Pipeline corrosion : Cathodic Protection

 

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Cathodic protection is a method by which corrosion of the parent metal is prevented. The two main methods of cathodic protection are galvanic anodes and impressed current systems . For offshore pipelines, the galvanic anode system is generally used.

Corrosion is an electrochemical reaction that involves the loss of metal. This is due to the fact that the steel pipeline surface consists of randomly distributed cathodic and anodic areas, and seawater is the electrolyte that completes the galvanic cell. This causes electrons to flow from one point to the other, resulting in corrosion. By connecting a metal of higher potential to the steel pipe line, it is possible to create an electrochemical cell in which the metal with lower potential becomes a cathode and is protected.

Pipeline coatings are the first barriers of defense against corrosion . However, after coating the pipe the process of transportation and installation of the pipelines results in some damage to the coating. Cathodic protection uses another metal that will lose electron s in preference to steel. The main metals used as sacrificial anodes are alloys of aluminum and zinc. By attaching anodes of these metals to the steel pipe line, the steel area where the coating is damaged is protected from corrosion .

Zinc anodes are not normally used in deepwater pipe lines because they are less efficient and therefore require a larger mass for protecting the pipeline. However, zinc anodes can be cast onto the pipe joint and therefore no cables need to be used for electrical connection to the steel. Zinc has been used in projects where the pipeline needed to be towed along the seabed and cast-on zinc anodes were less liable to be knocked off in the process of installation. Zinc anodes do not perform well for hot buried pipelines and are subject to intergranular atta k at temperatures above 50 8C. There is also a tendency for zinc anodes to passivate at temperatures above 70 8C.

Aluminum anodes, on the other hand, perform much better. They are better suited for hot buried pipelines. Generally, for deepwater pipelines, aluminum alloy anodes that are attached to the pipeline are bracelet anodes. These anodes are normally attached to the pipe joint at the coating yard for S-lay and J-lay installation methods. Electrical contact to the pipeline is made by thermite welding or brazing the cable from the anode. In the case of installation of pipeline by the reeling method, the anodes are installed on the lay vessel during unreeling and straightening. In this case, bracelet anodes are attached to the pipe by bolting and attaching the cable by thermit/cadweld to the pipeline.

The design of cathodic protection systems must consider the potential detrimental effects of the CP system such as hydrogen embrittlement of steel and local stresses that may lead to hydrogen induced stress cracking (HISC).

 Source:
 Guo, Boyun. (2005) Offshore Pipeline. Gulf Professional, U.S.A.