Effect of Soil Particle Size on Corrosion Behavior of Buried Carbon Steel
Authors: Ohki Shota , Mineta Shingo , Mizunuma Mamoru , Tsuda Masayuki 
Published: February 2020

Introduction

The fundamental cause of the deterioration of buried steel structures is soil corrosion. Soil corrosion is a complicated phenomenon because various environmental factors are related to each other. Additionally, due to the complexity of soil and its pores constituted by a solid phase, water liquid phase, and air and gas phases, the soil corrosion mechanism is still unclear. In spite of the inherent complexity of the soil environment, corrosion of carbon steel in soil is considered to be similar to that in a neutral solution. That is, soil corrosion is considered to be promoted by water and dissolved oxygen (DO) in soil water. With this nature of soil corrosion, environmental factors in soil can be represented as two principal ones: soil particle size, which influences the state of water in the soil and DO diffusion behavior, and soil water content. In this study, we evaluated the effects of soil particle size and water content on the corrosion rate of buried carbon steel by electrochemical impedance spectroscopy in a simulated soil environment.

Experimental

Carbon steel with an exposed electrode area of 10 × 10 mm was polished with #800 emery paper. The soil samples used for the experiment were prepared from reddish gardening soil, which was sifted with sieves into coarse- (0.5­­­­–2 mm) and fine- (<0.5 mm) grains. By mixing the coarse grains with the fine grains, the soil samples were classified into three levels coarse: fine = 1:0, 1:1, and 0:1. The soil was placed in a glass container and the electrode was buried in the soil. At the beginning of the experiment, enough water was supplied so that the soil was immersed. Water was gradually drained with time through a filter installed on the bottom of the container. The AC impedance measurements were carried out at constant time intervals. In the test chamber, the temperature was held constant at 25ºC with relative humidity of 60%.

Results and discussion

Figure 1 shows Nyquist plots obtained from the AC impedance measurement of carbon steel in the coarse: fine = 1:0 soil with water content of 43%. The equivalent circuit is also shown, where Rs is the solution resistance, Cs is the soil capacitance, Rct is the charge transfer resistance, and Cdl is the electric double-layer capacitance. As shown in Fig. 1, capacitive loops appeared in the high- and low-frequency regions. In the Nyquist plots, the low-frequency represents Rct. Figure 2 shows change in reciprocal Rct, calculated at each water content while water content was decreased. The horizontal axis is water content and the vertical axis is 1/Rct, which is directly proportional to the corrosion rate. The 1/Rct showed a maximum peak at a certain water content in all soil samples, and that water content showed smaller values as the particle size was smaller. Further, the maximum corrosion rate increased with decreasing particle size. It is considered that the maximum corrosion rate appears when the condition that the water film is thin and the effective wet area is large. As the particle size decreases, the pressure applied to capillary water becomes stronger. So, the effective wet area doesn’t decrease and thinner water film is formed. Accordingly, the water content satisfying the above condition becomes low, and the maximum corrosion rate increases.

Conclusion

The effects of soil particle size and soil water content on the corrosion rate of buried carbon steel were evaluated by the AC impedance technique. The maximum corrosion rate appeared at a certain water content, which varied depending on the soil particle size. The measured trends were rationally interpreted in terms of the balance between the effective wet area of the steel surface and the DO diffusion path distance depending on soil particle size and water content.

Figure 1