Letter 40097

Tri Chrome Coating For FE/ZN 12  

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My customer needs to replace a cadplate coating with an equivalent coating for corrosion protection but doesn't need the lubrication properties. I have talked to two platers who are reluctant to make a recommendation other than zinc. From online research I think I need Fe/Zn 12 Type 3 class 2 per ASTM B-633 [link is to spec at TechStreet]. Does that sound reasonable? How do I ensure that trichrome is used rather than hexavalent chrome?

Dale Neff
mfgr - Los Angeles, CA, USA

Hi, Dale. As you already know, zinc is not cadmium and does not offer the combination of properties that cadmium offers including corrosion resistance, freedom from gummy corrosion products, lubricity, crimpability, toxicity to certain funguses and molds, radiation absorbtion, etc. So a plater (or an internet responder) simply can't accept responsibility for recommending an equivalent coating; the designer is stuck with that job.

As far as I know, Mil specs and ASTM specs have not been written for trivalent conversion coatings, so I think you need to write something like: "Conversion coating to be applied from proprietary hex-chrome free baths designed for WEEE/RoHS compliance, and coating to be in accordance with WEEE/RoHS requirements". That's just an off-the-cuff attempt; I haven't written such a spec myself yet.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Thank you for your comment. Rohs compliant has greatly aided my getting the information I need.

Dale Neff
mfgr. - Los Angeles, CA, USA