New regulatory requirements for coating systems

Richard Norsworthy of Polyguard Products examines the new US regulations governing non-shielding coated systems.

“Coating on pipes used for trenchless installation must be non-shielding and resist abrasions and other damage possible during installation”
-Richard Norsworthy

The United States Department of Transportation (DOT) has called for the use of non-shielding pipeline coatings on natural gas pipelines that meet the new regulations for increasing Maximum Allowable Operating Pressure (MAOP). When selecting a pipeline coating, the "non-shielding" characteristics may be more important than other issues that are normally considered.

The Pipeline and Hazardous Materials Safety Administration is now responsible for implementing the regulations for the US DOT. The Code of Federal Regulations, Title 49: Transportation, Part 192 - Transportation of Natural and Other Gas by Pipeline, has recently added Part § 192.112 which says:

"Additional design requirements for steel pipe using alternative maximum allowable operating pressure. For a new or existing pipeline segment to be eligible for operation at the alternative maximum allowable operating pressure (MAOP) calculated under § 192.620, a segment must meet the following additional design requirements. Records for alternative MAOP must be maintained, for the useful life of the pipeline, demonstrating compliance with these requirements."

Under this new section pipeline operators are allowed to raise the MAOP of certain natural gas pipelines if they meet or exceed certain listed requirements. One of these requirements is part "(f) Coatings".

The pipe must be protected against external corrosion by a non-shielding coating.

Coating on pipes used for trenchless installation must be non-shielding and resist abrasions and other damage possible during installation. These are significant statements that require those companies that want to raise the MAOP of these pipelines for more through-put to meet or exceed these requirements.

"Non-Shielding" in this context means if the coating system adhesion fails and water penetrates between the pipe and the coating, corrosion on the pipe is significantly reduced or eliminated because cathodic protection (CP) current is able to protect the pipeline in these disbonded areas. To adequately protect underground pipelines, a coating must conduct CP current when disbondment occurs. Today's pipeline coatings are effective and provide a dielectric shield to CP when properly adhered to the surface. The problems begin when the coating does not adhere to the pipe surface.

Shielding on the other hand diverts CP current from reaching the pipe surface allowing corrosion to occur. When disbondment or blistering occurs, most coating types divert current from its intended path, therefore, CP current can not adequately protect the external surfaces of a pipe. The so-called "cathodic protection shielding effect" prevents cathodic protection current to flow to areas submitted to corrosion risk under disbonded coatings.

Each coating manufacturer attempts to make coatings that will not fail. The problem is all pipeline coatings fail for one reason or another. Those that fail by disbonding are the most susceptible to CP shielding. Each type has particular properties that allow it to be shielding or non-shielding to CP current if disbondments occur. 

There have been, and continue to be, many articles written concerning the problem with pipeline coatings that shield CP. Though no coating system can be totally immune to CP shielding, some have a proven track record of being non-shielding.

Disbonded coatings that shield CP have been and continue to be a problem for the pipeline industry. The failure mode of the coating is critical. This is the reason for these US DOT regulations for the companies that want to increase the operating pressure for certain natural gas pipelines. Of course, there are other requirements that are not discussed in this paper that must be followed to meet this regulation change.

Many in the pipeline industry recognise the importance of using pipeline coatings with proven non-shielding characteristics. To minimize or eliminate the CP shielding issues encountered in the past, more pipeline companies are using and requiring these non-shielding pipeline coatings no matter the MAOP. Does your company?

Richard Norsworthy has published numerous papers on corrosion control, and authored the chapter on Coatings for Underground or Submersion Service in Uhlig's Corrosion Handbook. He is a NACE instructor for courses in basic corrosion, CP, and Coatings Used in Conjunction with CP. In1995 he started Lone Star Corrosion Services, which became part of Polyguard Products in 2007.