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RoHS Compliant Gold Alodine

2006

Hi

The gold chromate conversion (aka "Alodine") finish, as specified on sheet metal enclosures, does not comply with RoHS due to the presence of hexavalent chromium (Note: the hexavalent chromium imparts the 'gold pigment' to this finish). Our local metal finishers both offer an RoHS-compliant finish (Alodine 5900T®). However, this finish is 'clear' in colour and isn't available with gold pigment. I was told by one of them that parts finished within this specification would exhibit a 'blue hue', on top of the base metal.

To validate the cosmetic appearance of this new finish, we'd recently sent some parts to our local finisher. Upon receipt, and subsequent examination of the metal work, there was no indication from a cosmetic perspective that the samples were even processed .The 'finish' was clear with no 'blue hue' present. Any imperfections, including handling damage and machine tool marks, were highly visible. In essence, the metal work resembled those of prototypes rather than production-ready parts. That said, our current gold Alodine is matte in appearance and is quite effective in masking small scratches and other surface flaws.

The Alodining process serves three purposes:
to improve cosmetic appearance (no longer true with RoHS-compliant process)
to improve corrosion resistance
to improve adhesion between organic topcoats/paints

Based on the appearance of these sample parts, it would seem that this new RoHS-compliant Alodine is cosmetically unacceptable on exterior metalwork. Is this true? There is no way to convert to a RoHS compliant Gold Alodine finish?

Daryoush Moayed
Kongsberg - Port Coquitlam, BC, Canada

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Please note that "Alodine" is a Henkel trademark for a range of conversion coating chemicals. The supplier can, and in fact does, use the name Alodine to refer to many different processes, some of which are hexavalent, some of which are RoHS-compliant; some are clear, some are yellow or other colors; some are for aluminum, some I believe are for zinc, etc. The fact that local finishers offer Alodine 5900TA® does not mean that this is the only RoHS-compliant Alodine that Henkel offers or can ever offer. But it is apparently true that most of the best RoHS-compliant finishes for aluminum today are clear; the TCP-approved conversion products (Henkel is one of the suppliers) are essentially invisible.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006