Ask the Expert: Measurement of Adhesion

Q: I need to test the adhesion of the epoxy primer applied on the blasted surface. The DFT of the primer is 60 microns, and Ry5 roughness of the blasted surface equals 60 microns. I suspect that the primer has been applied incorrectly.
For the adhesion testing I am going to use X-cut method (ASTM 3359). But I think that the test results may be wrong because of the surface roughness (I will not be able to cut the primer down to steel in all points along the incision). Is it reasonable to use X-cut method (or cross hatch method) in this situation? What other methods may be used in this situation?

A: For your application the tape adhesion test, ASTM 3359 is probably not the best method. Unfortunately, there is no perfect adhesion test that will yield reproducible results and you will have to make do with approximations. I suggest that you consider one of the pull-off adhesion test methods in which you apply an aluminum dolly to the coating by means of an adhesive. You then pull the dolly off using some form of tensile instrument such as is described in ASTM D4541. If you go to http://www.gardco.com/navigation.html you will see many different types of pull-off adhesion testers. Their results do not necessarily correlate with one another, but at least you will get a good idea if the adhesion in your situation is good or poor.