We have seen many pipeline failures, ruptures and even explosions due to shielding of cathodic protection current by the coating in corrosive soils and water tables.
Pipelines are protected from external corrosion by both coating and cathodic protection (CP). While the coating provides the first line of protective barrier, CP serves as a backup to prevent corrosion attack either at coating faults such as pinholes and holidays or under coating disbondment. However, pipeline corrosion and corrosion induced cracking have been widely documented . When coating is dis-bonded at small faults, such as pinholes or holidays, CP current can be shielded, either fully or partially, to reach the dis-bonding crevice, especially at the crevice bottom. As a result, the CP could not protect the area that is in a corrosive environment. This is called “CP shielding”. Another scenario encountered in reality to cause CP shielding is the disbondment of a defect-free coating due to either an inadequate coating application process or the lost adhesion of the coating to the steel substrate during service. For example, spirally wrapped tape coatings can experience dis-bonding over pipeline welds. In this situation, the CP shielding is attributed to the coating property. It has been reported that up to 85% of all the external corrosion of pipelines is under dis-bonded CP shielding coatings.
Non-shielding coatings are those which do not prevent distribution of CP current to the steel substrate through the dis-bonded coating. It is highly recommended that non-shielding coatings to be used in cathodic protection applications.
Matergenics can perform testing to determine if the coating shields cathodic protectIon i specific application. Please call us if you have any questions or concerns.