Alan Kehr provides some guidance and perspective on an article published in Pipeline and Gas Journal: “Fusion-bonded Epoxy’s Effects on Cathodic Disbondment,” by Emad Behdad. Behdad points to problems of surface preparation, whereas Kehr focuses on the more pervasive role of incorrect cathodic protection design.
It is well-understood that the criteria and design of cathodic protection (CP) systems vary as a function of pipe coating selection. For example, coal-tar coated pipelines typically require much higher cathodic protection voltages than FBE-coated pipelines.
As a result, the CP procedure required for one coating system, such as coal tar, may result in severe overprotection, as well as possible coating damage and increased cost if it is applied to a different coating system, such as fusion-bonded epoxy (FBE).
Some end users and engineering firms believe that you can mix coating technologies on the same pipeline. We do not recommend this practice if required CP levels result in excessive voltage applied to one of the coatings.
Cathodic disbondment of FBE can be accelerated at voltages greater than -1 V relative to a silver/silver chloride reference electrode. Since it is impossible to coat and install a perfectly coated pipeline, there will be damage, pinholes and exposed steel that the CP must protect. Higher voltages accelerate and increase the disbondment area around a damage or pinhole site. And as the disbondment increases, so will the CP current required to maintain the desired voltage.
Cathodic Protection Design
For any CP system, it is essential that its effectiveness be assessed via an appropriate criterion. There are a number of criteria used to design sufficient cathodic protection in pipelines, including:
- The absolute potential criterion
- The E-Log I criterion
- The 100 mV depolarization/decay criterion
Absolute Potential: This procedure attempts to evaluate the absolute electrochemical potential to which the metal surface is polarized. Specifically, the IR corrected potential, where IR is derived from Ohm’s Law that states that V = IR. In this equation, V stands for voltage, I represents current (amperage) and R stands for resistance.
The goal is to achieve an applied potential more negative than the potential at which iron oxidation becomes thermodynamically viable—approximately –0.8 V relative to a silver/silver chloride reference electrode.
Note: The applied potential should be mapped along the pipeline—this is not a single-point measurement. The idea is to make sure that the entire pipeline is protected, not just a single point.
100 mV Depolarization/Decay: This criterion assesses the amount of depolarization that the cathodic protection system is achieving. It is accomplished by measuring the difference between the instant-off voltage (IR corrected applied voltage) and the measured voltage after the pipeline has decayed back to steady state. As the name of this criterion implies, 100 mV of polarization needs to be attained throughout the protected area. Again, this is not a single-point measurement.
E-Log I: This criterion looks at the current-potential relationship of the protected structure. Essentially, an increasingly negative potential is applied until there is a linear relationship between the applied voltage and the log of the applied current.
Which Criterion is Most Accurate?
The most rigorously correct, from a scientific point of view, is the absolute-potential-based criterion. The others are more highly dependent on the local environmental conditions, and can be much more difficult to apply effectively.
An improperly designed cathodic protection system, resulting in overprotection, can cause loss of adhesion of any pipeline coating over time. However, the CP will still prevent the onset of corrosion due to the non-shielding nature of FBE.
The Cost of Overprotection
Unfortunately, an overprotected system will result in CP current that increases with time. This increased current will translate into an increased cost of operation of the CP. Thus, the proper combination of coating technology and CP procedure is essential in guaranteeing the long-term, cost-effective protection of a pipeline.
Fusion-bonded epoxy coating, combined with a properly designed CP system, virtually guarantees that the desired level of long-term performance and cost control can be achieved.