Page 267 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CORROSION OF UNDERGROUND GAS AND LIQUID TRANSMISSION PIPELINES 245

Figure 4.7 External corrosion on a buried pipeline (6).

(holidays), and/or the disbondment of external coatings. Figure 4.7 shows typical
external corrosion on a buried pipeline. The 25 mm (1 in.) grid pattern was placed
on the pipe surface to permit sizing of the corrosion and nondestructive evaluation
(NDE) wall thickness measurements.

4.11.1 Stray Current Corrosion
Corrosion can be accelerated through ground currents from dc sources. Electrified
railroads, mining operations, and other similar industries that utilize large amounts
of dc current sometimes allow a significant portion of current to use a ground path
return to their power sources. These currents often utilize metallic structures such as
pipelines in close proximity as part of the return path. This “stray” current can be
picked up by the pipeline and discharged back into the soil at some distance down
the pipeline close to the current return. Current pick-up on the pipe is the same phe-
nomenon as CP, which tends to mitigate corrosion. The process of current discharge
in the pipe and through the soil of a dc current accelerates corrosion of the pipe wall
at the discharge point. This type of corrosion is termed as stray current corrosion
(Fig. 4.8).

4.11.2 Microbiologically Influenced Corrosion (MIC)
This form of corrosion is defined as corrosion influenced by the presence and activi-
ties of microorganisms including bacteria and fungi. About 20–30% of all corrosion
on pipelines is MIC related. MIC can affect either the external or internal surfaces
of a pipeline. Microorganisms located on the metal surface do not directly attack