Cathodic Shielding – Phorgotten Phenomenon in Pipeline Corrosion Coatings

When I arrived at Polyguard Products in 1987, I already had background with the materials and production process being used, but I had no background in the use of corrosion coatings for steel pipelines, which was Polyguard’s original market. Fortunately there were old timers around who helped me learn by sharing their years of experience.

Of all the things I heard about pipeline corrosion coatings during that early tutorial period, the one I remember most was:

"With pipeline coatings, it takes about 15-20 years to find out if a coating has problems.  That's about how long it takes for widespread failures to occur, at which point people begin to stop using the coating".

In 1987, one of those 'widespread failure' cycles was coming full circle.  For many years, polyethylene film backed tape coatings had been used worldwide.  The problem which developed with tape coatings was that they were not compatible with cathodic protection systems.

At this time cathodic protection systems had become firmly entrenched in the pipeline coatings market.  Cathodic protection (CP) systems serve as backup protection to the pipeline's exterior corrosion coating system.  The idea is, if the corrosion coating fails, and the steel surface of the pipeline is exposed to corrosion, electrical current from the CP system will seek out the exposed steel, and, finding it, will travel through the steel body of the pipeline to complete the circuit, back to a place where the steel pipeline is wired to an anode.  By this process the current from the CP system protects the pipeline in two ways.  First, it overpowers away the weak electrical current in the area of corrosion (corrosion is an electrochemical process).  Second, it raises the pH of water adjacent to the steel surface to a level of 9 or above.  With pH above 9, corrosion rarely occurs on steel surfaces.

Are you still with me?  If not, remember this rule: the protective electrical current generated by the CP system has to reach the exposed steel surface in order to protect the steel from corrosion.

The problem with tape coatings was (and still is) that they use a solid film backing.  Solid film backings have high dielectric strength (resistivity), which means that the backings block the CP's protective electrical current from reaching the steel pipe surface.

Graphic 1

Protective current from CP system being blocked from the steel pipeline surface by the solid film backing of corrosion coating

Graphic 2

Protective current from CP system has access to the steel pipeline surface when a non-shielding coating is used.

So, in the late 1980s the corrosion community was realizing that for years they had been installing cathodic protection systems along with external corrosion coatings which by their solid film, dielectric nature were stopping the CP system from protecting the steel pipeline.

Word got quickly around the North American corrosion community that tape coatings were bad news because they shielded cathodic protection currents, and the use of solid film backed tape coatings declined rapidly. 

But what happened next is that cathodic shielding became a "phorgotten phenomenon" (I lifted that phrase from the title of an article on cathodic shielding in Materials Performance, published in 2000 by ARCO's D.P. Moore). The forgotten phenomenon is turning out badly for many pipeline companies.

Starting in the late 1980's a consolidation trend began in the pipeline industry which is still going on. Large companies merged into even larger companies. One prominent consolidator was of course Enron.

As companies consolidated, they looked for ways to eliminate waste and duplication.  One by one, the respected corrosion labs and corrosion departments within pipeline companies were consolidated, reduced, or sometimes even eliminated. 

With so many corrosion personnel gone, the choice of what to use for corrosion coatings often fell to managers who were less familiar with the corrosion process, and for this reason did not appreciate the somewhat technical problem of cathodic shielding.  Most of those managers knew about the dreaded 'tape corrosion coatings' from earlier reports, so they rarely specified tapes.  But in many situations, especially outside of North America and on offshore pipelines, companies began to specify other solid film backed corrosion coating systems.  These coatings often have impressive physical strength and thickness, and may even have an excellent corrosion coating element.  But, if these coatings become disbonded from the steel pipe surface, as even the best eventually will in spots, the solid film backings of these coatings will block most protective CP currents from access to the steel surface.  

And so, throughout the 1990's, and into the 21^st century, an odd parallel situation evolved.  On the one hand, pipelines across the world (we estimate over 50%) have been installed using cathodic protection systems in combination with solid film backed corrosion coatings which reduce the effectiveness of those cathodic protection systems.

Throughout this whole period the corrosion technical community has continued to publish technical articles warning of the dangers of cathodic shielding on pipelines.  (Search the web for "cathodic shielding" and you will be led to a number of these technical references).

Additionally, since the technical corrosion community has a presence on standards setting committees, their concerns about shielding are reflected there as well.  Take for example NACE SP0169:2007 - Standard Practice - Control of External Corrosion on Underground or Submerged Metallic Piping Systems (available from www.NACE.org for no charge (members) or $42.00).  Throughout the NACE standard, the reader is warned about using coating systems which create electrical shielding.  (See sections 2, 4.2.3, 5.1.2.3, and 10.9).

