Corrosion Behavior of Steel Buried in Soil Under Natural Rainfall Conditions at Outdoor Test Field
Authors: Mineta Shingo , Ohki Shota , Mizunuma Mamoru , Tsuda Masayuki 
Published: February 2020

Introduction

Buried steel structures generally deteriorate due to soil corrosion. The corrosion rate depends on various soil environment factors such as the water content, pH value, resistivity, and oxygen concentration (1, 2). It is well known that these factors in shallow ground usually change with rainfall. However, the relationship between these factors and the corrosion rate of buried steel under natural rainfall conditions has not been clarified. Therefore, in this study, we used electrochemical impedance spectroscopy to evaluate the corrosion behavior of steel buried in shallow ground under natural rainfall conditions at outdoor test fields.

Experimental

The electrode cell for the AC impedance measurements was fabricated from a carbon steel plate with dimensions of 10 × 10 mm and embedded in epoxy resin. It was buried at a depth of 90 cm in the gley soil of the experimental system at outdoor test field. The soil conditions changed with rainfall. The AC impedance measurements were carried out at constant time interval to evaluate the changes in the corrosion rate with rainfall. They were conducted over the frequency range of 0.05 Hz to 10 kHz at the AC voltage of ± 50 mV.

Results and discussion

Figure 1 shows the Nyquist plot of the buried steel obtained from the AC impedance measurement with different voltage conditions. The equivalent circuit is also shown in Fig. 1, where R
s, R
ct, C
dl, and Z
w are the resistance in soil, charge transfer resistance, double-layer capacitance, and Warburg impedance, respectively. At the low AC voltage of ± 5 mV, the Nyquist plot could not be measured due to exogenous noise. On the other hand, as shown in Fig. 1, it could be obtained by increasing the AC voltage to ± 50 mV. It has been confirmed that the influence of the applied voltage of ± 50 mV on the Nyquist plot is small. Therefore, we set the AC voltage during the impedance experiments to ± 50 mV. Figure 2 shows the changes in the 1/R
ct value of the buried steel over time under natural rainfall conditions. As shown in Fig. 2, the value of 1/R
ct, which is proportional to the corrosion rate, decreased during rainfall and gradually increased over time after the rain stopped. During this experiment, the soil water content and oxygen concentration were kept almost constant at around 54% and 0.9%, respectively. In extremely low-oxygen soil, it is considered that the cathodic reaction on the steel surface is a reduction reaction of water, not a reduction reaction of oxygen. Since both the soil water content and oxygen concentration were almost constant, it is suggested that the cathodic reaction stayed constant during experiment. Therefore, in this study, the change in the corrosion rate with rainfall was thought to be due to changes in anodic reactions.

Conclusion

The corrosion rate of steel buried at a depth of 90 cm in gley soil under natural rainfall conditions at outdoor test fields was monitored by electrochemical impedance spectroscopy. It was observed that the 1/R
ct value, which is proportional to the corrosion rate, decreased during rainfall and gradually increased over time after the rain stopped. From the changes in the AC impedance and soil environment factors with rainfall, it was found that the corrosion rate in gley soil was closely related to anodic reactions on the steel surface.

References

(1) R. Akkouche, C. Rémazailles, M. Barbalat, R. Sabot, M. Jeannin and Ph. Refait, J. Electrochem. Soc., 164(12). C626-C634 (2017).
(2) S. Mineta, S. Ohki, M. Mizunuma, S. Oka, ECS Trans., 85(13), 599-604 (2018).

Figure 1