ABSTRACT
With the broad use of three layer polyethylene (3LPE) coatings for external corrosion protection of oil and gas pipelines in the world, more and more failures mainly relating to coating adhesion problem were observed. Substantial on-site failure case studies and investigations showed that 3LPE coatings generally retained good adhesion to steel pipelines without being put into service, but complete disbondment of FBE primer to steel substrate have been observed, with some of them noted even after 2-3 years in service. A review of related literatures and broadly used 3LPE standards revealed that current standards are inadequate in assessing the long term performance of 3LPO coatings and the longer term hot water immersion test at elevated temperature was recommended. In this study, a comparative study was conducted by the laboratories on thirteen (13) different 3LPE coatings from ten (10) coating suppliers. Peel strengths of the applied coatings were firstly examined, and the coatings were then soaked in water at temperatures of 50, 70, and 80°C for 28 days. After that the hydrothermally aged coatings were tested for peel adhesion again. In addition, edge disbondment of the selected coatings was periodically assessed during immersion. The paper specially focuses on the disbondment issues on 3LPE coatings and is aimed to establish an applicable and effective test program to identify the problems. The disbondment noted on the tested 3LPE coatings showed good agreement with the failure mode observed in field. The test program can be used to screen weak 3LPE coatings during coatings selection and also it can be used to identify potential problems existed in coating application.
INTRODUCTION
With the high growth in global pipeline industry, the use of pipe coatings is increasing significantly. Among the versatile pipe coatings, 3LPE coatings grow in popularity in the oil and gas industry and are frequently used in most areas of the world because of their combining advantages from polyolefin coatings and fusion bonded epoxy coatings (FBE) 1. A typical 3LPE coating comprises of three layers: an inner FBE primer, a polyethylene (PE) topcoat, and a co-polymer PE adhesive middle coat to bond the epoxy primer with PE topcoat. The tough PE topcoat offers the enhanced mechanical protection to damages caused by rough handling, installation, and operation, while the FBE coating provides the long-term corrosion protection owing to its broad range of desirable characteristics, such as excellent adhesion to steel, high electrical resistance, and low oxygen permeability, etc.2 Despite the advantages of the 3LPE coatings and the improvements in the coating technologies, unexpected adhesion failures of the coatings have been reported around the world where the coatings have been used3,4,5 Even though it is debatable whether these adhesion failures put the pipeline at risk of corrosion, many believe the disbondment of 3LPE between the FBE and the steel creates potential high risk of CP shielding6. For the demonstration purpose, Figure 1 shows the examples of the disbondment failure of 3LPE coatings that were observed in field.
