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Buried and submerged pipelines are protected from external corrosion by a coating system, which is considered passive. Coatings are practically always applied to pipe lengths in specialized coating plants, and continuity of coverage is ensured after girth welding through field joint coatings (FJC). Cathodic protection (CP) is an essential, complementary, "active" protection system aimed at preventing corrosion at coating defects, where the pipe steel surface is exposed to the corrosive electrolytic environment. As long as coatings remain bonded to steel and CP is correctly applied, monitored, and maintained, no corrosion risk exists.
The majority of known corrosion cases result from disbonding of coatings, which may prevent access of the cathodic protection current to steel. The cause of failure, known as the "CP current shielding effect," appears to be a concern limited to buried pipelines onshore. Cases of corrosion and stress corrosion cracking (SCC) on old buried pipelines coated "on the ditch" with coal tar or asphalt enamels or cold-applied tapes have been known for a long time. No case of external corrosion of pipelines immersed in seawater has been detected so far using in-line inspection (ILI), despite, most likely, the existence of some coating disbondment. Despite the assumption of some coating disbondment in seawater, corrosion protection is maintained, probably because the high conductivity and homogeneity of seawater make it easier for the CP current to access the exposed steel and protect it.
Summaries of our company's past experience with various kinds of pipeline coatings have been presented in previous papers.1-9 In particular, at the 16th International Conference on Pipeline Protection in Paphos, the authors presented a paper on failures recently discovered on "newer" coatings such as heat-shrinkable sleeves, threelayer polyethylene (PE) coatings, and fusion-bonded epoxy (FBE) coatings.10 The present article first updates and completes the information presented at the Paphos conference on the recent cases of coating failures encountered. It also summarizes the results obtained from some laboratory test programs aimed at trying to explain the problems for improving the future choices and the specifications of the company.
Recent Feedback on Disbonding of Pipeline Coatings
Various practical case studies follow. Cases related to heat-shrinkable sleeves (HSS) used for field joints and which overlap the factory-applied 3- layer PE/polypropylene (PP) are the most important as far...