Chemical corrosion by certain soils (corrosive soils) is a geologic hazard that affects buried metals and concrete in direct contact with soil or bedrock. Metals are chemically attacked by chloride solutions while high sulfate levels are harmful to concrete. The Mancos Shale is considered corrosive to both metals and concrete because of its clay content, saline nature, and pyrite content. Through complex oxidation geochemistry in the presence of calcium carbonate, the Mancos Shale forms abundant secondary sulfate minerals (gypsum) in near-surface weathered zones.
Corrosion of metal is an electrochemical process that involves oxidation (anodic) and reduction (cathodic) reactions on metal surfaces. Corrosion occurs as a result of contact with soluble chloride salts found in the soil. Water is required to form a solution of these salts. Several key factors that influence the severity and rate of metal corrosion include: the amount of water available to make a solution, the conductivity of the solution, the pH of the solution, particle-size distribution, and how aerated the soils are. Organic content can also influence corrosion in soil. Furthermore, chloride ions from salt-enriched waters, soil, or even 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.
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 chemically react with the lime in concrete to form soluble reaction products that 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.
Sulfates can cause significant damage to concrete. Sulfates chemically react with lime in the concrete to form expansive products that cause the concrete to soften and crack, thus weakening the concrete and causing it to crumble. Sulfates may also lead to accelerated corrosion of steel reinforcement if converted to the more corrosive sulfides by anaerobic sulfate-reducing bacteria. Water with other destructive ions can percolate through cracked concrete, attack the reinforcement steel, and speed up the corrosion process.
In general, the most corrosive soils contain large concentrations of soluble sulfates, chlorides, and bicarbonates, and usually are characterized as being very acidic or highly alkaline. 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 installed through different soil boundaries or soil layers is more susceptible to corrosion than the steel or concrete installed within one kind of soil or within one soil layer.
For uncoated steel, the risk of corrosion is based on soil drainage class, total acidity, electrical resistivity near field capacity (an agricultural term that indicates the water content remaining in a soil 2-3 days after having been wetted thoroughly), and electrical conductivity of the soil saturation extract (the fluid or solution extracted from a soil when it is saturated). For concrete, the risk of corrosion is expressed as low or moderate to high. It is based on soil texture, acidity, and amount of sulfates in the soil saturation extract.
White, Jonathan L., T. C. Wait, and Matthew L. Morgan. “OF-09-01 Geologic Hazards Mapping Project of the Uncompahgre River Valley Area, Montrose County, Colorado.” Hazards Mapping. Open File Report. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2011.