We have been asked to quote chromate conversion coating for an aluminum sand casting prior to powder coating. My understanding is that all normal powder curing temperatures exceed the temperature limitations for chromate conversion coatings, and the coating will simply dehydrate. We suspect that the chromate conversion coating is inappropriate and perhaps even detrimental for this application, leading to poor powder adhesion. What would be an appropriate pre-powder coating preparation, if any, other than just normal cleaning of the sand casting? G.F.
Chromate conversion coatings are normally used to pretreat aluminum substrates prior to powder coatings. In fact, this is the preferred method of pretreating aluminum when high corrosion resistance is required using powder coating topcoats. Achieving corrosion resistance of 5,000 hr salt spray is typical when using this form of pretreatment. The outdoor furniture market has used this technology for more than 25 years and normally warrants its paint finish for 20 years in very arduous conditions.
You are absolutely correct in your statement that chromate conversion coatings will dehydrate when exposed to temperatures above 180F. Dehydrating the conversion coatings can cause a chalky residue on the surface of the pretreated aluminum substrate. This situation has prompted many others to ask the same question you are asking: “If chromate conversion coatings dehydrate at 180F, then how can I use this pretreatment in a powder coating process where dry-off temperatures exceed 212F and cure temperatures can exceed 325F?”
Here is what I have found to be the empirical results over the many years I have been in the industry:
If the powder coating process is carefully controlled to provide adequate drying without overexposure to high drying temperatures and if the product is fully covered with powder or tightly masked, then there is no measurable difference in corrosion protection or coating adhesion when using a chromate conversion coating under a powder coating. Violate the above-mentioned principles and all bets are off, and you may not be satisfied with the results.