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NACE Testing Non-shielding Properties Rev 5

Oct 22, 2014



Richard Norsworthy
Polyguard Products, Inc.

PIPELINE COATINGS and CP SHIELDING CORROSION 2012Richard Norsworthy Polyguard Products, Inc.


IS THERE REALLY A PROBLEM WITH DISBONDED COATING SHIEDLING CP?YES! Internal line inspection tool (ILI) companies have stated that up to 85% of all the external corrosion and all the SCC found is under disbonded (CP shielding) pipeline coatings! Oil and gas companies have confirmed that well over 50% of the external corrosion and most if not all SCC located is under disbonded CP shielding pipeline coatings.2

Recent Comment from a Major Gas Company Cathodic protection shielding is in fact a root cause in our companys group root cause process. Not only is it an option for root cause but it is in fact the pre-dominant root cause that we see during in-line inspection prove-up processes.3


SP0169-2007 defines electrical shielding as: Preventing or diverting the cathodic protection current from its intended path. 4.2.3 Materials and construction practices that create electrical shielding should not be used on the pipeline. Pipeline external coating systems shall be properly selected and applied to ensure that adequate bonding is obtained. Unbonded coatings can create electrical shielding of the pipeline that could jeopardize the effectiveness of the cathodic protection system.5

Electrical Shielding

CP Shielding NACE Publication 6A100 Coatings Used in Conjunction with Cathodic Protection says; Shielding Effects If a coating has high dielectric strength or electrical resistance, the coating shields the structure from cathodic protection currents. If water migrates between the coating and the steel, a shielding effect could prevent the cathodic protection current from protecting the steel substrate.6

Non-Shielding Definition SP0169 (Last Revision from TG 360 Committee) Non-shielding Coating System: A coating system with a failure mode (loss of adhesion, etc.) that does not prevent distribution of cathodic protection current to the metal substrate. This means in the event of adhesion loss, CP can protect the substrate.


New DOT Part 192.112 PHMSA has adopted a new ruling for those pipelines that want to increase maximum allowable operating pressure (MAOP) to 80% for more through put must meet several requirements beyond what they are now required to operate at this level. One of these requirements is to use nonshielding coating systems.8

New DOT Part 192.112

The pipe must be protected against external corrosion by a non-shielding coating. Coating on pipe used for trenchless installation must be non-shielding and resist abrasions and other damage possible during installation. A quality assurance inspection and testing program for the coating must cover the surface quality of the bare pipe, surface cleanliness and chlorides, blast cleaning, application temperature control, adhesion, cathodic disbondment, moisture permeation, bending, coating thickness, holiday detection, and repair.9

Coatings Used in Conjunction with CP All coatings used with cathodic protection must have good dielectric strength when adhered to the surface of the pipeline. Well documented external corrosion failures have occurred when certain coating types lose adhesion and water penetrates between the coating and substrate allowing corrosion to occur because the CP continues to be shielded. Some types of coatings allow CP to reach the substrate if adhesion is lost and water penetrates.10

Another Definition of a Non-Shielding Coating Non-Shielding means if the coating system adhesion fails and water penetrates between the coating and pipe, corrosion on the metal is significantly reduced or eliminated because adequate CP has a path to protect the steel under the disbonded coating system.11

Fusion Bonded Epoxy is such a Non-Shielding coating. Corrosion is rarely found under disbonded FBE. At this time stress corrosion cracking (SCC) has not been found under disbonded FBE.


