Corrosion is a natural process where refined materials react with their environment, and revert to a more chemically-stable form.
The rate of corrosion is dependent on a number of environmental variables, but for corrosion to occur four basic elements are required (Anode, Cathode, Metallic Path, and Electrolyte). Mediating these elements are the key to pipeline corrosion protection.
In the fight against corrosion, integrity engineers focus on removing the presence of one or more of these basic elements. The first defense against corrosion is a coating system which helps to protect the asset from the surrounding electrolyte.
Coatings are typically used as a barrier coating, protecting the substrate from contact with the Electrolyte. In addition to serving as a barrier, some coatings also use inhibitive, or sacrificial pigments, causing passivation, or a sacrificial anode to form at any Coating Holidays or defects.
Integrity engineers must also utilize cathodic protection (impressed or passive) to protect assets, as coatings can be damaged, which can amplify corrosion locally at the defect (Coating Holiday, cracking, etc.)
Cathodic protection systems combat corrosion by converting all of the anodic (active) sites on the metal surface of an asset to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source to equalize the potential on the surface of the metal structure (left).
When two metals are coupled, the more active will share electrons with the more noble material causing an oxidation reaction (rust) at the electronegative material, and a reduction reaction at the more electropositive (noble) material.
Corrosion protection does not eliminate corrosion, but actually transfers corrosion current from the protected structure (asset) to the cathodic protection anode. This is the case in Galvanic (Sacrificial) anode systems, and impressed current system.
Materials are listed from most Noble (passive), to most electronegative (active) in the Galvanic series. In reference to the below graphic, Magnesium is the most active.
Galvanic systems are used in specific applications (offshore, within vessels, or in an area with many other metallic structures). Galvanic anodes are commonly used in combination with impressed current systems at problem/ repair areas, shorted casings, cathodic interference / stray current discharge points, areas influenced by electrical shielding, and locations with significant coating damage.
A sacrificial anode system is easily installed, requires no external electricity, and minimal right-of-way cost. The limitations of sacrificial anodes are current output, require anodes to be replaced once an anode has been spent / consumed, and a higher cost per unit ampere than impressed current.
Impressed current systems are comprised of an external power source and anodes. The power source (rectifier) forces current to flow from the anode to the structure to through the electrolyte.
Galvanic protection is not sufficient to protect pipelines from corrosion. It is not economically feasible to protect and entire pipeline and all the connecting assets through Galvanic systems. Isolation is an important part of the design of a corrosion protection system.
In actual practice, pipes and the metallic pipeline components, such as valves and fittings, are manufactured with coatings. Following welding and inspection exposed weld bevels must be covered with field applied epoxies.
In addition to coating, Cathodic protection (CP) systems are used to supply extra electrons causing the asset to be a passive location or a cathode. In order to control what is protected by these electrons, isolation is used as the ‘bookends’ of a CP system.
Isolation will allow you to protect specific assets, and eliminate any dissimilar metals connection. An example would be a pipeline coming into a compressor station. The role of the CP system is to protect the pipeline, but not the compressor station. Isolation would be used on either side of the compressor station to avoid the large metallic compressor station draining the CP current.