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Computer Chassis Pre-Plating

 

Would like to reference a letter #11070 and re-issue the question. Did not see any responses in the archives. I'm also looking for a common call-out for pre-plated steel; the type used in computer chassis. Would also like to know the difference between this type of pre-plating done in the states vs. overseas. Does the pre-plating process involve any form of chromate?

Thanks for the help,

Pre-Plated Steel

Looking for a call-out for pre-plated steel (crs), Like the type that is commonly used in computer chassis. Is there an ASTM number or some other standard? Or is it best to call out a specific brand like GALXC. I would prefer to use a generic description because some specific brand names available in the US, are not available over-seas.

Michael Ward
TecWest Engineering - Tucson, Arizona, USA

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Steel is zinc plated because zinc is a sacrificial material that will prevent the steel from rusting. Preplating is not quite as good as plating after assembly because the sheared edges of preplate will have no zinc on them, nor will any spot welds, screw holes, etc. But it is more economical to plate strip than a fabricated chassis.

Zinc itself will corrode (white rust) almost immediately unless it too is protected by painting or chromating. If the parts are not to be painted they must be chromated.

Traditionally, chromate coatings came in two common varieties: the "clear" or "blue bright" which were usually based on trivalent chromium compounds and the "yellow" or "iridescent" which were based on hexavalent chromium compounds. Selection of which to use was based on which color was preferred (the clear looks sort of like silver or nickel plating and the yellow looks sort of like brass) and the fact that the yellow chromates were significantly more corrosion resistant.

Hexavalent chromate is a material that is toxic and we would like to get out of the environment. The European Parliament has greatly restricted the amount of hexavalent chromium that can be on autos, which has precipitated a worldwide revolution towards hastening the move away from hexavalent chromate. Europe is ahead of the U.S. on this, so to that extent the practice may be a little different.

For the relatively benign environment that computer chasses see, it's probably as easy as simply specifying trivalent chromating and understanding that it will be clear rather than yellow.

In more difficult service, it is tougher because it is necessary to find a trivalent chromate that offers the corrosion resistance of hexavalent chromate; this is not the normal state of affairs so the new hex-free chromates with good corrosion resistance are more complicated and expensive than the two traditional types. Further, if you need the yellow color from a solution that is not naturally yellow, you're adding another complication.

I don't know an ASTM standard for preplate myself, but I think what you need to find is one that specifies clear or blue-bright trivalent chromate on the zinc plating.

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