Pull-Off Adhesion Test for Coatings on Large Pipes: Possible Variations in Failure Location and Mode

Abstract

Pull-off adhesion testing per ASTM D-4541 is a commonly used quality control check for coatings on large-diameter steel water pipe. During the testing a metal dolly is glued to a pipeline coating then pulled off to assess adhesion of the coating. In practice, results are very sensitive to circumstances and have large standard deviations compared with their mean value. It is clear from a simple inspection of Griffith's equation for the strength of materials that the pull-off stress at failure increases with the stiffness of the materials and the interfacial failure energy and is diminished by the presence of existing cracks or flaws. Finite element stress analysis explored the effect of the coating and glue stiffness and some of the other possible reasons for variation in the results, i.e., pipe curvature, misalignment of the dolly, and the effect of scoring through the coating and glue around the dolly. The tensile modulus of three polyurethane coatings, an epoxy adhesive, and a cyanoacrylate adhesive were measured and showed that it was possible for a polyurethane coating to be stiffer than an adhesive used to fix the dolly in place. The location of the maximum strain, in the glue or the coating, was used as an indicator of where the adhesive failure was likely to occur within the overall joint. Results indicate that the influence of dolly misalignment on the coated pipe is greater than the influence of the pipe curvature itself. If the dolly was aligned perfectly, the greatest strain and thus most likely location of failure was on the crest of the pipe at the pipe-coating interface. The value of the strain was not a strong function of the curvature of the underlying pipe surface. If the dolly was misaligned, then the locus of failure shifted to where the glue was thinnest, but remained at the coating-pipe interface, if the glue is stiffer than the coating. However, if the coating was stiffer than the glue, then the location of the two greatest strains indicated that failure was more likely to be at the glue-dolly junction in all cases, rather than at the coating-pipe interface. Users should be aware of how these, and other possible, variations affect the pull-off results if they rely upon single dolly pull values to assess the overall adhesion or use the test method to assess the likely reliability of a coated steel pipe in service.