Rainbow color zinc plating

A. Hello, Shantanu!

I work only with blue and black chromate, but have you checked your chromate concentration and pH? Working outside the parameters might cause a discolored chromate.
Also, try reinforcing your acid dip before the chromate or changing it.

Regards,

A. I believe you may be referring to "yellow iridescent" or yellow chromate. If you get a nice coating, it will give you a rainbow type effect. The spec for yellow chromate allows for a yellow to this iridescent color. I would suggest that you purchase the prepared chromate chemical from a proprietary house (see our supplier directory). The formulas are a little complicated, and much research goes into making a good chromate.

A. Generally, the longer you immerse the parts in the yellow chromate solution, the more pronounced the iridescence will be (although too long of a dip time will cause the chromate to rub off. Also, subjecting the parts to heat, such as a heated forced air dryer, will magnify the iridescence. Finally, using a chloride zinc system will generally give brighter parts than cyanide or alkaline non-cyanide based zinc.

 Readers: Keith's response was very insightful and seemed to answer Srimath's inquiry well. But you can search the site to find other threads on rainbow passivation of zinc if this didn't answer the question you have in mind :-)

Regards,

A. Combronde are you alkaline zinc or acid chloride zinc plating? The colors you describe sound as if you are putting a yellow dichromate on the parts. Yellow dichromate is characteristic of red, blue, green, purple hues. You can experiment with the dichromate bath by varying pH, temperature, concentration, time. Alkaline zinc will allow more of a consistent yellow or gold color than acid zinc. Acid zinc is a finer and smoother deposit and brighter. Experiment with beakers [beakers on eBay or Amazon [affil links] in the lab. The yellow dichromate is doing what it is supposed to do.

A. I agree with Bill. Plated parts with yellow dichromate have that "rainbow" hue to them. That is how it is. We make parts that look like this too. Your customer has to either accept the color how it is, or look at specifying a different metal finishing process.

A. Sir,

The iridescence in yellow passivation is akin to colors on thin oil films or when a bright steel strip is heated to different temperatures. This is known as "interference colors"

The colors that appear on the zinc surface depends on the thickness of the passivation film, the thinner films interfering in the blue, yellow, and green regions and the most thick film appearing reddish.The iridescence that appears shows that the passivation film is o.k. If a very thick film is applied, e.g., a lacquer coating, then the interference stops and the iridescence disappears. Thanks

Ramajayam

A. Dear Christian,

The change in colour is due to refraction from a thicker chromate layer on top of the Zn plating. The difference can be due to a number of factors such as time, concentration and pH of the chromating solution. The organic content of the deposit can also effect the colour.

In fact the type of finish you require (blue) is the worst finish from a corrosion protection point of view. These panels will corrode very quickly once they are in the general environment. A yellow chromate would be a much better choice of finish, if you are concerned about corrosion resistance. Zn deposits are not meant to look pretty, they are used to delay corrosion of the substrate. By choosing the most attractive Zn finish, you are limiting the corrosion protection of your profile.

You say that your plater uses an acid Zn bath. I would recommend an Alkaline Cyanide-free bath, as this will give much better deposit thickness inside your profiles.

Hope this can help you a little.

Regards,

A. Dear Christian,

Sometimes excess addition of brightening agents gives this type of problems or organic breakdown can also bring problem in . If you are using acid chloride bath check for iron build up in your bath .

Regards,

A. Christian,

The discolorization on the parts is due to inconsistent plating time. You noted the work is being done manually. Your plater needs to develop a procedure (work instruction) documenting precisely the required plating time to obtain a uniform overall finish and the required thickness on every part.

Regards,

A. Hi Michael. Iridescence is a common and natural occurrence with both clear and yellow chromates, but it usually poorly controlled and rather random. I suspect some other chemical was applied on top of or in lieu of chromate in some of your photos. As usually happens, once a particular look becomes accepted and wanted, people come up with new ways to achieve it. I don't know the chemistry of the coating, but YouTube has videos of rainbows being applied by simply dipping several times into a vat; I think this approach will become generic in the not too distant future.

If you are looking for a controlled iridescence on a decorative plating, and cost is not critical, I think these subtle rainbows on nickel plating may be preferable though -- zinc plating is not generally considered to be a reliable aesthetic finish like nickel is.

Luck & Regards,