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INTRODUCTION ABSTRACT Fusion bonded epoxy (FBE) has been used for quite some time now either as a stand-alone single/multi-layer coating or as a primer for the three layer polyolefin coatings. Interfacial adhesion of coating to steel has been recognized as an important property of coatings to prevent steel substrates from corrosion. However, the coating industry has not been able to reliably measure quantitatively the interfacial adhesion of FBE coating to steel. FBE coating usually has a high initial interfacial adhesion to steel (estimated to be higher than 6000 psi) that exceeds the conventional pull-off adhesion test equipment capability. Its wet adhesion strength, though expected to be lower, has not been successfully measured either. Glue failure at the coating to dolly interface is common due to poor bonding of the dolly to the damp coating surface. In several international standards, different types of knife test are utilized to qualitatively assess the adhesion of FBE coating. Unfortunately, the knife test is often useful only for distinguishing the difference between good and poor interfacial adhesion. To quantitatively assess the interfacial adhesion of FBE coating to steel, a more reliable and repeatable test method is needed. In this paper, the notched coating adhesion (NCA) test method is presented. This test method has been used to quantitatively assess the interfacial adhesion degradation of FBE coating, from an initial high value to a lower value due to aging in water and air at high temperature. Disbondment at the coating to substrate interface is a common failure mode of a coating. The coating to steel substrate interfacial adhesion strength is usually measured using various adhesion tests that range from simple knife tests to more sophisticated tests based on fracture mechanics and/or contact mechanics. Unfortunately, different adhesion tests often provide inconsistent test results. Fusion bonded epoxy (FBE) has been used for pipeline coating for quite some time now either as a stand-alone single/multi-layer coating or as a primer for the three layer polyolefin coatings. Good interfacial adhesion of coating to steel has been recognized as a critical property for FBE to function as a protective coating to prevent the steel substrate from corrosion. ASTM D45411, is one of the most popular adhesion test methods used by the coating community to measure the dry adhesion of liquid cured coatings. However, the coating industry has not been able to use this test method to reliably measure the interfacial adhesion of FBE coating to steel. This is due to the high initial adhesion of the FBE coating to steel (estimated to be higher than 6000 psi), which exceeds the capability of conventional pull-off adhesion tester. The wet adhesion strength of FBE to steel, though expected to be lower, has not been able to measure successfully either. Glue failure at the dolly to coating interface is common due to the poor bonding of the dolly to the damp coating surface.
Residual Stresses In 3Lpp Pipeline Coatings
Guo, Shu (The Boeing Company) | Lo, K. Him (PolyLab LLC) | Chang, Benjamin T.A (PolyLab LLC )
INTRODUCTION ABSTRACT Three layer polypropylene (3LPP) coating system has been widely used for external protection of newly constructed pipelines. In the past few years, two critical service issues relating to coating disbondment and cracking have been observed in the field. Two possible causes of failure, due to high residual stresses and adhesion loss, have been reported in the literature. In this paper, an attempt is made to evaluate the thermally induced residual stresses that can be developed in a 3LPP coating system at locations away from the pipe ends and at the coating cutback. Both analytical and finite element methods have been used for the residual stress analysis. The potential impacts of the residual stresses and coating cutback angle on the observed 3LPP coating failure modes are assessed. Three-layer polypropylene coating system (3LPP) has been widely used in the world for external protection of newly constructed pipelines. A 3LPP coating system is made up of a layer of FBE primer, a functionalized polypropylene adhesive midcoat, and a polypropylene (PP) topcoat. The FBE primer provides a good oxygen barrier while the PP topcoat provides a good water barrier to the pipeline. PP topcoat also has mechanical properties that help to protect the coating from potential damages that might incur during transportation and installation of the pipeline. The commonly recommended upper service temperature for a 3LPP coating system is about 110 °C for onshore; though a service temperature as high as 140 °C for offshore has been suggested. In the past few years, some critical service issues relating to coating disbondment and cracking have been observed in the field1. Two possible root causes of failure, high residual stresses and adhesion loss, have been suggested in the literature2,3. The dry film thickness (DFT) of FBE primer in a 3LPP coating system is usually small (4 - 10 mils or 100 - 250 µm) while the PP topcoat is normally much thicker (80 - 240 mils or 2 - 6 mm). After the coating system has been applied onto the pipeline, it will be left to cool from a high processing temperature (in the range of 200 - 250 °C) down to the ambient temperature. Since the FBE primer has a higher thermal expansion coefficient than the steel substrate, thermal induced residual stresses will be developed in the FBE primer due to high thermal expansion coefficient mismatch with the steel substrate. The PP topcoat has a thermal expansion coefficient even higher than that of the FBE primer. High residual stresses will also be developed in the PP topcoat despite the more compliant characteristics of PP. At the coating cutback, the magnitudes of the induced residual stresses will be more pronounced due to the geometric discontinuities introduced at the cutback. These high residual stresses could contribute to the cracking observed in the PP topcoat during service and could lead to high peeling forces to initiate coating disbondment at the cutback. This paper addresses the residual stresses within the coating system as well at the coating cutback.
- North America > United States > Texas (0.28)
- North America > Canada > Alberta (0.28)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.94)
- Energy > Oil & Gas (0.92)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)