Understanding Internal and External Corrosion in Oil and Gas Pipelines
Oil and gas pipelines are susceptible to both internal and external corrosion damage. For internal corrosion, the environment would be water containing sodium chloride (salt), hydrogen sulphide, and/or carbon dioxide. In regards to external corrosion, the environment would be groundwater or moist soil, and /or atmospheric for onshore pipelines and seawater for offshore pipelines. Forms of internal corrosion encountered include general corrosion, pitting, stress corrosion cracking , microbial corrosion, galvanic corrosion, erosion corrosion, corrosion fatigue and crevice corrosion. External corrosion can manifest itself as general or localised corrosion and/or coating breakdown.
NACE RP-0502 details pipeline external corrosion direct assessment methodology. NACE SP-0602, NACE SP-0204, NACE SP-0208 and NACE SP-0110 cover internal corrosion direct assessments for dry gas, liquid petroleum and wet gas environments.
Mitigation of internal corrosion can be achieved by the application of chemical inhibitors, biocides, scavengers (for oxygen, carbon dioxide and hydrogen sulphide) and pH control. Externally corrosion is retarded by the application of coatings and for buried or submerged pipelines by coatings and cathodic protection. NACE SP-0207 covers “Performing Close-Interval Potential Surveys and DC Surface Potential Gradient surveys on Buried or Submerged Metallic Pipelines to establish protection levels.