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chemical resistance of polyurethane field joint coating
Posting by alan jefferson on April 24, 2008 at 17:13:33.
Can anyone support my supposition that large bulges about 30cm radius forming in a 12mm thick polyurethane (PU) coating above the weld in a steel pipe are due to liquid diffusion into the polyurethane, followed by adhesion failure, corrosion and osmosis. Sections of polyethylene (PE) covered steel pipe have been welded together. A 6mm thick polyurethane PU layer covers the polyethylene PE coated steel pipe upto just short of the weld, and then continues on as the sole 12mm coating upto the next section of PE covered pipe where it again reduces to 6mm.
The pipe is 135 metres below the ocean and the contents are at 90 degrees C. I presume that the polyetyhylelen PE coating is crosslinked PE (PEX) The lack of differential pressure seems to rule out mere water ingress as the problem, without a driving force to produce the bulges.
The steel has CP applied well away from the weld and bulges so hydrogen is free to escape.In my opinion adhesion failure could set up osmosis and Fe3+ ions could form to induce water through to form the bulges, which were noted after 2 years service. I am checking out the polyurethane PU grade because some polyurethane PU grades degrade in water at > 50 degC.
Any helpful suggestions would be appreciated. The proof will be in the pudding when we sample behind the bulges!
follow up posts
On 05/08/2008 chen chi posts:
Hi, could you help me with the corrosion / chemical degradation rate of undiluted high concentration nitric, hydrochloric and sulphuric acid in coating materials for tank of stainless steel (temperature 5 to 75 degrees Celsius, maximum pressure 3 Bar). Is there also a problem with diffusion of the acidic solution to the interface between coating and metal? What sort of coating do you recommend? We are thinking of a fluoropolymer (ETFE, Ethylene tetrafluoroethylene) based coatings. Regards,
Chen Chi
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On 04/24/2008 dilwyn jones posts:
The high temperature of the pipe contents will be a factor. All the temperature drop will occur across the polyurethane (PU) field joint layer, so its average temperature will be around 60 C. This will accelerate diffusion through it. Also, polyurethane PU has quite a high thermal expansion coefficent (around 1.5 x 10^-4 per deg C) so there will be a significant compressive stress set up. Once the interface weakens, the polyurethane PU covering will spring out to relieve the strain. There may have been some stress relaxation while intact, explaining why the bulge doesn't collapse once the polyurethane pipeline field joint gets cold again. I have seen a figures of 0.5 to 3.5% water uptake in different polyurethanes PU's, so the figure quoted (and presumably D also) may be too small. I offer consultancy on thermomechanical properties of polymers if this would help. Regards,
Dilwyn Jones
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On 04/24/2008 composite agency posts:
Dear All: Extensive polyurethane research shows that the temperature gradient of 92 degrees Celsius to around 25 degrees Celsius had a large influence on the solubility (S) of water in the polyurethane and the diffusion rate (D) of water in the polyurethane coating. The two factors combined, results in a relative small diffusion time lag and a considerable flux through the coating, reaching the coating - steel interface. The solubility figures Dilwyn mentioned, have the right order in the mentioned temperature and pressure conditions for a rigid polyurethane. For elastomer polyurethanes (for field joints), the solubility figures will be higher in the stated conditions. If there is a lack of adhesion, water may accumulate at the interface. Lack of adhesion is most probably due to insufficient wetting in combination with a local precipitation or pollution on the steel pipeline surface, forming the blister nucleus. The dilution of precipitation or polluted particles by the osmotic driven water flux, will probably lift the surface, exceeding the interfacial surface energy (the release of elastic energy exceeds the work of interfacial adhesion times the distance). Usually hydrolysis of a polymer, may be polyurethane crosslinked by polyether or polyester, leads to mass loss of the polymer: a corrosion front moves into the material gradually. Polymer chains are cut, and holes appear. The holes that appear are - perhaps paradoxally -so large that an eventual local osmotic pressure is reduced so fast, that a blister will not appear. In the sixties the polyester and gelcoats used for boats was so bad, that blister did never appear, from the moment they improved the gelcoats - making them more tight - the blister problem came in. Nowadays they probably solved this boat issue by using a better structural resin, e.g. vinyl ester resin or epoxy resin, or perhaps by making the gelcoat more permeable? (who knows this, please post me a follow up, many thanks in advance ;-). So lack of adhesion and the presence of pollution or precipitation on the surface is causing this blister phenomenon. The same problem may arise in high temperature/high pressure isolation applications in offshore or civil equipment, such as high pressure - high temperature isolation of steel pipelines and flexible pipes for natural gas (methane) and oil transport.
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