99 Diseases of Pressure Equipment: Soil Corrosion

Soil corrosion (underground corrosion) is another one of those extensively researched and documented types of corrosion, since so many pipes and pipelines are buried and nearly all storage tanks rest on the soil. An entire industry/ technology is associated with preventing soil corrosion (cathodic protection). And like external corrosion, soil corrosion rates can range from very low to fairly aggressive, depending upon the type of soil, the amount of moisture, soil contamination, soil resistivity, drainage, and oxygen content. Soil-to-air interfaces (~12 inches above and below grade) are one of the 101 essential elements of pressure equipment integrity management because they are so often exposed to much higher rates of corrosion than either the buried piping or the section above grade because ofcoating damage, lack of CP, and increased exposure to moisture and oxygen. Just as is the case with CUI and external corrosion, properly selected and applied coatings are key to soil corrosion control. A combination of a well-designed and maintained CP system along with a good coating is the best way to control soil corrosion over the long haul. Though the pipeline industry has a well developed service industry for smart pigging of pipelines to detect soil corrosion, the process industry has very limited techniques for inspection of buried piping and structures. Magnetic Flux Leakage (MFL) techniques are becoming well established for tank bottom inspection for soil side corrosion; however there is no such wellestablished technique for typical buried piping that is not designed for pigging. Hence, just as is the case of insulation removal for a lot of CUI inspection, digging with visual inspection is the primary technique for buried piping inspection. Section 9 of API 570, Piping Inspection Code, is devoted to inspection of buried piping.

When was the last time you had a soil resistivity test at your facility to help determine the potential corrosiveness of your soils and your risk of soil corrosion leaks?