Aluminum Sand Casting Preparation

Question:

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.

Answer:

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.

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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:

• The chromate conversion coating will not degrade significantly in the dry-off process as long as the oven temperature is closely controlled to heat the substrate to 212F (the boiling point of water) for 10 min or less. Reducing the time the substrate is exposed to this temperature is as significant as the actual substrate temperature itself. Using blow-off devices and/or a heated final rinse stage will significantly reduce the time the substrate is exposed to this drying temperature.
• The chromate conversion coating will not dehydrate in the curing process in the absence of air. Even though the substrate temperature is exposed to heat of 325F or higher, the powder coating protects the substrate from dehydrating by shielding it from air. Those areas not coated with powder are normally protected by masking devices (tape, plugs, etc.) that perform the same function as the powder coating with respect to shielding the substrate from air.

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.