Discoloration of black zinc plated parts

A. Hi Simon. Yes, black chromating can be successfully done in a barrel plating operation.

But your question involves expectations rather than being solely a technical question. The issue is that zinc plating is usually considered a functional finish rather than a decorative one ...

To offer an example of what I mean before discussing your problematical black parts: some shops do alkaline zinc plating and some do bright acid zinc. If you had had half of the the blue parts supplied by a shop doing alkaline zinc and the other half by a shop doing acid zinc they would not come close to matching in brightness, yet you'd have no basis for complaint as long as each batch was functional.

So, if the case were that the general color and shine of the black parts doesn't match your hopes & expectations, that would probably be solely on you. But if the parts actually look lousy and inconsistent you probably have grounds for complaint; I would suggest a call to the shop owner or manager telling him why you are unhappy and seeing what if anything can be resolved. But the fact that they oiled them means they already knew what they looked like; some engine parts might look perfect dark gray and oiled :-)

Luck & Regards,

A. I would suggest that you speak to your designers and your plating shop about the possibility of switching to zinc-iron alloy plating. This alloy not only offers better corrosion resistance than zinc plating but, presumably because of the iron content, it can be more reliably black chromated without light-sensitive silver-bearing chemicals. Good luck.

A. Silver nitrate IS what makes the chromate black.
UV might damage the black zinc finish but often the discoloration is due to a poor chemistry (--too much silver: dull, powdery finish; --too little silver: copper to brown color, poor coverage or blotchy; --not enough zinc plating: odd colors; --wrong pH).
The greenish-rainbowy flash over a nice black is OK, it is the chromate showing.

A. Hi Justin!

Did you measure thickness where you have this shading? It seems to be very low or none.

Maybe it's the photo, but I would check it.

Keep posting!

A. Hi. I work in a small plating company. It seems to me that the problem could be a poor rinsing after passivation. If they are not rinsed well some solution could remain causing a drip on the piece that will cause discoloration. The overall finish looks good. If it was a chemical problem there would be more discoloration overall on the piece.

A. Hi Hank. We didn't want to post your question about "light patches in black zinc plating" on our thread about "Blisters in Nickel Plating of die castings when heated" because no one will be able to find anything if we let the threads wander all over -- so we added your question to this black zinc thread, and copied the pics you mentioned here.

Light patches are not normal; and in general, defective parts can be stripped and replated ... but is the substrate zinc die castings or some other material like cold finished steel? Please send pics of your own parts to mooney@finishing.com for posting here because although a picture is worth a thousand words, 'like the other picture except ...' brings us back to the need for a thousand words again :-)

Regards,