Commentary on SP0169

COMMENTARY ON SPO169 PROPOSED REVISION
BY
RICHARD NORSWORTHY

ABSTRACT

As important as cathodic protection criteria is to external corrosion control, it may not be the most important issue facing those concerned with proposed changes to SP0169-2002(7). The document clearly states the intent of the document is effective control of external corrosion. Many seem to ignore the intent of this document and concern themselves only with cathodic protection and related criteria. Changes are needed in this document, but most are not in criteria section.


As we struggle with the proper revision of this very controversial Standard Practice we must not lose site of the purpose of this NACE Standard. This standard practice presents procedures and practices for achieving effective control of external corrosion on buried or submerged metallic piping systems. Confusion remains over the intent of this Standard Practice.

As important as cathodic protection is to buried and submerged structures that are also coated, it is not the most important factor in controlling external corrosion. Some consider design along with electrical isolation to be the most important. Pipeline coatings actually protect more pipelines from external corrosion than cathodic protection and electrical isolation. Coatings are often thought of as the “first line of defense” in the war against corrosion.

Each protection method is important in controlling external corrosion on pipelines and related structures. Each has a particular role, but must be used in conjunction with the others to successfully protect these structures. Nearly all companies must deal with pipeline systems that are over tens years old. Most companies have operating pipelines over fifty years old, which means the electrical isolation, the cathodic protection system and the coating systems (if used) have out lived their design life.

DESIGN

In the design phase of a project, the proper use of all these factors must be considered to control external corrosion. Electrical isolation when properly used allows the cathodic protection to be effective and economical on that part of the structure it is intended to protect and not be consumed by foreign or un-intended metal structures.

COATINGS

If coatings are properly selected and applied to the pipeline before and during construction, the amount of cathodic protection needed is considerably less. This section of the SP0169 should be stronger to provide guidance for selecting pipeline coatings for new pipe, girth welds, rehabilitation and repair. Since corrosion under disbonded coating is a major cause of external corrosion, one must consider how effective the CP system will be if the coating adhesion were to fail. Will the CP system be effective in controlling external corrosion under disbonded coating if electrolyte penetrates? Most pipeline coatings shield cathodic protection when disbondments occur. Some coating systems are non-shielding and compatible with CP if a disbondment occurs therefore the corrosion rate is significantly reduced or eliminated. Ideally, these two systems work together so that if the coating disbonds allowing ground water to contact the pipe surface the CP system will continue to function. These are called non-shielding, CP friendly, CP compatible, fail safe or partially shielding pipeline coatings.

CATHODIC PROTECTION

CP systems are designed around the coated pipeline using all the related equations and past experiences. CP current is only effective where it has a path to the pipe. Pipeline coating systems must have electrical insulating or dielectric strength in order to divert the CP current to the areas of the pipe where the coating has holidays or damage that exposes the metal to the current. Disbonded, shielding coatings do not allow sufficient current to the pipe steel therefore external corrosion becomes a problem.

Non-shielding coatings, will allow enough CP current to significantly reduce or eliminate corrosion on the pipe metal if the coating disbonds and electrolyte penetrates. This requirement has lead to the distinction between coatings that shield the pipe from the CP system and those that are classified as permeable or CP-compatible.

SHIELDING VERSUS NON-SHIELDING PIPELINE COATINGS

There have been numerous articles written about CP shielding and the corrosion problems that develop. Many articles have also been written about the value of using pipeline coatings that are non-shielding. Only a few are referenced in this article. The relative tendency of pipeline girth weld coatings to shield cathodic protection (CP) current was studied in the laboratory. A key consideration should be "Will the coating shield CP if the bond fails? '' However, all coatings experience some disbondment and, therefore, the behavior of a disbonded coating is important in the overall performance of a coating system. Even with adequate cathodic protection (CP), corrosion can occur under most disbonded coatings. With adequate CP, fusion bonded epoxies (FBE) do not totally shield CP currents ; therefore corrosion is not a major problem. However, FBE maintains its insulation properties in the presences of moisture and cathodic protection current.

TODAY’S TECHNOLOGY

Internal line inspection tools (ILI) and External Corrosion Direct Assessment (ECDA) allows companies to see the condition of pipelines, no matter the age. In some cases, external corrosion is a significant issue. CP may not have been adequate, especially in the early days, because of a lack of knowledge, improper design and monitoring. Too many companies use CP as a “cure all” for external corrosion on coated pipelines, but find out they still have active external corrosion through ILI or ECDA. Today, inadequate CP is rarely the cause of external corrosion.

Stray currents from DC or AC sources cause corrosion problems. Shielding of the CP current by soils, rocks, and other non-conductive materials may be the reason for external corrosion. Even though the US Department of Transportation regulations call for use of ‘non-shielding coating’ most pipeline coating companies do not understand or test coatings for potential CP shielding problems. Slight water absorption only corrodes steel if the cathodic protection is not adequate, or if electrical shielding is present. The industry is beginning to recognize the importance of selecting pipeline coatings that allow CP to be effective, if there is disbondment.

