The key points from this module are:
Coating of pipelines creates an additional barrier around the pipe to protect it from the substance running through the pipeline as well as the corrosive substances. Well-coated pipelines are 99% effective in corrosion control.
The drop in current and potential is called attenuation. To increase the protection distance, the source current is increased leading to over-protection. Coating decreases the attenuation.
The following are requirements for coating pipelines:
Effective electrical insulator.
Effective moisture and corrodent barrier.
It should not be toxic to the environment.
Antifouling for marine applications.
Should be resistant to damages during storage and transportation.
There are two types of coatings of engineering pipelines.
Mill-applied coating: It is a coating condition of a metal product applied at the location where it was produced.
Field-applied coating: Once pipes have been fused, a new coating is applied to the welded seam to protect them against corrosion.
The following are steps when preparing a surface for coating
Preheat the surface.
Blast the surface.
Grind surface defects.
Apply Phosphate coating.
The current demand for cathodic protection can vary with time from the following:
Stray current corrosion
The internal corrosion of oil and gas pipelines is not directly related to cathodic protection. The methodology adopted for cathodic protection engineering is entirely different from methodology to control corrosion of pipeline's internal surfaces.
Carbon dioxide corrosion is generally termed sweet corrosion while hydrogen sulfide corrosion is also termed sour corrosion.
The internal corrosion of pipelines constitutes 25% of the failures and the capital cost of corrosion prevention is around 10% of the project cost and 5 - 15% of the operating costs.
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