Defining Corrosion Exposure

Corrosion manifests itself on metallic structures in contact with electrolytes.  Therefore the concern for corrosion exposure applies to the interior and exterior of piping system, the interior and exterior of storage facilities and the mechanical equipment utilized in the treatment of water.  To ensure that the proper corrosion control method(s) are employed and the infrastructure service life is realized, corrosion exposure must be understood.  This is important in the planning and design phases of major infrastructure projects.  For the purpose of both design and operations, it is important to define the corrosivity of the installation.  Where experience or analysis indicate that the environment is corrosive and aggressive, corrosion control methods need to be developed and initiated. 

For the purposes of this paper, internal corrosion of various pipe materials results from the corrosivity of the raw and finished water.  Corrosion control methods for this environment include treatment, pipe lining and others.   The primary focus of this paper is the external surface of the pipe and the corrosion effects from soils and groundwater.  However, some of the non-destructive methods that will be described are applicable to assessment of internal conditions of a pipe and other infrastructure.

For the external pipe surfaces, the effects of soil, water, and air vary depending on the pipe material that exists in the ground or being considered for installation.  The following conditions should be understood to facilitate definition of environment corrosivity. 

Soil Assessment

The measurement of soil resistivity has been used for years as an indicator of the corrosivity of soil.  Soil resistivity is the reciprocal of conductivity, the lower the resistivity, the easier current will flow through the soil.  Of the measurable soil characteristics, resistivity is generally accepted as the primary indicator of soil corrosivity.  Resistivity is a property of the bulk volume of soil and electrolytes. 

Although no standard has been developed and accepted by such organizations as the American Society for Testing and Materials or the National Association of Corrosion Engineers, it is generally agreed that the classification shown below, or other similar classifications, reflect soil corrosivity.

The above table provides qualitative insight to the expected corrosion exposure of a metallic structure in a soil of known resistivity.  Accordingly, deterioration can generally be expected to be rapid and relatively severe in soils below 1,000 ohm-cm.  This does not mean, however, that severe corrosion will not occur in soils of higher resistivities.  In fact, depending on chemical conditions, severe corrosion can occur in soils above 10,000 ohm-cm.  The table only indicates that the latter occurrence is generally not observed.

Not only is the resistivity useful in predicting relative corrosion rates, but it is equally important to identify whether soil resistivity varies along a given route.  Structures such as  pipes, which are electrically continuous along significant portions of their length, will be susceptible to long line galvanic influences arising from variations in soil resistivity along the pipe route.  Portions of a structure in the lower soil resistivity environments tend to become anodic, and therefore corrode, relative to other portions of the same structure. 

Soil resistivity can be measured in several ways, and all may be utilized during an evaluation as conditions warrant.  Resistivity can be measured in-situ, typically via the Wenner four-pin method (ASTM G-57).   This method measures the average resistivity of large volumes of soil based on the spacing of the measuring pins.  Resistivity can also be measured in-situ with a Collins rod and Whetstone bridge.  This method measures a small volume of soil in the area surrounding the tip of the rod. 

Laboratory testing can also be performed to assess soil samples obtained through drilling and excavation operations.  Sample conductivity would be determined in the laboratory.  Conductivity is the reciprocal of resistivity.

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