Soil Characteristics and Their Impact on Pipeline Systems

The corrosive nature of soil is influenced by various factors, including the type of soil, moisture content, pH, and the presence of substances such as H2S, sulfides, coal, chloride, and sulfate ions. These factors affect the soil’s electrical resistance, which in turn influences its corrosive properties. Dry soils with high resistance are generally non-corrosive, while soils in low-oxygen environments, such as waterlogged areas or coastal regions, may lead to corrosion due to bacteria that reduce sulfates to sulfides. The soil’s electrical resistivity is a key indicator of its corrosivity. Corrosion typically occurs in the form of pitting corrosion in such soils.

Pipeline materials are designed for a service life of 30 years or more, and coatings are used to determine the lifespan. Pipes must be made of durable materials to resist damage and corrosion when buried underground. The effects of soil on pipeline corrosion can lead to significant financial and operational losses.

The duration of corrosion, or inadequate corrosion prevention, plays a significant role in the deterioration of pipeline materials. Several characteristics of soil indicate its corrosivity, including electrical resistance, salt solubility, moisture content, total acidity, and bacterial movement. Studies such as those from USEPA [2001] have defined these characteristics and highlighted the synergistic effects between them. According to USEPA, soil with resistivity below 500 ohm/cm is considered corrosive. Furthermore, soil temperature inversely affects resistivity, meaning higher temperatures can increase corrosion reactions.

Other studies, such as those from Battle Research Institute [2002], have shown that polluted soils can undermine even plastic pipelines. The degree and type of pollution in the soil, as well as its composition, are critical factors.

Research from USDA [1998] indicates that soil with high alkalinity or acidity can corrode materials. For instance, soil with a pH of 5.5 or lower can cause significant corrosion in concrete materials.

Studies from Scott [1987] and Conat [2001] have shown that sulfate-reducing bacteria play a significant role in microbial corrosion, and soil aggressiveness can be evaluated based on moisture content and resistivity. The presence of sulfate-reducing bacteria is particularly important for assessing the severity of corrosion.

Methods for Determining Soil Corrosivity:

To accurately assess soil corrosivity, representative soil samples must be taken from all regions along the pipeline route. According to ASTM D420-69, samples should be randomly collected through drilling at depths of 3 meters, ensuring that at least three samples are taken from each test site.

Key testing parameters for soil include:

1. Soil Samples: Collected according to ASTM standards to ensure they represent the entire region.
2. Soil Softness/Hardness: Soil particle classification can be done through sieve analysis (ASTM D422-63).
3. Physical Properties of Soil: Determined by tests for void ratio, temperature, and dry density (ASTM D854-58, D1157-70).
4. Soil Resistivity and Oxidation-Reduction Potential: Measured using a conductivity meter and potentiometer, respectively (Hesse, 1981; Rump & Krist, 1988).
5. Soluble Cation and Anion Ions: Analyzed through soil-water-based methods, including calcium (Ca+2), magnesium (Mg+2), chloride (Cl-1), and sulfate (SO4-2) ions (AWWA, 1985).
6. Soil pH: Measured using a soil-water-based pH meter (Hesse, 1981).
7. Soil Bacterial Analysis: Determines the presence of anaerobic sulfate-reducing bacteria using iron-enriched agar culture (Rump & Krist, 1988).

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