Non-shielding coating systems now part of new Us regulatory requirement

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.

Regulatory Requirements

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 say:

"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."[1]

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". 

(1)   The pipe must be protected against external corrosion by a non-shielding coating.²

(2)   Coating on pipe used for trenchless installation must be non-shielding and resist abrasions and other damage possible during installation. ²

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 Coating Definition for Cathodically Protected Pipelines

"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.[2]  Today's pipeline coatings are all 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.[3]  The so-called "cathodic protection shielding effect" prevents cathodic protection current to flow to areas submitted to corrosion risk under disbonded coatings.[4]

Pipeline Coatings with Non-Shielding Characteristics

Each coating manufacturer attempts to make coatings that will not fail.[5]  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. [6]^{,8,9,10,11,12,13,14,15,16,17,18}   Though no coating system can be totally immune to CP shielding, some have a proven track record of being non-shielding.

Fusion Bonded Epoxy (FBE)

FBE has been a widely used pipeline coating system since 1960 and as with all coatings these coatings lose adhesion, blister and disbond.  There are thousands of miles of FBE coated and cathodically protected pipelines around the world, yet rarely do these pipelines have external corrosion problems when adequate CP is available.  However, the electrical resistance is low enough to allow cathodic protection to prevent corrosion on the pipe with disbonded or blistering coating - FBE is non-shielding. ¹⁹ Thus FBE has high enough dielectric strength to resist corrosion while the coating is bonded, but if the FBE is disbonded, the coating is low enough in dielectric strength to allow protective CP current to reach the pipe.  Both high pH and the presence of the magnetite film appear to support past work which suggested that FBE does not shield cathodic protection current.²⁰  

Mesh Backed Tapes

Though some tape products are responsible for much of the CP shielding that has been written about in the industry, one manufacturer (Polyguard Products, Inc.) produces a mesh backed coating that has been proven to have non-shielding characteristics after 22 years of use and several laboratory tests.  Most of the tape coatings and shrink sleeves that caused and still cause CP shielding problems are the ones that have solid film backings of polyethylene or other type plastic films that have very high dielectric strength. 

The mesh backed coating has sufficiently high dielectric strength when bonded to allow use of CP with minimal output.  Yet this same coating system has been proven to allow CP current to reach the pipe under the coating if loss of adhesion occurs and water penetrates.  Proof of the CP current being effective is shown in the adjacent photo with a high pH of the water under the disbondment of an improperly applied mesh backed coating system.   With 22 years of excellent service, few reports of disbondments, several lab tests proving the non-shielding and no known reports of significant corrosion, this coating system is also a proven non-shielding coating system.¹

Conclusion

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 recognize 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?

[1] U.S. Department of Transportation, Pipeline Safety Regulations, Enforcement Procedure Part 192.112.

[2] J. A. Beavers, N. G. Thompson - "Corrosion Beneath Disbonded Pipeline Coatings" - Materials Performance, April 1997, page 19.

[3] R. Norsworthy - "Coatings Used In Conjunction With Cathodic Protection -

Shielding VS Non-Shielding Pipeline Coatings" - CORROSION 2009 - Paper 5958

[4] M. Roche - "External corrosion of pipelines: What risk?", Paper # E 93600-PP, SPE International

[5] R. Norsworthy - "Study Examines Coating Compatibility with CP", Oil and Gas Journal, May 25, 2009, Pg. 52

[6] S. Papavinasam, M. Attard, and R. W. Revie - "External Polymeric Pipeline Coating Failure Modes" Materials Performance, October 2006, Page 28.

8 J. A. Beavers, N. G. Thompson - "Corrosion Beneath Disbonded Pipeline Coatings" - Materials Performance, April 1997.

9 "Coatings Used in Conjunction with Cathodic Protection" - NACE International Technical Committee Report Item  No. 24207, page 2.

10 D.P. Moore, "Cathodic Shielding Can be a Major Problem After A Coating Fails", Materials Performance 39, 4, 2000, pg. 44.

1¹ S. Papavinasam, M. Attard, and R. W. Revie - "External Polymeric Pipeline Coating Failure Modes" Materials Performance, October 2006, Page 28.

1² B. C. Yen and G. D. Tofani; "Soil Stress Assessment can Prevent Corrosion, Reduce Pipeline Coating Damage"; Oil and Gas Journal; August 26, 1985; page 63.

1³ G. Mills - "The Pipe Coating as an Engineered Part of the Cathodic Protection System"; Materials Performance, December 1988, page 13.

1⁴ J. Been, R. Given, K. Ikeda-Cameron and R. Worthingham; "Investigating Coating Performance"; Pipeline and Gas Technology, April 2007, page 36.

15 R. Norsworthy - "Establishing Compatibility" - World Pipelines, February 2007

¹⁶ R. Norsworthy - "Proven Protection" - World Pipelines, October 2007

17 F. King, T. Jack, M. Kolar and R. Worthingham, - "A Permeable Coating Model for Predicting the Environment at the Pipe Surface Under CP-Compatible Coatings", CORROSION 2004, Paper 04158, page 1.

18 G. R. Ruschau and Y. Chen -  "Determining the CP Shielding Behavior of Pipeline Coatings in the Laboratory"; CORROSION 2006; Paper # 06043

¹⁹J. Alan Kehr, "Fusion Bonded Epoxy (FBE) - A Foundation for Pipeline Corrosion Protection", NACE Press

20 T. Fore, and K. Varughese, "FBE Found Effective After 30 Years of Service", Pipeline and Gas Journal, October 2006, Pg. 65