[Article Sharing] Pipeline Corrosion

Understanding the Process of Corrosion in Pipelines

Source: http://primis.phmsa.dot.gov/iim/docstr/finalreport_pipelinecorrosion.pdf

Although there are various definitions of corrosion, the definition used by NACE International (NACE), the primary support organization in the corrosion industry, is “The deterioration of a material, usually a metal, which results from a reaction with its environment.” With respect to pipeline corrosion, the metal is line-pipe steel, primarily comprised of iron with one to two percent alloy for strength and toughness (alloys have been determined essentially irrelevant to the corrosion process). In regards to external corrosion, the environment would be groundwater or moist soil for onshore pipelines and seawater for offshore pipelines. For internal corrosion, the environment would be water containing sodium chloride (salt), hydrogen sulfide, and/or carbon dioxide. The deterioration would be dissolution of the iron into the environment, which reduces the strength of the pipeline.

When iron dissolves, it does so as a positively charged ion. The process, represented as follows, is referred to as an anodic reaction (see Figure 1):

 

Fe → Fe⁺⁺ + 2e^-

The electrons produced from the reaction move through the metal pipe to another location where they are in turn consumed in a reaction that produces hydroxyl ions. The specific reaction depends on the nature of the electrolyte, but typically is one of the following:

O₂ + 2H₂O + 4e^- → 4OH^-

2H₂O + 2e^- → H₂ + 2OH^-

The reactions represented above are referred to as cathodic reactions. Movement of the ions through the electrolyte completes the electrical circuit. The iron ions typically react with the water and/or oxygen to form a corrosion deposit of rust or some other iron oxide, but, in some cases, they may react with carbon dioxide or hydrogen sulfide to form iron carbonate or iron sulfide.

The anode and cathode components of a corrosion cell can be next to each other or separated by many feet.

Figure 1 Basic corrosion cell
Source: NACE Corrosion Training Material