Detecting and preventing pipeline coating failures

Paul Boughton

John Fletcher looks at a method to spot potential problematic areas of corrosion which can be detected and then addressed before causing expensive and time consuming repairs.

Failure of protective coatings on pipelines can result in corrosion or other deterioration of the underlying substrate. Possible problems include the formation of rust or pits and chemical attack.

The resulting repairs of the coating and the inability to use the affected equipment and plant can be very expensive. Often failure occurs due to the presence of flaws in the finished coating.

Typical flaws include pinholes, holidays, inclusions, air bubbles, cracks and thin spots.
All these problems are easily detectable either prior to installation of the pipelines, or as part of the inspection and maintenance program of existing pipelines.

There are several standards and test methods that can be used to ensure the pipelines are in good condition which can all be achieved using the Elcometer236DC Holiday Detector. These include ASTMG62-B, NACERP0188, NACERP0274 and NACERE0490.
The detector works by generating a high dc voltage that is applied to the coating surface through a probe. In addition, the detector is connected to the substrate via the signal return (earth) lead.

When the probe is passed over a coating flaw, the electrical circuit is completed and the current flows from the probe to the substrate.

As a result, audible and visual alarms are activated in the detector and a spark may be produced at the flaw.

Before using a holiday detector it is important to establish the thickness of the coating, as this will determine the level of voltage that needs to be used.
If the voltage used is too high for the thickness of the coating, the coating can burn or pop, causing irreversible damage. But if the voltage is too low, it will not detect the flaws.

Coating thickness

The Elcometer236 Holiday Detector allows the user to set the appropriate voltage parameters dependant on the dielectric strength of the coating (the voltage at which the coating will break down) and the coating thickness. This ensures the holiday detector can be used effectively and accurately without risk to the coating and without the risk of not detecting flaws.

Also, the sensitivity of the alarm setting is fully adjustable to suit conditions. If the sensitivity is too high, the effects of a prevailing electrical leakage, through the coating or moist air, can cause false alarms. If the sensitivity is too low, the alarm will not be triggered.

However, the probe handle neon indicator will still illuminate and sparking may occur at the flaw.

There are a wide variety of probes available for use with the Elcometer236 Holiday Detector, depending on the nature of the inspection being carried out. These include a band brush, right angle brush, circular brush probe and a rolling spring probe. The portability and ease of use, including the external battery pack allowing battery change in seconds, along with reliability, accuracy and durability make the Elcometer236 Holiday Detector a highly efficient gauge for use in the field. 

John Fletcher is with Elcometer Instruments Ltd, Manchester, England. www.elcometer.com

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