Estimating External Corrosion Rates

All corrosion defects found during each direct examination should be measured, documented, and remediated as needed.  At each excavation, the pipeline operator should measure and record generic environmental characteristics (such as soil resistivity, hydrology, drainage etc.).  These data have been used to estimate corrosion rates.  Average corrosion rates related to soil resistivity are below in the following table with no CP.

                                                         Corrosion Rates Related to Soil Resistivity

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Keep in mind that these are estimates and assumed with no CP or interference at the defect/holiday.  If there is some type of CP on the pipe i.e., 50% you can estimate as follows:

·        Use a CP multiplier times the corrosion rate i.e., 50% of less than 16 mils is 8 mpy

·       For unknown defects, it must be assumed that the maximum defect dimensions are twice that of the largest defect depth and length measured during direct examination.

·       Or use statistical analysis results of defect severity from the corrosion measurements performed during direct examination that can be used to estimate the defect severity at other coating faults. 

Joe Pikas