Determining the required voltages for holiday testing dual layer fusion bonded epoxy coatings

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A standard test for detecting coating holidays is to apply a high voltage to a coated pipe that would break down the air gap between the metal surface at the holidays and a test electrode travelling over the coated surface. Previous studies including those carried out in Australia have provided much insight into voltage levels required to reliably detect through thickness coating defects without damaging an intact coating. For instance the current Australian Standard AS 3862 ‘External FBE coatings on steel pipes’ has specified test voltages for field holiday testing, however these standard voltages are based on the single layer fusion bonded epoxy coatings. Its applicability to dual layer fusion bonded epoxy (DLFBE) coatings that have become commonly used on Australian pipelines in recent years is unknown. Thus, there is a need to confirm the appropriate holiday detection voltages for DLFBE coatings. Also, the effect of multiple high voltage holiday testing on DLFBE coatings also needs to be assessed to ensure that repeated holiday testing does not breakdown the coating causing ‘new’ defects on previously good pipeline coatings. This is needed because pipelines are typically subjected to repeated high voltage holiday testing during manufacturing, load-out, transportation and installation processes. This paper reports an extensive study of high voltage holiday testing of DLFBE that has been performed to determine the effects of temperature, moisture absorption and multiple passage of holiday detection over DLFBE coatings under simulated conditions that are often encountered on-site. Short-term and long-term tests have been performed to characterise the effects of moisture and temperature on DLFBE coatings. The test results confirm the criterial voltage value of AS 3862-2002 is applicable to DLFBE. They show that that subsequent coating holiday testing in accordance with AS 3894.1, using a detector with constant DC voltage output and with the voltage set to 65% of the specified values in AS 3894.1, will result in reliable detection of coating holidays. The results also indicate that most holidays can be detected at voltages below the 65% level when DLFBE coatings have been exposed to a combination of temperature and moisture. It has also been established that multiple passes of a holiday detector over DLFBE coatings does not create new defects/pinholes in DLFBE coatings.

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