Variability of Pipe Coating Pull-Off Adhesion Measurements on Cylindrical Steel Pipelines

One of the most commonly utilized, and stringent, quality assurance tests for coatings or linings on steel pipe is the destructive pull-off adhesion test according to ASTM D4541. The specified value of adhesion is generally in accordance with AWWA C222 but for some projects adhesion values have been specified at very high levels which can be problematic from a testing or verification stand point. The rationale for high adhesion values (those greater than C222) appears to be a perception that very high adhesion values correlate to longer term corrosion protection. Usually, destructive pull-off tests are done by adhering metal "dollies" to the curved surface of the coated pipe using a fast reacting cyanoacrylate adhesive, and then mechanically pulling the dolly off in a radial direction. Tests are done not only on the very top of the pipe where it is possible to align the dollies well, but are also done around the sides of the pipe, where it is much more difficult to achieve ideal alignment of the dollies. Another limitation is that the adhesion of the glue to the coating may fail, giving a null result. Additionally, when the actual adhesion of coating is very high, the device for pulling the dollies from the pipe has a limit on the force that it can exert, and in necessity will terminate the test without giving a datum. The paper will discuss testing that was completed to examine the consistency of adhesion testing to ASTM D4541 on steel pipe coating with polyurethane applied in accordance with AWWA C222. Pipe diameter, flat surfaces, glue type (cyanoacrylate and epoxy), dolly diameter, polyurethane formulations scoring around dollies, and type of equipment were all considered in the testing. Possibilities for improving testing reliability and accuracy are discussed.