Permeable Coatings and CP Compatibility

Protective coatings applied to the external surface of line pipe are designed to protect the underlying steel from corrosion over the life of the installation. These coatings are usually used in combination with cathodic protection (CP) to ensure that the steel exposed by coating damage or defects remains protected. The ultimate success of this approach very much depends on the nature, performance and failure mode of the coating. Disbondment of a high impedance coating such as a polyolefin tape can block CP while allowing water access to the underlying steel. Corrosion and near neutral pH SCC can result. In contrast, pinholes or impact damage on coatings such as fusion bonded epoxy allow both CP and water to penetrate to the pipe surface. Coatings that become permeable in service such as asphalt enamels similarly allow protection of the pipe despite a general loss of barrier properties. Because no corrosion or environmentally assisted cracking occurs where adequate CP potentials are sustained, these coatings can be regarded as being “CP compatible”.