In order to protect pipelines against corrosion and stress corrosion cracking NOVA Gas Transmission (NGT) has employed a dual system involving the application of an insulating coating and effective cathodic protection to all pipelines. Since the installation of pipelines in the 1950’s the coatings of choice have changed with time. Construction in the 1950’s and 1960’s employed asphalt products which were hot applied over the ditch. Subsequently in the 1960’s and 1970’s polyethylene tape coatings were used extensively. Today the coatings of choice on the NGT system are fusion bond epoxies and extruded polyethylene systems.
This variety in the types of coating employed to protect pipelines against corrosion has necessitated a requirement to understand the long term performance of coating products. Studies at NOVA Research & Technology Corporation have been aimed at developing an understanding of the modes of failure of coating systems. The mode of failure of a specific coating system can impact significantly on the nature and severity of the resultant corrosion scenarios. Different coating systems under identical field conditions can fail in completely different ways. Examples from field coating failures will be described which show the significance of the mode of failure with respect to corrosion and stress corrosion cracking.
Laboratory studies using polyethylene tape, asphalt enamels and fusion bond epoxy coatings will be described which clearly demonstrate that coating failure modes are a key with respect to the type of corrosion scenario that is ultimately developed at the steel surface.
Such information is important in the development of site specific risk assessment models for the management of pipeline integrity.