Corrosion and Conservation of Underground Structures

Corrosion is the disintegration of metal due to chemical action. Corrosion of underground metallic structures is caused by the action of chemicals in the soils, soil waters and other materials that are in contact with the buried metal; however, no appreciable corrosion can occur in the absence of moisture. A steel pipe may safely be buried in dry chemicals for an indefinite period, but if moisture is added, severe corrosion may result. Pure water will not corrode metal, but if a small amount of corrosive chemicals is dissolved in the water, it will become corrosive. We, therefore, have two major factors that must be considered in connection with corrosion: chemicals and moisture. In the presence of large amounts of chemicals that we know will attack metal when moist, moisture will be the controlling factor, but if the metal is constantly wet, the amount and character of the chemicals will control the severity of the corrosive action. There are also other factors that often tend to influence corrosive action to some degree, and in rare cases do exert a powerful influence. Among these may be mentioned temperature, physical characteristics of soil and differences in potential and stray currents. Temperature will affect the rate of a chemical reaction and we will have more corrosion from a warm solution than from one that is cold. The physical characteristics of the soil will affect the rate of percolation of soil waters and will determine the character of contact between the metal and surrounding meteriais. Heterogeneity of soils may lead to differences in potential within the soil itself and also between the soil and the buried metal. Differences in potential will probably accelerate corrosion to a degree proportionate to the current flow. Stray current electrolysis is an example of this type of factor. If we will consider a pipe line as our underground structure, we will find that a practical application of these data can be made.