Monitoring of Carbon Steel Corrosion in Soil Under Wet-Dry Cyclic Conditions
Authors: Murakami Shun , Hirata Ryo , Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: December 2020

Corrosion of carbon steel in soil is affected by various factors such as particle size, water content, soli depth, pH, temperature and microorganisms. Based on the results of exposure tests conducted in the past, it is common to design a steel material with a 1 mm "corrosion allowance" in 100 years. However, environmental factors of soil change depending on regions and seasons. Thus further improvement and rationalization of design are required to reduce material cost.

Water content is one of important environmental factors that determines the ratio of the gas and liquid phases in soil, and it is considered to have an influence on corrosion. In addition, the corrosion rate near the soil surface may change significantly due to cyclic wet-dry such as rainfall and daily humidity changes. In this study the corrosion rate of carbon steel in simulated soil under cyclic wet-dry was monitored by electrochemical impedance method, and the effect of cyclic wet-dry on carbon steel corrosion was examined.

A pair of identical carbon steel plates (10 × 5 mm) were embedded in an epoxy resin (40 mm ϕ) and used as a probe electrode for the corrosion monitoring in soil. SiO2 particles with 100 μm diameter were used as a simulated soil. An acrylic pipe with a flange was fixed vertically on the probe surface with a gasket sandwiched, and the silica powder was put into the acrylic pipe so that the height was 1 cm. Furthermore, 3% NaCl solution was poured until the soil surface was completely immersed (100%). The experimental cell was placed in a chamber set at 25 ℃ and 40% RH

During drying, impedance at two frequencies, 10 kHz and 10 mHz was continuously measured. The solution resistance R
sol and charge transfer resistance R
ct were monitored by Z
10kHz and (Z
10mHz – Z
10kHz), respectively. The R
sol increased with the drying time and reached a constant. The impedance measurement was terminated when the R
sol became a constant. In this study, the reciprocal of R
ct was used as an index of the corrosion rate.

The corrosion rate reached a maximum after 1 ~ 3 hours from the start of the drying. It is considered that the increase in the corrosion rate was due to the decrease in the water content by drying. At the onset of the drying, the spaces among the particles were completely filled with 3% NaCl solution. In such a condition, the corrosion will be strongly suppressed because oxygen diffusion through the soil is very slow. As the drying progresses, the gas phase in soil increases and enhances the oxygen diffusion. Thus, the corrosion rate can increase. However, further drying decreases the solution phase and decreases the average corrosion rate because the anodic dissolution of Fe could be hindered. Finally, when the simulated soil was completely dried out, the 1/R
ct became negligibly small.

After the dry-up, ultrapure water was added to the soil, so that NaCl concentration became 3% and the cell was exposed to the 2nd drying. The tendency of the change in R
ct
-1 during the drying was the same as the first drying process. However, the maximum value of the R
ct
-1 seemed to increase by repeated drying. This may be attributed that the corrosion product formed in the drying process becomes an oxidant in the next drying process.