EFFECTIVE EXTERNAL CORROSION CONTROL

As more and more companies begin to realize many of their external corrosion problems are not from lack of CP, but from other causes, they can more effectively spend their dollars and man power. There are great examples of this problem. A close internal survey found inadequate CP. The pipe is exposed, to find deteriorated pipeline coatings which have allowed pipe metal to be exposed to the electrolyte, but there is no external corrosion, because the CP could effectively protect the exposed metal even though it showed to be inadequate. Where criterion is achieved, external corrosion was located through ILI or ECDA. This corrosion is usually caused by disbonded, coatings that are shielding the CP therefore the CP was adequate. ILI on a newer pipeline coated with FBE show no corrosion except at the girth welds where a shielding coating was used. Adding more CP or changing criterion will not stop this corrosion! Field inspection to renew or repair badly deteriorated coatings is crucial in reducing pipeline corrosion.

DO WE NEED TO CHANGE THE CRITERIA?

Criteria as stated in SP0169-2007 are sufficient for controlling corrosion if CP current is allowed to be effective. Proper educate of corrosion control and pipeline integrity personnel to identify the actual cause of external corrosion is more critical than changing criteria.

Taking the pH under any disbonded coating is a very good indicator of CP effectiveness. An alkaline or high pH (9 to 13), on the pipe surface or under disbonded coating is an indication that CP current is effective and corrosion is reduced or eliminated. If less than a pH of 9, corrosion is possible. The lower the pH the more likely corrosion will be a problem.

Field data derived from pipe exposures with actual CP potentials taken at the time of the excavation will indicate if adequate protection is being achieved. These potentials along with pH readings (especially under disbonded coating) and proper evaluation of the coating will indicate if CP is adequate. ILI tools find possible points of external corrosion that must be properly evaluated before increasing CP. External corrosion is rarely present under coatings that are compatible with CP.

CONCLUSIONS

Adding more CP does not control all external corrosion problems. More CP may cause further disbondment exposing more pipe to possible corrosion because of the shielding affects of the coating. The industry is misappropriating many dollars on un-needed CP to meet a certain criterion, instead of rehabilitating pipeline coatings that shield CP, correcting other shielding situations, interference, shorted casings, metal shorts, or failed electrical isolation devices. Meeting CP criteria does not solve most external corrosion.

The role of effective pipeline coatings is often overlooked when evaluating external corrosion. When ILI tools or ECDA show no indications of external corrosion under disbonded or blistered coatings with electrolyte under the coating, these are non-shielding coatings. The pH of this water is usually 9 or above. Under shielding coatings corrosion is usually found and the pH is under 7. Therefore, the committee and industry should concentrate more effort on using non-shielding coatings and replacing shielding coatings that have disbonded to allow CP to be effective and effectively control external corrosion.

SP0169 - 2007
G. Mills, “The Role of The Pipe Coating as an Engineered Part of The Cathodic Protection System”; NACE CORROSION 88; Paper 237
T. Jack, F. King, M. Kolar, and R. Worthingham; “A Permeable Coating Model For Predicting the Environment at the Pipe Surface Under CP-Compatible Coatings”; NACE CORROSION 2004; Paper # 04158.
T. Jack, F. King, Y. Cheng, and R. Worthingham, “Permeable Coatings and CP Compatibility. Proc. IPC ’02 4th International Pipeline Conference, (ASME, New York, NY 2002), paper IPC2002-27267, pps. ,1889-1893.
G.R. Ruschau and Y.Chen, “Determining The CP Shielding Behavior of Pipeline Coatings in The Laboratory”; CORROSION 2006; Paper 06043
D.P. Moore, "Cathodic Shielding Can Be a Major Problem After a Coating Fails",
Materials Performance 39, 4 (2000): pg. 44
J. A. Beavers & N. G. Thompson, ""Corrosion Beneath Disbonded Pipeline Coatings",
Materials Performance April 1997, pg. 13
R. Norsworthy, “Select Effective Pipeline Coatings”; Hart’s Pipeline Digest, February 1997, pg 17
T. A. Pfaff, “FBE Serves a Broad Market”; Hart’s Pipeline Digest, October 1996, pg. 2
J. Alan Kehr, “Fusion Bonded Epoxy (FBE) – A Foundation for Pipeline Corrosion
Protection”; NACE Press, pg 471 vii Page 55
R. Norsworthy, “Proven Protection”, World Pipelines, October 2007, pg. 49
“Coatings Used in Conjunction with Cathodic Protection”; NACE Technical Committee Report; July 2000; pg. 4
D. Song, F. M. Song, D. W. Kirk, and D. E. Cormack; “Barrier Properties of Two Field Pipeline Coatings”; Materials Performance, April 2005, pg 26