I don't understand the problem. Chromate conversion coating is usually
about 1 µm thick. Less than 1/3 the weight of zinc chromate is chromium.
The density of ZnCrO4 is roughly 1/2 that of steel. So if your steel is
more than 0.33 mm thick (assuming both sides are conversion coated),
then you will be under the 0.1% limit for flat surfaces. As hardware
with a convoluted surface, such as screws, is usually pretty bulky, I
see no problem there. You could always weigh your parts, chemically
dissolve the conversion coating and analyse to determine the weight of
chromium: I'll bet a pound to a penny that very few items (very small,
thin, convoluted parts) would exceed your 0.1%.
If you have transparent, colourless, chromate conversion coatings, these
must be very thin (less than 1 µm). Zinc chromate is a quite deep and
intense yellowish colour.
If you're still scared, there are other conversion coatings, such as
oxidation and phosphatation.
Brian
to Paul Taylor wrote:
> Dear All
>
> this question is a more RoHS orientated, however it is associated with
> metallurgy, I'm a bit confused as I'd not heard of it before.
>
> What chromate conversions involve Hexavalant Chrome (this applies to
> fixings, screws etc). I've been told that both the Nickel Plate and
> Zinc Chromate platings use Hexavalant chrome. Is this true? or have I
> been miss-informed.
>
> The issue fo us especially concerns steel parts that have a zinc
> chromate finish. This is also known as zinc clear and /or zinc clear
> chromate. The problem is that this process results in the part having a
> coating of Hexavalent VI chrome. I am not a metallurgist, but have been
> informed that when tested, the directive of having no more than 0.1% by
> weight will be exceeded, by parts large in surface area.
>
> regards
>
> Paul
> Paul R Taylor
> Technical Advisor
> Pitney Bowes Ltd email: [log in to unmask]
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