Other Non-shielding Coatings The combination of a particular mesh backing and compound formulation has also been proven to have Non-Shielding characteristics. After over 24 years of service life on thousands of projects around the world, this company has had no reports of measurable corrosion under the rare disbondment areas that have been located when adequate CP is available. ILI runs and ECDA have proven this fact.13

We cannot say that Fusion Bonded Epoxy (FBE) or other coatings are 100% Non-Shielding, but we can start to understand that some coatings systems do provide us with a definite advantage when coating adhesion failure occurs.14

Integrity Advantages of Non-shielding ECDA processes such as DCVG and ACVG can successfully find disbonded areas since the current is being allowed into the disbonded area. With CP shielding coatings, disbonded areas are not as likely to be located with these surveys. If a company knows the coating is non-shielding, there is not an urgent need to replace the coating since CP can be effective.15

FIELD DATA IS CRITICAL The most important information about whether a coating is CP shielding or not comes from field evaluations of disbonded coatings. Presences of measureable corrosion usually indicates a CP shielding coating system, but further investigation is required. A pH less than 8 under the coating typically indicates a CP shielding coating. Between 8 and 9 further investigation is required. A pH over 9 under the coating typically indicates that it is non-shielding to CP.16

FIELD DATA IS CRITICAL There are many issues that must be considered when evaluating a coating in the field to determine if the coating is shielding or nonshielding to CP. If the pH is over 9 and there is corrosion, then the history of the pipe at that location is important. The corrosion may have developed before the CP was applied to the pipeline system or before adequate CP was applied, etc. pH in the corrosion will also determine if the corrosion is active or inactive.17

CP and Coatings Non-Shielding coatings allow CP current to protect the substrate when water is present between the coating and the substrate even if there is no opening or holiday. CP requirements may increase slightly for Non-Shielding coating systems. The benefits of Non-Shielding (little or no corrosion under disbonded coating) outweigh any additional CP cost. Adhered coatings are not usually a problem!18

Water under blisters on FBE coated pipe used for gas transmission in central USA. Water under the blisters had a pH of 12.


Even though blisters and poor adhesion were a problem, the steel under the coating shows to be in excellent condition.


How to determine if a coating is Non-Shielding The best test is to use actual field data. If a failed coating is found, one should always take pH readings of any water between the coating and the substrate. If the pH is over 9, CP current is probably adequate enough to control corrosion of the pipe under the coating. Check several areas if the disbondment is large. De-ionized water can be used to wet the pipe surface if there is limited moisture present.21

Checking pH under the improperly applied Mesh Backed Tape Repair Coating after three years in service.


pH check indicates a high pH (9 to 10) in water under the improperly applied mesh backed tape showing it is NonShielding.


pH check under disbonded coal tar with significant corrosion present.


pH of 5 to 6 under disbonded coal tar coating.


When Corrosion is found on an FBE or other Non-Shielding" coated pipeline Check previous CP records to see if CP has always been adequate or provided. Could corrosion have happened before CP was applied or during periods of inadequacy? Was or is AC or DC interference with a foreign system possible? Is the pipe being shielded by some object? Always take potentials and pH to determine if CP is adequate in this area.26

Laboratory Non-Shielding Test Lab testing can be another effective method of determining if a coating will be NonShielding. Proper testing requires successfully duplicating the condition of disbonded coating as encountered in the field.


Laboratory Non-Shielding Test INVESTIGATION OF THE SHIELDING BEHAVIOR OF TWO PIPELINE COATING SYSTEMS Performed by CC Technologies Laboratories, Inc. in 2001 Report R335-01 This test was performed using the test procedure developed by Polyguard Products, Inc.28

Hydrogen evolution through this coating system during a Non-Shielding test proving CP is penetrating under overlap.29

Development of a Test Method Is it time NACE develops a test method for coatings used with CP to determine if they are shielding or non-shielding to CP when disbondments occur? Another issue is to determine the potential under a disbonded coating and current consumption. SCC occurs under disbonded coating in a particular potential range and pH range.30

Development of a Test Method The process of developing a test that will determine if a coating used with cathodic protection (CP) is shielding or non-shielding to CP is a critical issue for the pipeline industry. A single test method may not be adequate for each type of pipeline coating. Testing must relate to actual field conditions. The test method presented in this presentation has been successful for some types of coatings, but may not work for other types of coatings.31

Lab Testing After several attempts to find the best method for determining if a coating will allow CP current to be protective under disbonded coating. One method tried was to use a grooved pipe with water injected into the grooves after the pipe was coated. These was somewhat successfully, but hard to seal, etc. Coating a wet pipe was a