Pipeline Design and Construction Corrosion Control
Corrosion is a natural process that, under the proper conditions, can affect any metal or alloy. In simple terms, corrosion is a chemical reaction known as oxidation that reduces the strength and structural integrity of steel by re- moving its principal element—iron. It can be a problem both internally and externally along a pipeline system. Most corrosion failures on pipelines are not caused by rust or general corrosion but by localized attack—pitting, crevice corrosion, intergranular corrosion, or stress corrosion cracking. Cor- rosion control requirements can be found in 49 CFR Part 192, Subpart I for gas pipelines, and 49 CFR Part 195, Subpart H for liquid pipelines. Pipeline operators must also have written guidelines and procedures for most corro- sion-related activities. Like any tool, corrosion control systems must be main- tained to be effective. Types and Causes. Pipeline corrosion is most prevalent when the failure
of coatings, inhibitors, or cathodic protection occurs in a corrosive environ- ment. It may occur in both the interior and exterior of the line. Corrosion does not influence the pipeline equally at all locations and does not grow at the same rate throughout a pipeline.
External corrosion may be caused by damage to coatings, manufacturing
defects within the metal, or through the loss of cathodic protection. In addi- tion, stress from outside forces such as during excavation activities may dent or damage the pipeline or its protective coating, and eventually result in cor- rosion and the failure of a pipeline some years later.
Internal corrosion of pipelines is a concern to all pipeline operators. Causes
include chloride, carbon dioxide, hydrogen sulfide, oxygen, and microbio- logical activity. For example, a recent case of internal corrosion caused several failures on a 20-inch oil pipeline after only two years of service. The cause of the failure was high carbon dioxide concentrations (9%) in combination with high levels of brine.
Stress corrosion cracking (SCC) is the cracking of a pipeline from the com-
bined influence of tensile stress and a corrosive medium. It has been associ- ated with both liquid and natural gas pipelines. The tensile stress may come from directly applied stressors to the pipeline (e.g., pressure and outside forces) or in the form of residual stressors associated with the fabrication and construction of the pipeline. The corrosive medium has been associated with both high pH (9–11) and near neutral pH (6–8) environments.
Microbiologically influenced corrosion (MIC) involves microbes that can
produce corrosive conditions. MIC often works in combination with other forms of corrosion on or in the pipe. It can also accelerate corrosion produced by impurities in the pipeline system environment, such as hydrogen sulfide and/or carbon dioxide. For example, MIC can occur in the crevices produced by disbonded coatings on the outside of a pipe that are shielded from cathodic protection. Maintenance, Inspection, and Prevention. Pipeline integrity management programs use tools, technologies and strategies to ensure that pipeline
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