George
There are two issues here: the quality/thickness of the zinc plating and
the chromate conversion. Assuming the former is OK, the latter appears
to be your difficulty. The passivation forms essentially a layer of zinc
oxide admixed with a zinc chromate. There are three zinc chromates and
none of them are colourless. ZnCrO4 is yellow and is the result of most
forms of chromatisation. If it is truly colourless, then the layer must
be very thin, indeed. Remember that the light goes through the chromate,
is reflected off the metal and back through the chromate again, so that,
as a piece of coloured transparent plastic looks more intense in colour
against a piece of white paper than when held up to the light, so it is
with conversion coatings. The zinc oxide, itself, can enter into some
acid reactions.
One advantage of having coloured passivation, apart from thickness, is
that you can see it's there! :-)
I should imagine that the easiest rapid way of determining the quality
of the coating may be by Auger analysis with comparisons against known
samples.
Brian
Wenger, George M. wrote:
> If a cold rolled commercial quality steel (per ASTM A366) assembly that is zinc plated (0.0002" min per ASTM B633 Type 3) and supposed to be colorless chromate conversion coated begins to show finger prints and surface tarnish in a relatively benign environment, should you suspect that the chromate conversion coating is missing or improperly applied? And secondly, is there an analytical method to determine the presence and quality of the coating or would one have to resort to Salt Fog testing
>
> Regards,
> George
>
> George M. Wenger (908)-546-4531
> Reliability Engineer
> RF Power Amplifier Group
> Andrew Corporation, 40 Technology Drive, Warren, NJ 07059
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