EMAT, Pipe Coatings, Corrosion Control And CP Shielding

By Richard Norsworthy, Polyguard Products and Jörg Grillenberger, Stephen Brockhaus and Markus Ginten, Rosen Technology | August 2013, Vol. 240 No. 8

Benefits of knowing about disbonded coatings – Once a pipeline operator knows disbonded coatings are present, several decisions must be made. Is SCC or external corrosion present, and if so, to what extent? If there is SCC or corrosion, is the type of coating known to be CP shielding or not? When was the CP applied and what is the history of the CP system (was it adequate and for how long)? These questions will help determine what direction the company needs to take to solve the problem.

CP shielding types of coating – Certain coating types have a reputation for being CP shielding when disbondments occur.

Solid film-backed tape – Solid film-backed tapes have been used since the 1950s as pipeline coatings, but have been known to have significant failures (for a variety of reasons), leading to external corrosion and SCC because they usually shield the CP current when they disbond. If the coating (polyethylene- backed tape) disbonds, CP’s ability to protect the pipe surface from external corrosion is severely limited.

Shrink sleeves – This approach has been used since the 1970s mainly for girth weld and repair coatings. Many have disbonded and shielded the CP current, allowing corrosion to become significant in just five years in some cases.

Coal tar – Coal tar has been used since the 1920s as a pipeline coating. Depending on how coal tar fails, it can cause many problems with CP shielding. Many times the coal tar coatings were applied in the field on poorly prepared surfaces. Coal tar is also affected by soil stress.

Three-layer – This coating system has been used since the 1980s and has documented disbondments with CP shielding issues. The issue is that the perceived savings during construction from the use of 3LPO (three-layer polyolefins) must be offset against the additional monitoring and maintenance required during the operational life of the pipeline after the coating adhesion has degraded.

Sometimes there is little or no FBE primer at the edge of the cutback, therefore there is no adhesion at the edge of the coating which can allow penetration of water under the coating, causing further possible disbondment and potential CP shielding (Figure 2). This problem will also affect the girth weld coating performance. Poor adhesion does not necessarily constitute an integrity risk to an operational pipeline but, once identified, poor or no adhesion requires more expensive monitoring to prove the integrity of the pipeline has not been compromised. EMAT provides this monitoring tool.

Figure 2: Three-layer coating without FBE primer

If the distance between the pipe and the FBE (although the disbondment may be large) is very small, there will not be much electrolyte present and more importantly, the electrolyte will not exchange easily. The wider the gap, the more the electrolyte can exchange and bring in fresh oxygen, bacteria and other potential corrosion, accelerating the rate of corrosion under the disbondment. Because these three-layer coatings have high dielectric strength polyolefin outer layers, CP will not effectively protect the pipe in these disbonded areas. The problem may exist where the lack of CP in these areas will allow the potential and pH levels to drift into the SCC range.