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? Please advise. Thank you. M.G.
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.