But who - outside of technical people - closely reads technical articles and standards?  Not all that many, apparently, because the use of shielding corrosion coatings on pipelines with CP systems continues worldwide.

Today, late in the first decade of the 21^st century, the corrosion bill is finally coming due for many pipeline operators.  At the beginning of this article I quoted the veteran pipe coater who said; "With pipeline coatings, it takes about 15-20 years to find out if a coating has problems..."  Today the widespread failures he talked of have begun for this second generation of coatings which have the built-in CP shielding problem.

There are excellent alternatives available for pipeline operators who insist on non-shielding coatings.  The most prominent of these is FBE (fusion bond epoxy).  With a field track record of over 30 years, most knowledgeable corrosion people agree that FBE's non-shielding properties are proven.  Also Polyguard Products makes a non-shielding pipeline coating, RD-6, which is widely used for girth welds and for rehabilitation of other failed coating systems.  The RD-6 product was introduced in 1988 by Bob Nee, who is the veteran pipe coater quoted previously.  RD-6 has a track record of over 20 years without reported corrosion problems.

Polyguard Products began a worldwide advertising campaign several years ago aimed at raising awareness in the pipeline community about the cathodic shielding problem.  Perhaps in part as a result of increasing awareness, we are beginning to see manufacturers of a variety of different types of corrosion coatings add to their claims that their coating is "compatible with cathodic protection".  This is an encouraging industry trend, but it would be well for pipeline operators to remember that cathodic protection, and the shielding of cathodic protection currents, are fairly technical subjects.  For this reason, pipeline operating companies will do well to closely question any salesperson making the claim that their coating is "compatible with CP". Ask them to explain how their coating is compatible, and to provide proof that if their coating disbonds from the steel, the protective CP current can reach the steel and raise the pH of any water in the area of disbondment to 9 or above.  See what 3^rd party testing they have to support their claims.  With all the corrosion taking place underground and offshore, there is a need to be watchful for vague and unfounded generalizations in the corrosion coatings area.

References and Links:

- 1995 report tying shielding coatings to ruptures and failures on a Manitoba gas line www.tsb.gc.ca/en/reports/pipe/1995/p95h0036/p95h0035.asp
- Extensive report by National Energy Board of Canada tying 76% of stress corrosion cracking (SCC) failures to solid film back corrosion coatings https://www.neb-one.gc.ca/ll-eng/livelink.exe/fetch/2000/90463/90474/90513/1996-12-01_Reasons_for_Decision_MH-2-95.pdf?nodeid=90538&vernum=0
- A Transportation Safety Board of Canada report on a 20,600 bbl. Saskatchewan oil spill. The Analysis section contains a good definition of CP shielding.www.tsb.gc.ca/en/reports/pipe/1999/p99h0021/p99h0021.asp
- US Federal regulations calling for corrosion coatings to have properties compatible with cathodic protection
- §49 CFR 192.461(Natural gas pipelines) http://www.setonresourcecenter.com/49CFR/Docs/wcd0000d/wcd00d52.asp
- §49 CFR 195.559 (Petroleum and products pipelines) http://www.setonresourcecenter.com/49CFR/Docs/wcd0000e/wcd00e75.asp
- A 2005 NACE paper identifying all forms of solid film back corrosion coatings being tied to the shielding problem http://www.davidnormancorrosioncontrol.com/html/papers/PDF/DavidNormanNACE2005.pdf
- A 2005 report on Stress Corrosion Cracking (SCC) by Michael Baker Engineers which recommends the use of non-shielding coatings (sec. 5.7.2) http://primis.phmsa.dot.gov/iim/docstr/SCC_Report-Final_Report_with_Database.pdf
- A 2008 Q&A concerning disbondment and cathodic shielding on three layer systems. www.cisoilgas.com/pastissue/printarticle?art=268860
- A graphic "Flash" explanation on cathodic shielding http://www.polyguardproducts.com/flash.htm
- A collection of technical references concerning the cathodic shielding problem http://www.polyguardproducts.com/products/pipeline/TechReference/index.htm
- 2004 article concerning Polyguard Products RD-6 non-shielding pipeline corrosion coating"Coatings used in conjunction with cathodic protection - "Fail/Safe" Pipeline Coating Systems"