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topic 5541

Explain the Different Finishes


(2000)

I'm investigating different finishes suggested in an automotive restoration guide. First, I'm no chemist. So I'm trying to make heads or tails out of the different finishes, processes and language used to describe the finishes I've heard when talking to service providers. Essentially, there are four finished called out in the guide, zinc or cadmium, cadmium dichromate, black phosphate and dark green phosphate.

I've heard the words 'clear' and 'gold' for the zinc plating but I don't know if the 'gold zinc' finish is cadmium. I assume not given zinc is obviously a different metal than cadmium. Would someone be able to explain the differences, the processes for each finish, and an organization in the San Francisco Bay Area who might provide a service for each finish?

Walter Muta
- Cupertino, California, USA


(2000)

That's an awfully tall order, and a week with some metal finishing texts is what you probably really need, but...

Zinc and cadmium are two silvery colored metal platings that offer sacrificial corrosion protection and a reasonably bright finish. Both corrode with a white rust very quickly, so they must be protected themselves with a chromate passivation process. The chromate can be bright blue-ish or iridescent yellow (brass-ish, gold-ish color). You probably can't tell the difference between zinc and cadmium by eye, so zinc is used unless cadmium is necessary for special cases where its compatibility with aluminum, good lubricity, and freedom from gummy corrosion products is necessary. Cadmium is toxic and never used anymore except when absolutely necessary. The yellow chromate is, more strictly speaking, dichromate--hence that term.

Phosphating is a process that is used before painting, and sometimes as an oiled final finish with limited corrosion resistance. Oiled phosphate would be grey to black. I'm not familiar with the term 'green phosphate' myself.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

 



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