Soil corrosion is a geologic hazard that affects buried metals and concrete that is in direct contact with soil or bedrock. Metals are typically attacked by chloride solutions, whereas high sulfate levels are harmful to concrete. In western Colorado, the saline nature of the Mancos Shale with its abundant secondary sulfate mineralization (gypsum) is considered corrosive to both metals and concrete. Chloride content, electrical resistivity, and pH level are indicators of the soil’s tendency to corrode ferrous metals.
Concrete damaged by corrosive soils derived from the Mancos Shale in downtown Montrose. Photo by Laurie Brandt.
Corrosive soils contain chemical constituents that can react with construction materials, such as concrete and ferrous metals, that may damage foundations and buried pipelines. Electrical resistivity, chloride content, and pH level are indicators of the soil’s tendency to corrode ferrous metals.
Corrosive Soil Damage
Corrosion of metal is an electrochemical process that involves oxidation (anodic) and reduction (cathodic) reactions on metal surfaces. Corrosion is typically a result of contact with soluble chloride salts found in the soil or water, which requires moisture to form solutions of these salts. Several key factors that influence the severity and rate of corrosion include: the amount of moisture in the soil, the conductivity of the solution, the pH of the solution, and the oxygen concentration within the soil (aeration). The organic content of the soil, soil porosity, and soil texture indirectly affect corrosion of metals in soil by influencing the key factors listed above.
Water Main Failure
This cast iron water main failed after 8 years of service. Although primed and tape-wrapped for corrosion protection, the pipe surface showed extensive pitting and corrosion damage. Based on resistivity measurement of the soil, it was found to be corrosive. Cathodic protection was recommended to prevent future failures. Image courtesy of James Aleszka of Fracture Investigations
The rate of corrosion of uncoated steel is related to such factors as soil moisture, particle-size distribution, acidity, and electrical conductivity of the soil. Furthermore, chloride ions from salt-enriched waters, soil, or from anti-icing salts can lead to corrosion of steel reinforcement in concrete and steel structures by dissolving the protective layer of oxides present on the steel surface.
Sulfate ions may also lead to accelerated corrosion of steel reinforcement and concrete. Sulfates react with lime in the concrete to form expansive products that cause the concrete to soften and crack, thus weakening the concrete. Water with other destructive ions can percolate through cracked concrete, attack the reinforcement steel, and speed up the corrosion process.
The presence of high acidity (pH ≤ 5.5) in water or soil is considered a corrosive condition for concrete and certain metals (carbon steel, zinc, aluminum, and copper). Like the corrosion of metals described above, acidic soils or waters can react with the lime in concrete to form soluble reaction products that can leach out of the concrete, resulting in concrete with greater porosity and weaker condition. Concrete that has been affected by acidic conditions will often have yellowish or rust colored areas on the concrete surface.
Copper Pipe Failure
If the soil is corrosive enough it can even attack copper water lines. Typically, the surface of the water line at the bottom of the trench experiences the most corrosion. The difference in oxygen concentration between the soil which was dug up to create the trench and then replaced (thus high oxygen content) and the undisturbed soil at the bottom of the trench (low oxygen content) creates a corrosion cell. This cell, in addition to the soil’s general corrosiveness, causes the copper tube at the bottom of the trench to corrode. Image courtesy of James Aleszka of Fracture Investigations
Characterizing the corrosivity of a soil is complicated by the interaction of the variables described above. For example, metal buried within an aerated or disturbed soil with a particular resistivity and soluble chloride concentration generally will not experience the same amount of corrosion as a similar metal placed in the same soil in a compacted, less aerated state.
Special site examination and design may be needed if the combination of factors results in a severe hazard of corrosion. Uncoated steel or concrete in installations that intersect soil boundaries or soil layers is more susceptible to corrosion than the steel or concrete in installations that are entirely within one kind of soil or within one soil layer.
According to a resident of Delta, Colorado, a one-inch-thick steel pipe was nearly dissolved after only one year of burial two feet below the ground. The resident lived near Sweitzer Lake State Park in an area known for its corrosive soils that are derived from the Mancos Shale.