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-----Decorative Chrome Plating and Cr6 (Hexavalent Chrome) Rules
Mr. Mooney;
I saw your response to another reader in regards to chromium vs. chromate. My question is somewhat related and is as follows...
The European Union has issued a directive adhered to by automakers, which eliminated the use of Cr6 in zinc/chromate surface treatments. This did not, however, affect the products which utilize a nickel/chrome surface treatment (i.e. decorative chrome). Why? were the decorative chrome parts not affected by this?
Thank you.
QC Coordinator/QE, automotive fasteners - Berne, Indiana, USA
2007
Hello, John. Admittedly this issue is quite confusing for people who are not personally involved in metal finishing because two very different metal finishing processes use chromium, and either trivalent chromium or hexavalent may be used in either, and whether hexavalent chrome was used is very important to the end user in one case ... but doesn't at all in the other 🙂
Let's talk chromium electroplating first. One step in the process is to immerse the part into a chromium plating tank, apply electricity, and electrochemically deposit the bright chrome metal that we see on truck bumpers. Either trivalent or hexavalent chemicals can be used for the plating. But once the part leaves the tank and is throughly rinsed, the only chromium it has on it is metallic chrome -- not hexavalent chrome, not trivalent, just metal. Metallic chrome doesn't have valence, and it makes no difference to the end user whether the manufacturer created that metal deposit by dipping the part into a tank that used trivalent chromium chemicals or hexavalent chemicals.
While there are advantages to trivalent chromium electroplating baths (like lower fuming, and reduced operator exposure and water treatment costs) and advantages to hexavalent chromium baths (like lower cost, thicker deposits possible, more consistent color), they don't affect the safety of the product or the end user.
But in the case of chromate conversion coatings on aluminum or on zinc plating, the deposit is not chromium metal-- it is a rather complex mixture of "salts" of chromium. At the risk of slight simplification, if the tank it was dipped in had hexavalent chromium salts in it, the deposit on the part contains hexavalent chromium salts, whereas if the tank it was dipped in contained only trivalent chromium salts then it has only trivalent salts in it. Hexavalent chromate salts that remain on the part in this case are considered hazardous.
Ted Mooney, P.E.
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