study, we focused on the influence of soil water content on the corrosion rate of buried steel because it is one of the most important parameters in soil corrosion. According to our previous investigation, the soil water content is not constant and usually changes with precipitation. Here, we evaluated the corrosion rate of carbon steel buried in soil by electrochemical impedance spectroscopy in a simulated soil environment with changing water content.

Experimental

The electrode cell for the AC impedance measurements were fabricated from a carbon steel plate with dimensions of 10 × 10 mm and embedded in epoxy resin. The electrode was polished with #800 SiC emery paper. The soil used in this study was reddish gardening soil. The soil was sifted and then classified into several levels of particle size. The electrode cell and soil were placed in a glass container. At the beginning of the measurement, enough water was added to the container to thoroughly wet the soil, and the water content in the soil decreased with elapsed time. The surrounding temperature of the container was maintained at 298 K during measurements. The AC impedance measurements were carried out at constant time interval in order to evaluate the corrosion rate of the carbon steel with changing water content in the soil.

Results and Discussion

Figure 1 shows a Nyquist plot obtained from the AC impedance measurement of steel buried in the reddish soil with water content of 33.4%. The equivalent circuit is also shown, where R_s and C_s are the resistance and capacitance in soil, and R_ct and C_dl are the charge transfer resistance and the double layer capacitance. As shown in Fig. 1, a capacitive loop appeared in the low-frequency region. The radius of this loop represents the charge transfer resistance R_ct, which is inversely proportional to the corrosion rate. Therefore, the integration of the 1/R_ct versus elapsed time curve provides the total amount of corrosion that occurred during the electrochemical measurement. The monitored water content in the soil and the 1/Rct values with elapsed time are shown in Fig. 2. The water content in soil gradually decreases with elapsed time. However, the change in the value of 1/R_ct that is proportional to the corrosion rate was not monotonic. Figure 3 shows the dependence of the 1/R_ct changes on the water content in soil. The value of 1/R_ct showed a maximum peak at nearly 35% water content. This indicates that the corrosion rate and water content are not in a simple proportional relationship. It is considered that the water liquid and oxygen gas phases in soil are competing with each other. Then the corrosion rate shows the maximum peak when the proportions of these two phases are balanced.

Conclusion

Corrosion rate monitoring by electrochemical impedance spectroscopy was carried to reveal the quantitative relationship between the corrosion rate and the water content in soil. It was found that there is no proportional relationship between them, and the corrosion rate has the maximum value at a certain water content, which is determined by the balance between the water liquid and oxygen gas phases in soil.