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Definition of "homogeneous" for pre-plated steel

We use a considerable amount of preplated steel typically ASTM A653. Basically hot rolled steel with a light zinc coating and a Trivalent chromium wash to prevent white rust. We have been assured that some hexavalent chromium is unavoidable as a side effect of the trivalent chromium wash and that it converts over time from hexavalent chromium to trivalent.

Two questions

1) can the steel with the zinc and chromate wash be considered a homogeneous material or do we have to split the plating and the steel apart and consider each a homogeneous material?

2) if we split them what do we use as the measurement, the concentration of hex in the bath at the time of plating or the concentration of a piece that has the plating chemically stripped and analyzed?

William DeBerard
Mechanical engineer - Thornton, Colorado, USA
2005

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July 3, 2010

Hi, William.

With reference to your introductory remarks: I keep hearing secondary or tertiary references to this idea that "it converts over time from hexavalent chromium to trivalent", but I haven't seen a primary reference that actually makes that claim. If you can point me to where you read that, or where the people who offered you assurance read it, I'd appreciate it.

Your question 1: Unfortunately, this must be placed in context because my own definition of "homogeneous" and yours probably isn't going to have much to do with your problem :-)

Presumably, you are concerned with whether the product complies with European RoHS standards for export to and use in Europe. And for that situation you are going to have to go with their definition. To them, I think you will find that the chromate is "homogeneous" with the plating (the two are taken as a whole), but the plating & chromating are not "homogeneous" with the substrate.

While this strikes me personally as ludicrous (since you can rub the chromate off of the plating with a simple pencil eraser, whereas you can't remove the plating from the substrate with a chisel), you still have to go by their intentions since they will be the judge :-(

Your question 2: RoHS isn't involved in what is in your plating bath back in the States, they are concerned with what is on the parts in Europe.

Good luck!

Regards,

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