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.
