Holiday detection | testing
Pulsed DC detection devices, such as Elcometer’s 280 model, provide a versatile method for detecting and locating a variety of holidays in internal and external coated steel pipe
In-fi eld detection of coating defects
Testing cured coatings for fl aws and defects is often a key part of the acceptance process for a coating assessed against its specifi cation. This is particularly the case for steel pipeline coatings and for coatings applied to pipeline hardware for corrosion protection, where any discontinuities can lead to damage to the substrate, disbondment of the coating and ultimately the premature failure of the metal. Even small fl aws or defects – holidays - in pipeline
coatings can result in corrosion damage, leading to shortened service lifetime or additional servicing cost. This is especially so where the pipe is immersed or partially immersed in water. Whether the coating’s function is to protect the
internal surface of the steel pipe from the substance it is transporting or to shield the external surface from environmental corrosion, it is essential it is free of any potential defects. Discontinuities, fl aws and defects in coatings can take many forms, but they all have one common property in that they reduce the effective strength of the coating and hence its durability. Some of the more frequently encountered discontinui- ties in applied organic coatings include pinholes, runs and sags, cratering, cissing and incorrect coating thickness. Each can be caused by a combination of errors and inconsistencies in the coating application process: Pinholes are typically caused by air or blasting media trapped in the coating during application; Runs and sags are usually caused by the application
of an excessive thickness of a liquid coating; Cratering is caused by air being released from the surface of the coating in the partially cured state, where the viscosity is too high to allow the coating to “fl ow” to cover the depression; Cissing is characterised by surface breaks in the fi lm that expose the substrate and often results from the steel substrate being contaminated by grease or oil (cissing is also known as “crawling” or “fi sheyes”); Incorrect coating thickness- whether too thick or too thin - can be detrimental to the service life of the coating even when other defects are not present and is caused by poor control of the coating process. One key measure of the effectiveness of an
organic coating is its insulating property - its ability to resist the fl ow of current when a voltage is applied. UK-based Elcometer’s 280 Pulsed DC Holiday Detector uses pulsed DC voltages to detect holidays in coatings on metal pipeline surfaces. Designed specifi cally for use on coated steel pipes, the unit offers a voltage range from 0.5-35kV and is said to provide a safe, simple and reliable testing solution for the
Right: The 280 confi gured with a rolling spring for pipe testing
November 2012 | PIPELINE COATING 39
Main image: Elcometer’s 280 pulsed DC holiday detector
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