What Does Soil Corrosion Mean?
Soil corrosion is a geologic hazard that affects buried metals and concrete that is in direct contact with soil or bedrock. Soil corrosion is a complex phenomenon, with a multitude of variables involved. Pitting corrosion and stress-corrosion cracking (SCC) are a result of soil corrosion, which leads to underground oil and gas transmission pipeline failures.
In some respects, corrosion in soils resembles atmospheric corrosion in that observed rates, although usually higher than in the atmosphere, vary to a marked degree with the type of soil. For example, a cast iron water pipe may last 50 years in New England soil, but only 20 years in the more corrosive soil of Southern California.
Three common indicators of a soil's tendency to corrode ferrous metals are:
- Electrical resistivity
- Chloride content
- pH level
Corrosionpedia Explains Soil Corrosion
Corrosive soils contain chemical constituents that can react with construction materials, such as concrete and ferrous metals, which may damage foundations and buried pipelines. The electrochemical corrosion processes that take place on metal surfaces in soils occur in the groundwater that is in contact with the corroding structure. Both the soil and the climate influence the groundwater composition.
Factors that influence soil corrosion are:
- Porosity (aeration)
- Electrical conductivity or resistivity
- Dissolved salts, including depolarizers or inhibitors
Each of these variables may affect the anodic and cathodic polarization characteristics of a metal in soil. The most corrosive soils have high content of:
- Electrical conductivity
- Dissolved salts
Corrosion rates underground have been measured by using:
- Stern-Geary linear polarization method
- Weight loss method
The former method has been useful in assessing the corrosion rates of footings of galvanized steel towers used to support power lines.
The corrosivity of soils can be estimated by measuring soil resistivity. Sandy soils are high on the resistivity scale and therefore considered the least corrosive. Clay soils, especially those contaminated with saline water are on the opposite end of the spectrum.
Soil corrosion can be controlled by:
- Using organic and inorganic coatings
- Applying metallic coatings
- Alteration of soil
- Cathodic protection