Corrosivity of a particular soil is also affected by several other parameters, including chemicals present in the soil, moisture content and soil type.  For this evaluation, there are a few other soil parameters that may warrant investigation.  This information is typically determined through laboratory assessment.  These are listed:

• soil moisture

• pH

• sulfide concentration

• chloride concentration

Without moisture, corrosion would not occur.  The hydrolyzed water molecule provides the ions required for anodic (oxidation) and cathodic (reduction) reactions to occur.  Moisture content of approximately 16% or greater is required to sustain corrosion.

Hydrogen ion activity is expressed in terms of pH.  Environments containing more of the OH^- ion than H⁺ ion exhibit a higher pH and are considered alkaline.  Low pH environments, termed acidic, contain more H⁺ ions then OH^- ions.  Generally, for a pH between 6.5 to 7.5 the environment is considered neutral.  Higher pH and lower pH environments would be considered more corrosive. 

Sulfides in the soil are indicative of the presence of sulfate-reducing bacteria.  Their presence are indicative of oxygen deficient soils and higher sulfate concentration that is detrimental to cement structures. 

Chloride ions facilitate the corrosion process.  Their presence in the soil may have resulted from snow de-icing operations on public roads, or the present or historical presence of salt water. There are many inland areas that contain concentrations of salt from past salt water exposures.  Determination of the chloride content of the soil is typically performed in the laboratory.  Concentrations greater than 50 ppm should be considered more corrosive.

Structure Electrical Continuity

As mentioned in the discussion of soil resistivity above, electrically continuous pipe can be subject to long line galvanic cells.  Electrically continuous pipe can also gather current along a long length, from a foreign pipeline.  These factors would therefore lead one to believe that discontinuous pipe would be desirable.  However, in order mitigate the effects of long line galvanic cells, and current flow due to foreign pipelines, cathodic protection would typically be used.  Cathodic protection relies on the piping being electrically continuous over its length, so that anodes in one location can be utilized to protect other locations.

Longitudinal pipe continuity is typically measured in several locations along the pipe under study.  Access to the pipe is made via existing test stations and test wires.  Temporary wires are run between two test stations in order to perform the test.

Individual test wires are used to establish a voltage circuit (E) and a separate current circuit (I).  An external battery is then utilized to impress a test current.  Voltage and current are read with the external current applied, and then with the current removed.

From this data, the longitudinal resistance of a segment of pipe can be calculated by dividing the resulting change in voltage by the impressed current (R=delta E/delta I).  The pipe resistance can then be used in further tests and/or compared to theoretical values.

Pipeline Excavation

Condition assessment described to this point provides qualitative information about the structure corrosion exposure, both for new design considerations and in the evaluation of existing infrastructure.  Much more quantitative information on existing infrastructure can be obtained through physical examination, non-destructive evaluation and statistical analysis.  For this type of study of an existing pipeline, the number of excavations is determined from a practical approach to assessing corrosion activity along the pipe route.   

In analyzing extreme values encountered in statistical samples, the deepest pit in the pipe is  of primary interest to the analysis.  This method of analysis, together with studies of factors influencing the corrosion of the structure of concern, such as soil resistivity and soil moisture content, allows the prediction of the magnitude of corrosion penetration problems that will be encountered on a given structure.  This can be done by inspecting only a very small portion of the structure.

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