Problems Black Chromating on Zinc Plating

A. Dear cousin Senthil,

I suggest that you look up plating process suppliers in your area, and ask a supplier to visit your plating shop. These processes are not "home brew" that you mix up yourself from commodity chemicals; they are proprietary systems of compatible electroplating processes together with chromate conversion processes and post treatments that a supplier who has developed them helps you install and use.

It sounds like you are using a light-sensitive silver-based black chromate, and these are known to turn greenish (see thread 32622). You may need to switch to zinc-iron alloy plating before you will obtain color stability. Blisters are most likely due to poor cleaning, but not definitely. Plating thicknesses range from 5 to 25 microns depending on exposure. 5 microns is typical for 'cheap' stuff, 8 more typical for automotive, and 13 for severe service.

But please tell us what you do know about the current system: What proprietary chromate or home brew formula do you use that is black? What kind of zinc plating do you do -- acid, alkali, or cyanide, or do you do alloy plating? What surface finish do the components have already? What thickness do you plate now? Do you have brake fluid compatibility now? What thickness ratio do you get between the HCD and LCD areas now with respect to plane and intricate surfaces? What NSS life do you get now?

Please don't be offended, but your questions are not so much specific questions as a request for tutoring, and a local chemical supplier will be in a better position than me to offer it. Thanks!

A. The plating system has two main parts to it: the main zinc plating, and the passivation. The thickness of the plating depends on your application. The auto parts we work with get around 8 microns of plating. The passivate is not specified by thickness.

Some of these technical issues should be addressed with your plating supplier. They should understand their processes well and should be able to educate you a bit on issues like this. You can also refer to the Metal Finishing Guidebook from Metal Finishing Magazine. This guidebook gives generic technical explanations of plating and of conversion coatings (passivation).

Sometimes is is very hard to produce an exact color consistently. The color is dependent on the passivation, not necessarily the plating part. As you make part after part, the chemicals in the bath deplete, and the concentrations change over time. It can be difficult to maintain the passivation chemical bath the same all day every day. You may want to relax your color requirements to make things much easier for your plater. The plater may not have the capability to meet tight color requirements.

A. Traditionally, there hasn't been a great difference between yellow and black chromates. However, with the worldwide move to using only trivalent chromates, it is not easy to get color choices, and different colors can represent fundamentally different processes.

Yellow chromates today can be thick-film chromates or they can be dyed films (although not all trivalent chromates are dyeable). Black trivalent chromates are more difficult and you may need to switch from zinc plating to zinc-iron plating to get a black color. Zinc-iron processes tend to more expensive, but they deliver greater corrosion resistance.

A. Mr.Karimulla,
Ted rightly mentioned about the trend in Trivalent chromate conversion coatings.

Trivalent passivations greatly enhances the salt spay life and it improves with respective topcoats.
Blue with topcoat can give up to 200-250 hrs white rust, and red rust up to 400 hrs in acid chloride with cobalt as alloy.
Black with topcoat gives up to 350 hrs white rust on acid chloride plated components and up to 600 hrs and more as mentioned above.
If passivated on Alkaline Zinc iron the SST life can go up to 600 hrs white rust and beyond 1200 hrs red rust depending upon the concentration of iron as alloy in the bath.

A. Dear Ravi Sankar,
Yes. there is Trvalent black passivation available.
There are supply houses in and around -Ambattur-Chennai.
Please look around.
Good luck,

Hi Bahiru. Specific questions often get good answers in this forum, but short, abstract requests for information that fills whole chapters in plating books usually don't. We have to give people a reasonable starting point :-)

Please try to detail what you already know how to do, and explain whether you are talking about chromating on zinc plating, and whether you do barrel or rack plating. If you can purchase a proprietary chromate and lacquer, you'll receive this required technical service from the vendor. Good luck!

Regards,

? Hi Stephen. Please email pics to for posting here. One person's white spots are another reader's pits, salt stains, or whatever. But edges usually get thicker plating, while not drying as well. So one problem may be incomplete drying. Thanks.

Regards,

A. Hi Janes. You may wish to review thread 37445 and thread 26622 which deal with 'white rust' on zinc plated and chromated components, which is what I suspect you are encountering.

But 8620 castings possibly exhibit porosity which results in later bleedout of acidic solutions. Alternating hot and cold rinsing will help, and making sure the parts are baked to full dryness will help. But in the end you may need to vacuum impregnate those castings to make them non-porous.

Luck & Regards,