Adhesion Problems

Question:

We are a custom coater and have a potential job should we be able to overcome adhesions problems. The part is an exterior part made from cast steel. It does have machined surfaces (initial adhesion test looked good on the cast surface and poor on the machined surfaces; however in the end, adhesion was poor on both). This part is zinc plated. We have run tests on plated parts with the zinc/chromate, and we have run tests on zinc plating with a brightener dip. Prior to both tests, we pre-baked the parts at 450F for at least a half hour. The geometry of the part is a 12-inch OD, nine-inch ID, and average thickness of one inch. The weight is approximately 15 lb. With the zinc chromated parts, the coating just flaked off with a crosshatch. In the impact test, the parts looked good, initially. On the zinc only with the bright dip, parts all looked good after the impact test but, in the cross hatch all peeled off. These parts did have a black soot powder between the coating and substrate (in both tests the parts went through a baking only and did not run through the three stage wash, for two reasons: 1) fear of taking on moisture; and 2) parts should be clean due to the plating process). So, what’s up with this? Why won’t it stick?

Answer:

As I always tell my clients: “Nothing good happens to a part once it has been cleaned, so coat it right away.” That is my advice to you. Even though this part was cleaned during the plating process, the surface can be contaminated as it sits around waiting to be powder coated. Also, the plating process can leave salts on the surface, so be sure to use DI or RO water as a final rinse.

If cleaning the part right before powder coating does not correct the problem, look at your cure cycle in the oven. The part you describe is quite massive and can be under-cured in your oven. Run an oven profile using an actual part to determine the correct dwell time and oven temperature. Under-cured powder coatings will often show poor adhesion test results.

 

Question

I would like to know why we have adhesion problem on some factory rims we powder coat. We wash the rim first with an aluminum cleaner, put the rim in an oven to dry, apply a primer coat to the rims with zinc-free epoxy primer, gel primer for 7 minutes, and then apply a polyester top coat. The rim is cured after the topcoat is applied at 200°C for 25 minutes. On some rims, the primer and top coat will delaminate from the factory clear coat. 

Answer

After correspondence with you, we established that you are applying the primer over a wheel that already has a factory clear coat applied. We are not certain what that coating is, but it is never a good idea to apply a fresh layer of powder over an unidentified coating. The clear-coat on the wheel may be an acrylic powder coating, because that is a very common coating for alloy wheels. But it could be something else, and you cannot reliably apply the primer over any clear-coat, especially when you do not know what it is for certain. Your best option is to remove the factory coating, thoroughly clean the metal, and add a step that will ensure a good bond of the primer coat. A conversion coating and a clean blast should do the job. After that, you can apply the primer. That will provide a reliable bond. You could try roughening the clear coat with a blast media or sand paper, but that will be less reliable than removal of the factory coating and treatment of the metal.