Quality assurance of coatings on steel water pipes commonly relies on tensile pull-off measurements of the coating-steel adhesion, according to ASTM D4541. These tests are performed by adhering a metal ‘dolly’ to the coating with an adhesive, then scoring around the dolly circumference, through the coating down to the steel surface, before recording the stress necessary to pull the dolly (and coating) away from the steel pipe.
Product Number:
41214-815-SG
Author:
S. G. Croll, B. D. Keil
Publication Date:
2014
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$20.00
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Quality assurance of coatings on steel water pipes commonly relies on tensile pull-off measurements of the coating-steel adhesion, according to ASTM D4541. These tests are performed by adhering a metal ‘dolly’ to the coating with an adhesive, then scoring around the dolly circumference, through the coating down to the steel surface, before recording the stress necessary to pull the dolly (and coating) away from the steel pipe. Usually, the adhesion value required accords with AWWA C222 but it may be specified higher, in the belief that higher values ensure that the coating will provide longer corrosion protection for the steel. Values of the failure stress from this technique are very prone to experimental uncertainty and there is no established quantitative connection between adhesion, measured by any technique, and corrosion protection. Since the tensile pull-off test is the only practical adhesion test in these circumstances, possible sources of its variation were studied using polyurethane coatings. Variables included pipe diameter (including flat surfaces), glue type (cyanoacrylate and epoxy), dolly diameter, polyurethane formulation and scoring through the coating, around the dolly. It is well known that, in any engineering material, flaws that exist in, or between, materials dominate the ultimate strength possible. Thus pull-off results may depend significantly on problems introduced in preparation of the adhesion test, and results may not be indicative of the intrinsic properties of the coating.
