Data Evaluation for Ductile Iron Pipe with Polyethylene Encasement and Cathodic Protection in Soil 10 Points

The maximum observed pitting rates for bare, as-manufactured, and damaged DIP with PE in corrosive soils was observed to range from 13.6 to 68.0 mpy, depending on the study. The maximum observed pitting rates for undamaged PE ranged from 4.0 to 52.5 mpy, depending on the study. These results indicate that, for whatever underlying reason (e.g., PE damage, damage to the asphaltic coating, MIC, and so on), DIP with PE installed in corrosive soils can be found to exhibit the maximum pitting rates characteristic of bare and as-manufactured DIP and DIP with damaged PE. The maximum observed pitting rates for DIP with PE and CP ranged from 3.0 to 86.6 mpy for the seven studies represented in Tables 3-1 through 3-10. These results indicate that, for whatever underlying reason (e.g., shielding, joint bonds, insufficient CP current, PE damage, damage to the asphaltic coating, MIC, and so on), DIP with PE and CP in pipeline installations can be found to exhibit the maximum observed pitting rates characteristic of bare and as-manufactured DIP and DIP with damaged PE.

These data indicate that DIP with PE and CP will not meet the target 50-year pipeline lifetime when installed in highly corrosive soils. For a nominal pipeline thickness of 250 mils, pipeline leaks can be expected in the worst case as soon as 3 years after installation. It should be remembered that lifetime is based on the behavior of the tail of the corrosion rate distribution, not on average values. There will be many pipe segments in a pipeline that will meet the target 50-year lifetime, but those in the tail of the distribution will not. The large number of high maximum observed pitting rates seen for this small number of total miles is indicative of the rates seen in the tail of the distribution and can be used to make risk management decisions.



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