Field joint coatings | technical report
stress. This layer connects the epoxy primer (anti-cor- rosion layer) with the polyethylene outer layer (mechan- ical protection layer). The soil stress resistance of polymeric backed mesh
coatings comes from the outer mesh layer, which is non elastic, strong and applied with high tension. The polypropylene mesh backing of the Polyguard RD-6 coating system contracts after application, increasing adhesion and soil stress resistance. The lap shear test methodology from ISO21809-3 conducted for heat- shrinkable coatings or polymeric tapes is, therefore, not relevant for polymeric backed mesh coatings. Real life results, or modified lab protocols with tubular sub- strates, would give higher results.
Long Term Performance: Cathodic Disbondment Resistance - When ionic current from the anode bed reaches the exposed steel surface
to be cathodically protected, gaseous hydrogen (H2) is produced. The gas pushes and disbonds the “healthy” coating - cathodic disbondment. Test methods for determining resistance to cathodic disbondment consist of boring a 6mm diameter hole into the coating and exposing the coated sample to ionic current during 28 days. After this period, the extension of the original hole is measured in mm. According to EN12068 Class C50, the maximum disbondment for heat-shrinkable coatings and poly- meric tapes is between 10 mm and 20 mm (10 mm if the peel strength is minimum 0.5 N/mm, 20 mm if the peel strength is minimum 1.0 N/mm). EN12068 recognises that high peel strength coatings have poor cathodic disbondment resistance; which is a typical rule for polyethylene coatings. ISO21809-3 allows a maxi- mum 15mm disbondment for polymeric tapes and 8mm for heat-shrinkable coatings (3LPE). Cathodic disbondment resistance can be considered
more critical than peel strength. Cathodic disbondment is applicable throughout the whole life of the coating and cathodic disbondment is tested during 28 days (a long term test); Peel strength is not applicable for the lifespan of the coating and the peel strength test takes place once just 24 hours after coating application (a short term test). The Polyguard RD-6 coating system scored particularly well at KTA Tator. With zero disbond- ment after 28 days exposed to ionic current, the coating system largely exceeds the required performances of heat-shrinkable coatings and polymeric tapes, and does this while displaying excellent adhesion (peel strength). Hot Water Immersion - The conclusions of the Pipeline Protection Conference of November 2005 in Cyprus was that hot water immersion, followed by adhesion testing, is probably the best screening/
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qualification test to determine long-term reliability of buried pipeline coatings. After 28 days immersed in hot water (at the maximum operating temperature) a peel strength (adhesion) test is conducted. ISO21809-3 requires a result of 0.4 N/mm for polymeric tapes and 1.5 N/mm for heat-shrinkable coatings. EN12068 does not require this test. The Polyguard RD-6 coating system scored very well at KTA Tator, with more than 3. 0 N/mm after 28 days in hot water.
Conclusions: According to the international standards EN12068 and ISO21809-3 used for field applied coatings, key performances of polymeric backed mesh coatings are better or equivalent to heat shrinkable coatings and polymeric tapes. These can be summarised as follows: l Mechanical Resistance (Impact & Indentation): Equivalent
l Adhesion to Steel and Plant Coating (Peel): Better l Lap Shear Resistance: Equivalent (but soil stress resistance higher)
l Cathodic Disbondment Resistance: Much better l Peel Strength after Hot Water Immersion: Much better
About the authors: Nathan Muncaster is Global Business Development Director for Polyguard Products. He holds a Masters degree in International Business Administration and has more than 10 years experience in selling Polyguard RD-6 and other products globally. Luc Perrad is Manager of ExParTech, Polyguard’s
representative for Western Europe and Africa. Based in Belgium, he holds a Masters Degree in Civil Engineer- ing and has more than six years’ experience in sales and marketing of field-applied pipeline coatings in Western Europe, Africa and the Middle East. He is also a NACE Coating Inspector Level 2.
September 2015 | PIPELINE COATING 27
Above: Holiday testing of an RD-6 rehabili-
tation project in the US
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