Disbondment mechanism of 3LPE pipeline coatings
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Both Fusion Bounded Epoxy (FBE) and 3-layer Polyoleftn (3LPO) coating systems have been widely applied in the world as new construction external pipeline coatings. Several incidences of coating disbondment at the FBE-steel interface of 3LPO have been reported in the literature. The coating disbondment issue has raised concerns over the use of 3LPO, which is regarded as a more damage resistant coating system than single layer FBE. The coating disbondment of 3LPO coatings is the result of loss of FBE-steel adhesion due to inadequate surface preparation and/or preheat temperature. The loss of adhesion is aggravated by the residual stresses generated by either thermal mismatch and/or polyolefin processing. The purpose of this work is to investigate the coating disbondment mechanism, which is related to the residual stresses caused by thermal mismatch between coating materials and steel substrate. Both analytical stress analysis and finite element modelling (FEM) have been used to calculate the thermally induced stresses. Particularly, at the cutback, FEM is powerful for analyzing the stress concentration at the corner of coating cutback. The stress analysis results indicate that the FBE and PE topcoat dry film thickness (DFT) will not introduce high stress to the FBE primer except at the coating cutback. At the corner of coating cutback, the peeling stress increases significantly with the increase of PE DFT. Coating disbondment may be initiated due to higher stress concentration at the corner of 3LPE coating cutback, particularly at higher PE topcoat thickness in combination with the environmental degradation of the FBE-steel iiiterfacial adhesion strength. BHR Group 2007 Pipeline Protection 17.
