Influences of Pore Water Content and Covering Depth on Carbon Steel Corrosion in Soil
Authors: Hirata Ryo , Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: February 2020

Carbon steel is widely used as structural materials in soil, but it suffers from corrosion. There are lots of environmental factors that may influence the corrosion of the steel structures in soil. For instance, water content, soil covering depth, soil particle size, pH, temperature, anions (Cl− and SO4
2−) and microorganism1 may be considered as the environmental factors. In order to understand the corrosion mechanism in soil, the influences of these factors must be systematically investigated. In this study, the corrosion of carbon steel has been monitored in silica sands of different water contents and covering depths by electrochemical impedance spectroscopy (EIS).
A pair of identical carbon steel (SM490A) sheets was embedded 2 mm apart in parallel in epoxy resin. The dimension of the carbon steel sheet was 5 mm × 10 mm. The silica sand of 100 μm particle size was used as artificial soil. The mixture of the silica sand and 3% NaCl solution was placed on the carbon sheet electrode using an acrylic pipe (inner diameter 30 mm). The soil covering depth was defined by the height of the silica sand. It was changed to 10, 30 and 50 mm. The pore water content W in soil was changed to 100, 90, 80, 70%. It was calculated by

W=(V
Water/
V
Pore)×100 [%]
where V
Pore is the pore volume in soil and V
Water is volume of the added NaCl solution. The V
Pore was determined from the volume of the solution when all the pores were completely filled with the solution. EIS was measured at the open circuit potential with amplitude of 10 mV and frequency range of 100 kHz to 10 mHz. The charge transfer resistance R
ct was determined by curve-fitting. The reciprocal R
ct
-1 was used as an index of the corrosion rate of carbon steel.
At all the pore water contents, the corrosion rate of carbon steel decreased with time at the early stage because oxygen in soil was consumed by the corrosion reaction, and then reached a steady value within several days. The corrosion rate under the steady state condition was strongly dependent of the pore water content. The maximum corrosion rate was observed at 90 % water content, which was about 19 times higher than 100%. This is attributed to the enhancement of the cathodic reaction (diffusion of oxygen) due to the increase of the gas phase. On the other hand, the corrosion rate decreased with decreasing the water content below 90%, probably due to the suppression of the anodic reaction by decrease of the liquid phase.
At 100% water content, the corrosion rate under the steady state condition was independent of the covering depth because the cathodic reaction of the corrosion changed from oxygen reduction to water reduction. At 90% water content, however, the corrosion rate decreased with the increase of the covering depth. The corrosion rate was inversely proportional to the covering depth. It means that oxygen diffusion through the silica sand controls the corrosion of carbon steel at 90%. On the other hand, at 80 and 70%, the corrosion rate was independent of the covering depth. This is because that the corrosion is controlled by the anodic reaction.
1) I.S. Cole and D. Marney, Corros. Sci., 56, 5 (2012).